Build this TV Typewriter

Build this TV Typewriter

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Description: Classic reprint displays 16 lines of 32 text characters on an ordinary tv screen.

 
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Domain:  High Tech Category: Displays Subcategory: Classic 
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Contents:
It's a super sales promoter, either lo cally or on a store-wide basis. It's easily converted to a title machine for a video recorder. It's a message generator or "an swer back" unit for advanced two-way cable TV systems. Tied to a cassette recorder, it's an electronic notebook and study aid, or a custom catalog. It's an an nunciator for plant, schools, and hospitals that tells not only that someone is needed, but why and where. And, if all that isn't enough, it's easy to convert into a 12- or 16-place electronic calculator. You can also make a clock out of it, and, with ex.tensive modification, you can even make a 32-register, l6-place serial digital computer out of the beast! Cost of the project'? Around S120 for the basic unit. This is slightly under two month's nonnal rental of commercial units that don't do nearly as much, and less than 1/10 the cost of anything commercial you could buy to do the same job. And we feel that this cost is finally low enough that a lot of new uses are now not only poSSible, but reasonable as well. The low cost comes about by using the latest available semiconductors, leaving the keyboard and case as ftex.ible options, and working in kit fonn. Printed-circuit boards and complete kits are readily available as are any special or hard-to-get-normally parts. A limited quantity of high-quality keyboards are also available from the same source. This is not the sort of thing you'd want to try as a first electronic project, but if you are wiUing to slowly and methodically work things out and carefully reason out any debugging problems, you shouldn't have an unreason able amount of trouble getting the thing to work. Once you're past a certain stage early in the construction, the TV set itself be comes a self-testing display that greatly sim plifies debugging.

To make things easier, you can get a complete copy of the entire story that in cludes additional design information, how it works, PC patterns, construction details, etc.

DO NOT AITEMPT CONSTRUCTION WITHOUT THIS ADDITIONAL INFOR MATION!

Construction is done in stages. Once each stage is tested, it is safe to go into the next, progressively working up to a com plete unit. The basic machine we'll show you works from a keyboard or a set of six switches and a pushbutton. Thanks to the plug-in construction, low-cost add-on circuit boards can let you talk to a computer or a cassette recorder, or adapt the unit for 12place calculation. These add-ons will be

picked up later if enough readers seem in terested. They're not needed for most of the possible applications of this TV Type writer.

Specs of the unit

Complete specs appear in Table l. The basic device generates and stores 512 char acters, arranged as sixteen lines of 32 char acters each. A second page of characters is easily added internally to bring the total up to 1024 characters. For more storage, a C90 cassette can store well over a hundred pages, so the total capability is quite large. The characters available are standard

ASCII ones that include the capital letters, numerals, and most punctuation. The TV Typewriter is self-powered and contains its own miniature TV transmitter

which simply clips onto the antenna tenni nals of an unmodified TV tuned to an unused channel. Several TV's may be driven simultaneously, and a direct video output is also available for industrial and experimental uses. \y"hile any TV can be "borrowed" and used with the typewriter, small, high.quality portables give the nicest presentation, and Slight size and position

HOW TYPEWRITER ELECTRONICS IS ASSEMBLED within the cue. The Umlng, cUrior and memory board. plug Into

the

mainframe and each other, cordwood fa.hlon.

2 RADIO-ELECTRONICS

adjustmenl" can further help the appear ance, although they are nOI neeJed. The characters are added one at a time and normally go on the screen JUI like you were typing. This is done b)' prwiding the proper ASCii character code on seven in

put lines nd tripping an c:ighth "key pressed" line to enter the character. A winking cursor tells you whelC: the next character is to go, bUI you can [urn this otf if you want to. Should the screen get filled, the machine starts over again on the top. rewriting over the old message. a Besides the normal openitloll you have complete editing capability. You can

A SURPLUS KEYBOARD Is used In this version of the TV typ. writer. Unl! on cover has a home made keyboard baled on the a, Ucle In the February Issue.

TABLE I
COMPLETE SPECIFICATIONS-TV TYPEWRITER STORAGE: 1024 Characters arranged as 2 pages of 16 lines of 32 charac ters each. Rf Output tuneable from channel 2 through 5; clips directly to the antenna terminals of one or more unmodified television sets. Optional positive-white video output. Parallel, TTL compatible, ASCII character code (Table II) is in put with positive logic on six lines; a seventh keypressed line is suddenly brought to ground to input character, Internal de bouncing. The full 8-bit ASCII code may also be used as an in put. If done, any CTRL input will be interpreted as a combined CAAAIAGE RETURN and LINE FEED, CTRL output available for code extension. Begins in upper left HOME position and proceeds as in normal typing. Carriage return and linefeed automatic at end of line. At bottom of screen, jumps to upper left HOME position and re writes over old text. Winking cursor indicates next character position. Cursor may be blanked and may be independently moved in any direction with or without changing text. One or more letters may be easily changed at any time. Internal, crystal controlled TTL divider. Basic video clocking rate = 4.562 MHz. 15,840 kHz noninterlaced horizontal scan rate; 60-Hz vertical scan rate. Easily converted to full interlace for Video Recorder titling applications.

OUTPUTS:

move the cursor either direction anywhere you want and then change only the charac ters or words you wish to, thu.. edIting something you already have on the screen. This nicely handles mistakes without having to stan over again. A REPEAT key is avail able for putting down a group of identicaJ characters or getting to a given position in a hurry. There's a KEEP-CHANGE switch to protect what you have written while you are moving around, and you can home the cursor to the upper left either by itself or erasing the whole picture on the way. Other switches control the direction the cursor goes, which page is being displayed, and optionally whether the mode will be a full screen one for typewriter use or a line scan one for calculator use. Computer people would call this a par allel input system with off-line Single machine command is available; this is the UNE FEED. Thus, any CTRL key moves you down a line. Other remote commands are easily added, but were left off to hold the cost down. The contents of the memory can be retransmitted with slmple circuit modifications, and the whole system is bus oriented to allow all sorts of add-ons with out major circuit rework. Character input rate is asynchronous and up to 30 characters per second, thus making the beast three limes faster and compatible with the industry standard ASR�33 teletype. Hard copy is via cassette recorder or Poloroidr photos.

INPUTS:

FORMAT:

EDITING:

TIMEBASE:

Organization 01 the Instrument

MEMORY:

page conversion, optional memory output, and optional exten sion to calculator, computer, and other functions. CONTROLS: Internal: Rf frequency (trimmer capacitor)

512 word by 6 bit MOS dynamic storage, bus orientAd for easy

Position-(Jumpers-4 horiz; 3 vert for 12 possible lo cations.) EXTERNAL:
KEEP-CHANGE ON-OFF

the circuit is arranged like :l set of snap-to gether blocks. This way, the only inter connect wiring consists of the line cord and the 300-ohm twinlead output. Since the in terconnect wiring is locked into the board and 6O-pin connector system. the biggest single headache and potential error source is eliminated. shows the h:'lsic blocks. The contains a power supply of + 12, +5, - 5, and - 12 volts; the control switches; the rf modulator; the internal test system; and connectors for both the key board and the other boards in the stack. There are three other essential boards. The MEMORY board is the most imponant and the most complex. II contains a dy namic MOS (Metal Oxide Semiconductor) shift register that stores S 12 words of 6 bits each. It also holds a Single-line memory; a character generator; and an output video register. We'll see later that the single-line memo,) is needed to get each character
MAINFR:AME

To keep things as simple as possible,

Fi g.

1

PAGE OR LINE SCAN

memory protect page select REPEAT or SINGLE character HOME or RUN cursor location
A

or

B

CURSOR ON-OFF ADD-SUBTRACT

cursor direction

CONSTRUCTION;

Modular mother-daughter boards. Mother board contains power supply, rf modulator, and control switches. Timing board, cursor board and one or two memory boards snap on as a stack. Add cns such as calculator and MODEM FSK unit snap onto same stack: not included in basic unit. 33 integrated circuits, of which 8 are MOS LS1. 7"x8'h"x3", not including keyboard or case.

SIZE.

back eight times in sequence for eight suc cessive TV scans. For a page-A memory. you need all of

RADIO-ELECTRONICS 3

this board. The additional page-B memory does not need a new single-line memory, character generator, and output video regis ter, as it can borrow the one in the page-A memory when the second page is in use. This is called bus organization. The charac ter generator will respond to anything that is enable on th bus, be it page-A memory, page-B r.:mory, a calculator add on, or whatever. Of course, we have to be careful to onl} enable one possible source of characters at a time, but this is easy. We can also use the bus optionally to output characters to the outside world. The output of the memory board also contains a video combiner that assembles the character video, sync signals, and the internal test signal into one composite video output. This output may either be used directly or routed to the rf modulator for clip-on operation of an unmodified TV. It can be optionally flashed or blinked. The TIMING board contains a crystal divider and TTL (Transistor Transistor Logic) countdown chain that generates all the needed signals to run the typewriter in proper sequence. It does not normally use interlace, but the timing chain is split so that the somewhat more complex TV full interlace system can be added if you need this sort of thing for video titling. There are two principal areas to the timing board, the MAIN timing. and the DERIVED timing. The main timing is the continuous waveforms obtained off the cry.stal divider, while the derived outputs combine portions of the main timing signals into properly coded sig nals needed to run the rest of the type writer. Two examples are the composite sync signal and a blinker used for flashing, cursor winking, and repeat functions. The third essential board is a CURSOR board. Anyone who ever lried to design and debug a simple one of these will easily understand why it is called a cursor board. Anyway, the cursor keeps track of where the next character is to go; runs the wink ing line that shows the character position; controls entry of the character; and option ally sets up characters for output It also contains an input conditioner, and deboun cer and a detector for CTRL commands that tells the typewriter to carriage return rather than enter a character. Many cursor systems are extremely complex. This one is relatively simple in that it uses a phase-shift counting technique. The cursor has a continuously running counter just like the main timing chain does. Its output drops suddenly in some relalive position, indkating where the next character is to go. To back the cursor up, we throw in another count pulse. To run it forward, we hold back one norm31 counl pulse. Thus, the re/ative position or phase of the cursor counter advances or backs up with respect to the system timing. Actually, to go for ward, we hold back two norm31 system tim ing pulses and throw in a new one. Thjs buys us a simplification of circuitry, but still ends up with the same result. An ADD-SUBTRACT switch on the main frame controls the cursor direction for edit� ing. LINEFEED is handled by adding and re� moving the proper number of counts in the proper position in the cursor counter so that the new position is reached. Just like most typewriters, the linefeed always re turns to the lefthand side.

In normal operation, each character entry moves the cursor over one character. When it gets to the end of the line, it starts again on the next line. When it gets to the bottom of the page, it starts again at the top. A CLEAR or HOME override also moves tbe cursor to the upper lefthand poSition. And, this is about 311 you need for a normal parallel entry type of TV typewriter. One possible optional board is a MODEM or frequency-shirt-keying interface. This would use a MOS chip and some TIL to convert to or from a serial tone input, suitable for computer or telephone line communication. A cassette recorder will work just as well with the modem for electronic notebook use. Another possible add-on makes the typewriter into a calculator. This is done by converting the scan from a complete frame to a single line of numerals and would use a surplus calculator chip to provide the fa miliar c31culator functions. If you already have or need the TV typewriter for some thing else, this add-on is far cheaper than a conventional calculator would be, and its display would be obviOUSly larger and more readible. Or, you can add most anything else you want onto the machine by tieing into the bus-oriented lines (b1 through b6). For instance you can think of the memory as sixteen registers of 32 numbers each, and those numbers are decimal numbers plus, not bits! With six bits per word, you can store 10 possible numerals and 54 machine comands in any word! Or, you can split the registers into 32 registers of 16 decim31 numbers each, building your own computer or programmable calculator.


Of course, this computer-add-on is very much an advanced experimenter project, but it really doesn't take much mOre tl)an a double handful of TIL to pull it off. While such a computer wiD be rela tively slow (around a 33-ms cycle time), it does provide an extremely accurate and "ery low cost computer approach, partic ularly when you are working directly with BCD numbers instead of binary. Some basics Before we turn to the actual circuitry, some basics of what a character is and how it can get on a TV screen is in order. Lets start with the characters: If we had six possible binary bits of ei ther I or 0, we would have sixty-four dif ferent possible combinations ranging from {)()(XX)(), 00000 1 , 000010, . through to 111110 and finally 111111. These 64 differ ent states can represent 311 the capital let ters, 311 the numbers, a blank, and most punctuation, following the standard ASCII code. In the TV Typewriter, all of the six bits of this code must be presented at once or in parallel form, and thls is the only code the circuitry shown can use. Other codes can be converted to ASCII before going into the TV Typewriter. A seventh bit is used to separate characters from internal commands. Our final presented character consists of an array of 5 x 7 dots. Since it only takes 6 bits to store a coded character and at least 35 bits to store a generated one, its obviously much better and cheaper to gen erate the characters a er they are stored, ft rather than before. For other keyboards and encoders, the
CURSOR INHIBIT 28 VIDEO CLOCK 47

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reduc.. circuli complexity and makea poaslbte a very compact

FlO. 2-SCHEMATIC OF THE CHARACTER GENERATOR. the u..

4 RADIO-ELECTRONICS

add it for video titling or other places where you must superimpose the TV Type� writer's output onto an existing program. We assign 48 possible character posi tions across the TV screen, but we only use 32 of them. The remaining 16 allow for re trace and the extreme overscan used on economy TV's, We assign 12 scan lines per row of characters. The uppennost scan line is blank except possibly for a winking cur sor that appears as a bar above the next character to get input. The next 7 lines form the character as a series of dots 5 dots wide by 7 dots high, and the final 4 lines
INSIDE VIEW SHOWS STACKED BOARDS conltllnlng the electronic clrculta In the TV

arrives, for, just like a typewriter, we don't want to enter anything while the carriage goes back to the beginning of a line. The only time we actually enter a character is once when the cursor board tells us it is the right time, then only if we are working on this page. want to Change the character, and are not trying to return the carriage.

About keyboards
So where do we get our characters? The simplest and far cheapest source is from six switches and a pushbutton, ar ranged to get us + 5 for a "1" and ground for a ''0'' and set up so the pushbunon gives you a sudden + 5 to ground "Key pressed" output. This is handy (almost es sential) for testing. and can be used for message generation, although it takes quite a bit of practice to get any speed. A keyboard is the next best bet. such as the low-cost keyboard in the February 1973 issue of Radio-Electronics, and its low-cost companion ASCn encoder (April 1973). Many of the surplus keyboards of fered in the back pages of Radio-Electron ics can also be either used directly or read ily adapted. The input code must be in ASCll. Any keyboard that consists of one or two make contacts per key must be converted in a suitable encoder, again such as the low-cost ASCII encoder described in the March 1973 issue of Radio-Electronics. Further, the ASCU output must never exceed the in ternal + 5 volt supply of the typewriter, nor should it go below ground, even by a small amount. Jumpers 00 the timing band allow for selection of 12 possible display positions so that internal TV adjustments need not be changed.

typowrt.....

are blank. These allow for the space be tween character lines. We likewise assign 22 possible charac ter lines but only use 16 of them, this time saving 7 for vertical retrace and overscan. By picking the right timing frequencies, we obtain a horizontal rate thats so close to the normal rate the TV doesn't know the dif ference, and a vertical rate of exactly 60 Hz. The latter is especially important to keep hum bars out of the display. Since each frame is stationary and ends with a buncb of blanks, Equalizing pulses are nei ther needed nor used. Eacb character takes six bits of storage, arranged as one parallel 6-bit word. The storage is used to hold the character from time of entry until it is no longer needed,

typewriter gives you lots of + 5 and a choice of + 12 or - 12 volts at relatively low current. The original Radio-Electronics ASCII encoder needs the + 12; a mechani cally encoded keyboard will only need + 5, while a MOS encoded keyboard probably will need + 5 and - 12 volts. A limited quantity of suitable key boards is presently available from the kit source. Other soures of input material in clude computers, calculators, the phone line, or a cassette recorder. Many of these signals will be in serial or one-bit-at-a-time form and have to be changed to tbe paral lel ASCII code. This takes an add-on board using a MOS terminal-receiver chip. Re gardless of tbe source, your input must be in parallel form when presented to the typewriter, with a "I" near but not ex ceeding the internal + 5 and a "0" near but not going below ground. Internal debounc ing is provided on the cursor board for manual keyboard entry.

which can range from seconds to days. For a single-page memory of 512 characters, we use six 512-bit recirculating MOS shift reg isters. These go around once each TV frame. The timing and cursor boards to gether decide where in each of the six regis ters a new character is to go. Once in memory, the cbaracter stays in the same relative slot until it is cleared or replaced. The memory is vioiatile, meaning that you lose the message if power drops for more than a half a second or so. Note that an ASCII blank is 100000. All l's is a "1" and all O's is an "@'. This is helpful when troubleShooting, for its ra ther difficult to get a totally blank screen by accident. On the other hand, this means we have to be careful when we clear our memory to erase the screen. Here, we posely have to set up the 100000 code. way to do this, is to hold down the board's space bar during the clearing pur One key pro

Interfacing with the TV

To use an unmodified TV, we have to

build a miniature transmitter and arrange the signals so they compare as closely as possible to a normal broadcast set of scan ning standards. This way, any TV can be driven by the typewriter simply by clipping onto the antenna tenninals and tuning to an unused low channel. A TV starts in the upper lefihand cor ner and sweeps a dot rapidly to the right and slowly downward, taking around 62 ps to get across the screen and 33 ms to get to the bottom. It then repeats the process again and again, presenting a series of dots that assemble into a series of still pictures that the tube phosphor and your eye in tegrate to get the effect of a complete and usually moving picture. BrighUless is changed on each dot by controlling the picture tube's cathode cur rent, which in tum follows the input rf sig nal seen at the antenna terminals. Very low signal is seen as white. The stronger the sig nal, the blacker the picture. The sync sig nals are the strongest of all, or "blacker than black", and are used to synchronize the scanning of the television set to the transmitted signal. To provide sync, we need one horizon tal sync pulse at the beginning of each scan line, and one vertical sync pulse for the be ginning of each frame. To keep things simple, we make all the frames identical in the TV Typewriter instead of using inter lace. lntedace has no advantage on a sta tionary message presentation and simply adds parts. If you have to you can easily

Character generation
We use raster-scan dot-matrix charac ters providing an array of 5 dots wide by 7 dots high for each character with one "un dot" between characters for spacing. Seven passes of the TV raster are needed to gen erate each line of characters. This says we must borrow one line's worth of characters (32) from the memory and put it into a new line register memory, use it over again at least seven times, and then later on, go get a new line of characters, To do this, we need a line memory, a single IC consisting of six. 32-bit recirculating shift registers. Let's go through a typical scan and see what happens. Most of the circuitry is shown in Fig. 2. Suppose we just retraced to the upper left hand comer. We're now on line I of the top of the characters. On line No. I, our line� register is connected to the memory and it samples the next 32 charac ters to be presented. The main memory

cess. A better way is to remove the key board encoder power (via the UNCLEAR or not-clear line from HOME switch S6) from the encoder to get all O's out. Then a "I" can be force-fed to the a6 input line to set up the proper code. This gives us a one button clearing operation. Other schemes can easily be worked out, but the essential thing is that the H)(XXx) code gets set up during the time you want to erase what you have. We normally put a character into memory and leave it tJtere for a relatively long time. The memory usually is in a re circulate mode where its own output is con nected 'internally back to its input. The memory is a serial device-the bits tAke turns coming out one at a time, and 512 pairs of clock pulses are needed to tum the memory over exactly once. The memory is nonnally in the re circulate mode. We keep it there if we are using the other page or if we have our
KEEP-CHANGE

switch in the KEEP position. We also force it to recirculate if a CAR RlAGE REllJRN command (a6 and a7 = 0)

thus fills the line register. For the next twelve scans, the memory is idle, but the line register brings the same characters back over and over again. On scans No. 2 through No. 8, the character is actually generated. The line register drives a charac ter generator. The character generator is also con nected to logic that tells it which part of each character it is working on. The output of the character generator goes to an output register that converts the characters into ac tual video.

RADIO-ELECTRONICS 5

Since line No. I is supposed to be all blanks, the character generalor is told this and we get all blanks, except possibly ror a brier cursor winking bar. On the second scan line, we ag ai n clock the line register 32 times, letting it go once around. The main memory just waits. This time, the character generator is told to work on line No. 2 and please put down the lOp row or dots on each character. For instance, ir a ';T" comes up, we get five ''I's'' in a row. An "S" would be 0 1 1 1 0 and so on. As the TV scans across, each top row or dots ror each rollowing character is put down. line register 32 times. This time, the second row or dots gets output, with a "T" being a 0 0 1 00 and an "S" being J(XX>I , and so on. Lines No.4, 5, 6, 7, and 8 are handled the same way, with the character generator working on the line it is told to and the line register going once around ror each line. By the end or the eighth line we have put down all the dots we need ror a line of 32 complete dot-matrix characters. The cir cuitry is blanked ror the next rour scan lines, providing us with a space between character lines. On line No. 13 (a new line" I"), our main character memory is once again clocked 32 times and the line register is si multaneously clocked. This fills up the line register with a new set or 32 characters. The same operation repeats ror each of the sixteen rows or characters that we want to put down. Notice that the timing runs in bursts and is not continuous. Thus, the line regis ter runs for 32 counts and waits 16 for re trace and so on. The memory does the same thing, but only on every twelfth line during the active scan. Carefully established internal timing delays take care of settling times between memory. line register, char acter generator, and the final video gener ating output register. The output register converts the five parallel outputs of the character generator into serial, high speed video. On the next pass, we again clock the

PARTS LIST
MAINFRAME Cl-5000-IlF 10-V electrolytic C2, C3-1000-IlF 25-V electrolytic C5, C7-470-pF disc for vhf bypassing C6-3-30 pF trimmer C8-27-pF mica Cl0-Gimmick attenuator m de of twin lead Dl to D6...... or equal D7-12-V, l-W Zener, lN4742 or equal D8-6.8-V, l-W Zener, lN4736 or equal D9-5.1-V, l-W Zener, lN4734 or equal Fl-I-A fuse and fuseholder Dl0 to D14-1N914 or equal ICl-7aOS regulator (Fairchild or Motorola) Jl, J2-Binding posts, one yellow, one black Ll-Coil made from 4" Of No. 14 solid wire transistor in metal can, (S way) C9-47-pF mica C4-100-IlF 6-V electrOlytic C14-0.1-J.(F disc ceramic IC8-2518 MOS hex 32-bit reCirculating regis ter (Signetics) IC9-2513 character generator, MOS (SignetIC10-74165 TIL 8-bit PISO register A34-220 ohms ohms A27 to A32, A4S A36-470 ohms ASl-330 ohms MISC: PC Board, ers; Sleeving; Solder. IC11-7401 TTL open collector NAND gate lcs)

C} 1 Nt,.(.h.n.

A33, A35, A38, R40 to A42, A43, R44-2200

to A50-6800 ohms

A37-47,000 ohms

4'12"
PC

x 6'12"; =24 wire jump Terminals (Optional-2),

01-2N918

do

not

substitute!
A3-22 ohms,

Cl to C4-01-pF 10-V disc ceramic C5, C6-16D-pF mica C7-0.001-IlF disc ceramic C8-100-IlF 6-V electrolytic C9-33-p.F 6-V electrolytic IC1-MC4024 dual astable (Motorola)

TIMING BOARD

AI, A2-47 ohms,

A4, A9, A10-2200 ohms, A7-4700 ohms, A8-470 ohms, SS to

'hoW II4_W '!.-W IJ.-W

'!.-W

S 1, S2, S3, S4, S7, S8-dpdl rocker switch S6-dpdt rocker switch, spring return

IC2, IC3,IC5-8288 divide by 12 (Signetics) IC4-7473 dual J K, IC6-8288

momentary

TTL

SOl to S06-connector, Molex 09-S2-3103 Tt-Power equal MISC:-PC Board, 83J. x 63J.; mounting brack ets and hardware (6); switch mounting hard ware (8 sets); line cord and cable clamp; hardware for T1; vertical heat sink for ICI; No. 24 jumper wire; sleeving; No. 14 wire for L1; fuse clips and hardware, 30D-ohm twin lead, 18"; PC terminals, optional-2; solder. S07-TV lead-in connector transformer, dual 12-V center

IC7, IC8-7432 quad OR gate, TTL IC9, IC12-7402 quad NOA gate, TTL IC10,IC11-7410 PI nector to P60-Pins to fit Molex 09-52-3103 con-

tapped secondaries, 1.5-A. Signal 24-1A or

A2, A3-220 ohms, A4-2200 ohms,

Al-330 ohms

VO-W

SOl to S06-Molex 09-S2-3103 socket MISC: PC Board, 4'12" x 6112": #24 solid wire jumpers; Sleeving; PC Terminals (optional21): solder. XTAL 1-4561, 920-kHz series-resonant crystal

*h-W V.-W

MEMORY BOARD
A" board. Memory "B" boards are optional. These parts are needed for a Page B memory: Note: Each system needs one "Memory CI-1200-pF mica C2-4300-pF mica C3-620-pF mica C4-6200-pF mica C5-100O-pF mica

CURSOR

either

a Page A or

C1, C3, C5, C7-100-J.(F 15-V electrolytic ramic 01,04, OS,06-1N914 or equal 02, D3, D7-1N4001 or equal

C2, C4, C6, C8, C9, Cl0, Cl1-0.1-J.(F disc ce-

C6 to C9, C12 to C15-0.l-J.(F disc ceramic C 10-1OO-J.(F 6-V electrolytic Cll, 16-10-pF 6-V electrolytic ICl-740a quad AND gate, TTL IC2, IC4-74197 or 74177 or 8281 or 8291 diIC3-7473 dual JK TTL IC5, IC6-7402 quad NOA gate TTL IC7-7474 dual0 flip-flOp, TTL IC8-7400 quad NAND gate, TTL IC9-555 timer, Signetics PI to P60-pins 01-2N5129 Al, AS, R8, A13,A16, A21-1000 ohms, A22-2200 ohms, nector vide by 16

ICl to IC 6-2524 MOS 512-bit recirculating shift register (Signetics) IC7-7406 hex driver, TTL Pl to P60-Connector pins to fit Molex 09-5201,02-2NS139 3103 connectors

About the memory bus
So far, we've assumed that we were us ing the page-A memory with the page-A character generator. Thanks to the memory bus ( b l through b6) we can connect any thing we like to the character generator, in cluding the page-A memory. the page-B memory, or anything else we want to hang on these lines. To run page-B, we simply disable the page-A memory and enable the page-B memory's output. The handy thing about bus organization is thaI no complex switch ing is involved. Whatever is enabled gets connected to the character generator; other Ihings tacked on just sit there. The only rc striction is that we have to enable only one character source at 0 time. We can also use the same memory bus optionalJy to output characters to a computer, a cassette recorder, or a phone line. This way, with suitable add-ons we have a choice. We can send one character at a time directly from the keyboard, or we can send an entire page at a time from the memory. The latter is faster and more com plex but has the advantages that you can

TIL

All

Al to A6, A25-2200 ohms

resistors If.-W carbon

R7, R8, AI5-2.7 ohms A9, A23-10,OOO ohms Al0, A12-22 ohms All, R13-4700 ohms A14, A18, A19, A20-1S0 ohms A16, A17-tOO ohms A21, R22-1000 ohms A24-330 ohms A26-470 ohms

10

fit Molex 09-52-3103 con-

A2, R3, R4, A6, R7, R9, All, A14, R17, Rl8, A10, A19-330 ohms, A12-100

'!.-W

A15-100.000 ohms. v.-W A20-150 ohms,

I!._W '!.-W ohms,I!._W -W

These parts are needed ONLY for a page A memory:
C12-68O-pF mica C13-100-pF mica

SOl to S06-Molex 09-52-3103 connector MISC: PC Board, solder. jumpers; sleeving; PC terminals (optional-8); 4 W' x 6Y.z"; No 24 wire

The following Items are avllable from Southwest Technical Products, 219 West Rha socty. San Antonio, Texas, 78216.

All circuit boards are etched and drilled

fix all the mistakes first and don't tie up
nearly as much outside equipment. R-E

No, TVT1, $9.75 Mainframe board: No. TVT-2. $5.75 Timing board: Cursor board: No. TVT3, 55.75 Page A or B board No. TVT-4, 55,75 Hlgh�quality keyboard, custom remanufactured for TV typewriter use (less encoder) No. TVT-5. $18.75

A complete or nearly complete kit of parts will also be offered. but pricing depends on semiconductor availability at time of publication. Write for a complete list of available parts and prices for as sembled units.

6 RADIO-ELECTRONICS

TABLE II

line register from recirculate to update each time a new line of characters is needed.

ASCII CHARACTER CODE USED IN
A6 AS A4 A 3 A2 Al 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1 0 1 0 0 0 2 3 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 1 1 0 0 0 1 0 1 0 1 0

T.V.

TYPEWRITER

18. 19.

Line Register clock. 32 clock pulses delivered every line.

normally high, TTL negative logic.
Self test.

Superimposes

Char

Char

A6 AS A4 A3 A2 A1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 31. 0 0 0 0 0 0 1 1 0 0 0 1 0 1 0 1 0 22.

connected to any TTL point in system. 20.
21. Video output.

"1"

'"

white on video picture when

@
B C

blank
i,

Composite video for outside world of rf

A

modulator. Sync'" ground. White'" maximum positive.
9�12 Blank. Goes high on lines 9�12. 21�24. 33�35, etc.

D E F G H I
J

# $ % &

to prevent register from clocking video when blanks are wanted,
Keypressed. Goes to ground when key is pressed. Signal

TTL

positive logic.

a
0 0

is conditioned by cursor and then used for update. 23.
Blink!r.

Source of 4�Hz signals used for cursor winking.

a

K L M N

0 0

1 1

0

comma

+

0 0 0 0 0 0

1

grounded. 25 goes to ground on clear. IS otherwise +5V. 26,29. Up-down direction control. If 25 is grounded. cursor moves up a line on linefeed. If 26 is connected to P clock on cursor moves down a line on linefeed.

24,25 Clear and UNclear. 24 goes to

repeat, or outside world special applications. TTL,

+5V

on clear. is otherwise

1

29.

0 0

27,30, Right left controls. If these pins are open, the character cursor backs up a space. If they are shorted. they go forward a space on command. NOT directly TTL compatible. 28.
Cursor ON. Ground turns cursor OFF, Blinking update

0

p 0
A

gives brief positive signal to turn cursor ON above indicated character, Not directly TTL compcltible.
Protect A. Prevenls character entry if grounded. Calculator control. Optional signal. Goes to ground when

S

a

1

1
1 0 0

1
0 0

32. 33. 34.

T

V

U W

1

S
7

0 0 0

a

line scan rather than full frame scan is selected.
Protect B. Prevents character entry if grounded. Enable A. Enables Memory A and connects it to output

6

35.
0 0 0 0 1 0 0 0 0 0 0 1 0 1 0

bus if +5; disables it if grounded.
Enable B. Prevents character entry if grounded.

y

X

8

36,37 Line/Frame scan select. Connects to clock on 37 for special scans. is open otherwise, P clock connected to pin optionally combined with logic for the display of one character through 12 whole lines of characters. Characters are continuously input or output i n line mode. Normal operation is in frame scan. and this clock is not used.
Sync. Composite

Z I I
/

1 1
0

9
< >

1

37

may be

/\

a

1

38.

undo

negative logic. Position controlled by jumpers on timing board.

V

and H sync. Normally positive TTL

39-42, 44. 46 Spare pins for add-ons. 43.
Flash Display. Optional. Ground puts out display for

emphasis of one word. one line, or full frame, Useful with external timing to flash information. particularly negative To Input a character, the proper code above is set up and the Keypressed input IS suddenly brought to ground. voltage Irom 0 to +0.8 volts.
+5 volts.

numbers In a calculation. 45.
Output load command.

A

"1"

A "0" is any

Loads characters into output

is any voltage from +3 to

register a SUitable time delay after each character slot. TTL. Falling edge provides load command, 47.
21 Video Clock. "A" clock of 4.561 MHz. Marches characters

If you are using a computer. a modem. or a commercial keyboard With the full eight bit code. the eighth or parity bit is ignored. bits 6 and 7 together are NOT "0", the characters above get input. IF bits 6 and 7 together are "0", regardless of the code. you get a simultaneous line feed and carriage return, and a "CTR L" output.

If

out of output register as serial video if not inhibited by pin 48�50 "What character line is it?" commands used to tell character generator which row of dots to work on. TTL POSitive logic.
000 51.
-"-

9

12 blank.

line 3

Update. Normally low TTL. goes high in propr slot for

9

line 1,001

=

line 2 (put down top row of dots.) 002 '"

put down next row of dots). etc, ..

TABLE III�A

character update. Generated by cursor. Used by memory. Inhibited at memory if protected or CTRL being received. Overridden at memory during clear. 52.
CTRL goes to ground if inputs A5 and A7 are grounded.

WHAT DO THE CONNECTOR PINS DO?
1. 2�8. Ground
-

all boards Sent to memory A and 53.

Indicates a transparent or machine command has been received. Interpreted as a carriage return and line feed In baSIC unll.
Horizontal out.

detection. True TTL logic.
9-14.

memory B as needed. A6 and A7 also go to cursor for CTRL

Input ASCII code from keyboard.

Optional 15.840 (with crystal specified) or

Memory buss. Memory A. Memory B, or outside world act

15.150 Hz (with external phase lock loop) output useful for

as sources. Memory A character generator or outside world act as loads. P channel, silicon gate MOS compatible. Only one source should be enabled at a time.
15.16 Memory clocks. Normally

interlace and video titling. 54.
Interlace Reset. Optional. Holds entire vertical counter

times per frame to clock main memory. 32 times on line No. 1: 32 times on line No. 13,32 times on line No. 25, etc. Negative TTL logic.
17. Goes low on lines 1, 13. 25 . normally high. Used to change

+5V.

at maximum count when grounded. First horizontal clock edge follOWing starts new frame.
55. V output. 50 Hz used by cursor for synchronizing. Handy

Each goes low briefly 512

for scope sync. Optionally useful for interlace, 56.
57. -12�V -5-V 25-mA available 25�mA available

RADIO-ELECTRONICS

7

58,59 +5-V

60.

250-mA available

REPEAT ISSI (momentary)

When forward . the blinker clock is applied in place of the conditioned keypressed input, adding characters or moving the cursor at 4 places per second. In its normal position the TV typewriter

Ground. Use both grounds on all PC boards.

TABLE I V KEYBOARD CONNECTOR PI NOUTS A. + 1 2 volts for keyboard if needed (or - 1 2 with jumper) KEYPRESSED Normally +. Grounding enters INPUT Al INPUT A2 INPUT A3 INPUT A4 INPUT A5 INPUT A6 INPUT A7 SPARE GROUND HOME IS61 (momentary)

advances one character for each character entered. This circuit provides one line thats normally +5V and goes ground and one that does the opposite. To clear the TV typewriter, three things happen: (1) power i s removed from the ke\o board, forcing all inputs to the 0 state; (2) a " 1 " is applied to input A6 via memory diode 04; (3) Flip flop IC3 in the cursor is clamped and held till atter the clear is released and system timing indicates a new frame is to begin. I f you are in the K E E P position. homing simply resets the cursor and keeps the message intact. I f you are in the CHANGE position when you home, the entire

8.

C. D. E. F.
G.

+5 volts for keyboard via CLEAR (HOME) switch

H.

J. K.
L.

I.

I

I nputs Al thru A5 Must be Grounded during CLEAR (HOME ) cycle.

All Inputs TTL Compatible. Fits any PC edge connector with 0.1 56" contact centers or may be directly soldered to flat cable or harness assembJy. CURSOR ON IS71 WA R N I N G : DO NOT ALLOW ANY I N P U T VOLTAGE TO GO BELOW GROUND. EXCEeD P I N B VOLTAGE ( I n ternal +5) OR

message is erased and replaced with spaces or a 1 00(x)() code. Moving this switch forward grounds pin 28. This keeps the winking cursor from appearing on the screen. AOO-SUBTRACT IS81 This controls the cursor and entry direction. I n the ADD position, you move forward or down a line. Forward because pins 27 and 30 are shorted to provide a big capacitor and Down because the down clock (P) is connected to pin 29. the line direction pin. I n the SUBTRACT position. you move backward or up a line. Backward because.only a small capacitor remains between pins 27 and 30 and up since only a brief clock pulse appears when pin 29 is grounded. PARTS LIST IMPROVEO ASCII ENCODER

TABLE V WHAT DO THE ROCKER SWITCHES DO? OFF-ON ISli When this switch is forward . 1 1 0 Vac power reaches the transformer. Power is removed with the switch backwards. LINE-FULL 1S21 This switch decides whether a normal full scan is to be displayed or whether a single line or a group of l i nes is to be shown. In the F U L L SCAN (normal operation); pin No. 36 is left open. I n the L I N E SCAN operation, pin No. 36 is connected to a selected optional clock of timing that forces reset of the one-and only-one IC7 on the cursor. Extra timing is needed for line scan and varies with the application. A ground is also optionally placed on line No. 32 when use, the switch should be left in the tv I I position. in the L I N E scan position_ For normal

Cl 0.1 - ",F disc ceramic Mount flat. 01 to D 4 l N9 1 4 or equal silicon computer diode I C l HD0165 encoder (Harris) IC2 7402 TTL quad NOR gate IC3 MC789AP hex inverter, RTL, do not substitute IC4, IC5 7400 TTL quad NAND gate 0 1 , 02 2N5139, sil icon pnp R l . R 2 Varies with keyboard, 1 (X)() ohms for mechanical contacts and +5V supply; 3300 ohms for elastomeric high resistance contacts and + 1 2V supply. R3, R4 1 000 ohms, 1 /4�W carbon resistor MISC: PC Board, Solder; No. 24 soldereeze wire. 20 feet for keyboard wiring. sleeving. No. 24 solid wire jumpers. The following are available from Southwest Technical Products, 219 West Rhapsody, San Antonio, Texas, 78216 Mainframe board: Timing board: Cursor board: Page A or B board No. TVT�l, $9.75 No., TVT-2, $5.75 No. TVT-3, $5.75

AlB IS31

This switch decides which page is to be displayed and which page is to allow character entry. Half the switch controls the output enable or bus access of a memory and half of the switch works with the page protect to allow character entry. Program ju mpers on memory A and memory B boards set up which board does what. You normally enable and load the same board. except for electronic notebook use. +5 on the NOTE:

No. TVT-4. $5.75 High.quality keyboard. custom remanufactured for TV typewriter usa (lessencoder) No. TV-5. $18.75

enable l ines connects you to the output bus. A ground on the PROTECT lines prevents character entry. KEEP-CHANGE 1S41 This is the memory protect switch and overrides S3 by forcing a ground on both the A enable and B enable lines in the PROTECT position.

Mainframe circuitry

The mainframe is shown in Fig. 3 with its PC and component guides in Fig. 10 and 1 1 .

8 RADIO-ELEC TRONICS

The power supply has its +5 volts regulated by IC 1 which must be heatsunk to a vertical radiator. The -5, - 1 2 and optional +12V supplies are Zener regulated. Note that the -5 is derived from the -12 by a series string of a 6.8 and a 5 . 1 = V Zener. Also, note that some transformers have split secondaries. If yours does, be sure to connect the secondaries aiding rather than bucking. The +5V supply is short circuit proof and good for an amp or more. Long term shorts on the other supplies can damage R l or R 2 by overheating. The control switches are mounted as shown and their operation is summarized in table V. REPEAT and HOME are momentary switches. The rf circuitry uses Ql as an oscillator and 010 as a modulator. It may be disabled for video-only by omitting it entirely or by removing R3. Coil L l is made up of 6 turns of No. 14 wire wound on a 3/8" form and spaced out to 3/4". Tuning range should be 55 to 80 MHz. Note the tap at exactly one turn. The current through R7 determines the high frequency rf resistance of the diode and thus the amount of carrier to be amplitude modulated and sent to R8. No current is the white level.

The diodes provide reverse polarity protection, for any of the MOS devices can be damaged if the supplies get mixed up. Even w ith the diodes, be careful. Fig. 4 is all you need for a page-B memory. For a Page-A memory, you also need the character generator of Fig. 5 . IC7 is the line register that samples the memory on every line NO. 1 and brings the characters back eight times in succession for generation. Pulldown resistors R27 to R32 serve for anything that's hung on the bus input B 1 to B6. IC8 does the actual character generation, receiving the ASCII character code at the right time from IC7 and getting a "which-line is-it?" command on the line address inputs LI, L2, and L4. Enable pin No. 1 1 gives you an all-white output if it is positive and normal operation i grounded. When you want a cursor, you f briefly make this line positive in the proper location on line No. 1 . ICg transfers its characters to the output register IC9, where they are converted into seri l video. An output load command transfers a the information when the character generator has been settled and the data is valid. Grounds are provided on the unused parallel and serial register inputs. On the last character of the line, "O"s are marched out for the next 1 6 character slots, effectively blanking the rest of the line during retrace. Additional blanking is provided on lines 9 to 1 2 and vertical retrace by inhibiting the output clock w ith this input. IC9's output consists of raw video, black where you want nothing and white where you want a dot. While you cart get opposite polarity video on pin No. 7 , it generally looks awful and is not recommended for use. Video is combined with the selftest and sync in the video combi er, getting the sync from the timing board. The output is n composite video. For special use, you can logically connect the blinker to the Oash terminal to flash or blink either the entire message or a

directly drive a TV. If we attempt to attenuate it here, coupling from the coil could hurt the TV white level. So, we add 8" of twin lead, and overlap anotheL piece of twin lead 2" from the far end, securing it with plastic hardware or tape. This cuts our signal down to size with capacitive coupling and still doesn't give us any excessively long output radiators. This, of course, is a TV transmitter and the only way it is legal is if it doesn't radiate much, and above all doesn't interfere with anything. Because of this, the TV typewriter should be housed in a metal case and attenuator cia should not be eliminated. Be sure to remove all antennas from the TV when operating. Frequency is adjusted with C6. Be sure to tune to an unused low channel.
Memory board schematics

A moderate current is black, and a high current is sync, or blacker than-black. Sync pulses should exceed the black level by 25 to 35% in rf amplitude. The signal across R8 is our output, but it is a bit strong to

this gets expensive. and a cassette storage system should be consid ered above 1024 characters. Fig. 4 shows the circuitry common to both the Page-A and Page-B memories, while Fig. 8 shows only the circuitry needed extra on PaJ!:e-A. This is followed by the parts tists, PC layouts (Figs. 1 2 and 13), and mechanical and component details. Each memory board stores 5 1 2 characters in the form of a six = bit ASCII coded character. The memory consists of ICI through IC6 which are 5 1 2-bit recirculating MOS shift registers, driven by the two phase clock driver IC? and QI, Q2. These transistors translate the TTL level clock pulses into MOS levels, giving us a swing from +5 to -12 on the clock lines. IC7 increases the power level so that the clock line capacitance can be wiven without degrading the clock waveforms. Note that IC7 has unusual supply connections. It has its

There are two types of memory board, A page-A memory includes the line register, character generator, and video output register. One Page-A memory is essential for the TV typewriter. A Page-B memory contains only storage and may be used as a 5 1 2 character addon. Additional page-B memories may also b e tacked on to extend the character storage is they are properly enabled, bu t

logically selected portion of the display. The self-test superimposes itself onto the video with positive black. It may be connected for test purposes on any TTL circuit in the TV typewriter, and you can use it as a pattern detector or a logic probe. Composite video output appears on terminal No. 20 and is based on sync-ground, black = 2 volt; white = 5 volts level. An external line driver is needed if you want to use the video directly with long cables, but the circuit mpedance is low enough to i drive a few feet of unterminated coax if you have to.
The timing board

The timing board may be thought of as two types of circuits: the main timing generated as a counter chain from a Crystal oscillator, and the derived timing that makes up the proper combinations of main timing signals to be useful The main

most negative pin 7 connected to -12V and its most positive pin 14 connected to �5V and operates on a 7 volt differential. The clock lines may be viewed at the test points with an oscilloscope. Any short, however brief directly on ICTs output can damage the device. Recirculation of the memory is controlled at pin No. 5 of lCI-6. If p n No. 5 is grounded, the memory recirculates. If i pin No. 5 is positive, new data gets entered. The UPDATE command normally allows entry via R26, provided that a CTRL (carriage return) command is NOT being received and that the page in use is not being protected.

timing schematic is shown in Fig. 6, while the derived timing appears in Fig. 7 , with PC and component layouts in Fig. 14 and 15. We'll start with the main timing. The circuit is nothing but a SUi g of "by two" and "by six" dividers that take the 4,561-MHz n crystal reference and divide it down to generate all the needed, locked together reference frequencies, f rnally ending up with our 6Q-Hz vertical output frequency. IC I is a dual oscillator. Half of it is a 4-Hz blinker. This is used to wink the cursor, and to provide the REPEAT key action where you want to put down a bunch of identical characters or rapidly move to a new location. It can also be used, with or without logic to flash the display or a portion of the message. This is handy to drive home an important message or change, and is a very simple way to show minus numbers in a calculation. The other half of IC I is the 4561 .920-kHz oscillator. It is the system reference frequency and the rate at which we clock the output register I. CI0 on memory board A. Thus, it sets the dot rate for our video put-down. For very fancy applications (video titling, recording, etc.. ) you can replace the crystal with a capacitor and lift pin 2 from ground. Pin 2 ow becomes a VCO input that lets you phase-lock the TV typewriter timing to external circuits or systems. Normally, you use the crystal.

Memory output is controlled at pin No. 3 of ICl-6. If pin No. 3 is grounded, you get no output. Make pin No. 3 positive to get connected to the bus lines B l through B6. Jumpers set up the proper Page-A and Page-8 connections for enable and protect; normally you protect the page you are not working on or looking at.

We next go through a dividc-bysix in IC2, dropping us to 760.32kHz. This is our basic character rate, and intermediate values

RADI O-ELECTRONICS 9

of this divider are used by the derived timing for register clocking pulses. 760.32kHz is also the rate at which the characters are loaded into the output register lCI0 on the Memory A board. From this point, we go into a divide-by-4B, made up of IC3 and parts of IC2 and IC4. This divider generates the 4B possible character positions across the tine, of which 32 are actuaJly used and 1 6 are reserved. for retrace and overscan. The output of this divider is our horizontal rate, or 1 5 ,B40 Hz. Note that this is negligibly faster than the usual interlaced 15,750 Hz. A divide-by-12 in IC4 and IC5 counts horizontal lines for us, directly giving us the "what line is it?" commands for the character generator. It aJso gives us inputs for derived timing involving the line I transfer. The output at this point is identified as an "0" clock on the internal test points on the timing board is 1320 Hz. FinallY, IC6 and IC5 do a divide by 22 to give us the 22 possible character rows on the screen. Feedback via lelO of the derived timing ("T" Clock) shortens what would normally be a divide-by 1 2 to a divide by 1 1 . We use 16 of the 22 lines for characters and save 7 for retrace and overscan. Our flnaJ output frequency is the 600Hz verticaJ rate. All of the possible main timing chain signaJs appear on internal test points A through U, with U being the s10west and A being the fastest. The compliment of G, or G is aJse brought to a test point since it is useful in the derived timing.

position of the pulses is ad justable by changing the jumpers shown on the memory board, giving you four possible horizontal positions and three possible vertical positions for a total of 1 2 potential locations on the TV screen. In cases where a TV badly overscans or when you have a color set or something else you don't want to make any internal position adjustments on, a simple changing of these jumpers will center the picture for you. These could be made continuously adjustable, but the extra complexity of four monostables and two controls didn't seem worth the benefits.
About interlace

The derived timing is on the same board as the main timing chain. It combines the continuous main timing chain waveforms into suitable "by bursts" signals needed for TV typewriter sequenc ing and control. Half of IC7 AND,s (Negative Logic) the N and S clocks to give us a 9-12 BLANK signal, an output that is high on each line 9-12, or counting from the top of the tv picture, on horizontal lines 9-12, 2 1 -24, 33-36, 45-4B, and so on. The wave fonn is used to generate the vertical space between characters as well as blanking the characters for the vertical retrace and overscan time. It works by i hibiting (stopping) clock pulses (A Clock) from n marching video out of the output register l C I 0 on the page A memory board. The same IC also ANDs (negative logic) the K, L, M, and N clocks to give us an output that is low only on each horizontal line 1 . (Lines I , 13, 25, 37 . . . . . etc. .) The output is used directly as a line-l transfer command that connects the line register to the memory only during lines 1 , 1 3 , etc.. so a new line of characters can be transferred from the memory to the line register. It is also used by IC10 to aUow clocking of the memory only on Lines chosen. leB generates our line clock by negative logic ANDing the J and o clocks. This gives us 32 clock pulses per line, used to march the characters through the line register. IC9 provides a suitable time delay after each register clocking and then provides an output load command to the output register. The time delay is essential, for after you clock the line register, its output takes a brief amount of time to change. This changes the input on the character generator, which also takes a while to get its output correct. Only after we have the right output do we want to transfer the valid new character information into the output register. Clocks for the main memory are called I and 2. We get t.hese from IC I 0, which suitable combines some high frequency clocks (8 and C) with the Line-l output, the 32 pulses per line logic, and some other signals. The net result is a pair of 32 pulses per line, present only on lines 1, 13, 25,_ . and of the proper width to drive the clock driver circuitry on the memory boards. Note the clock levels are only TTL here; the transistors and inverters on the memory boards convert them to the full swing MOS levels at very low impedance to drive the capacitance of the memory clock lines. The final third of ICIO does the shortening of the vertical interval for us, converting IC6 from a divide by 12 to a divide by 1 1 . IC I I and most of IC12 generate our horizontal and vertical sync pulses and combine them into a composite sync signal. The horizontal pulses are around 5.2 microseconds wide and happen once each horizontal line. The vertical pulses are around 1.5 milliseconds long and happen once each vertical frame. The
_

Derived timing

Interlace is not normally used, nor is it desirable on a stationary. words-only presentation. You can pick it up u you have to by using the INTERLACE RESET input, which when grounded, resets the entire vertical counting chain to its maximum count. When the reset is released, the next whole line of horizontal timing restarts the new frame. To synchronize the internal hor izontal with an external system such as a video recorder, you can either lower the crystal slightly to gct exactly 15,750 and use this as system timing, or else you can replace the crystal with a capacitor and voltage control lCI by applying a +3 to +5 volt control signal onto pin 2 of the oscillator. A very simple phase lock loop system then can lock the typewriter terminal to the video recorder or whatever you may be interfacing. The important point is that you don't need or want interlace for the majority of applications, and the only time you have to have it is when you must superimpose your message on top of some existing program material not under your control.

The cursor board decidcs where and when a new character is to be entered. It also conditions the keyboard inputs, and optionaJly lets us use a line scan instead of a frame scan, and optionally controls the winking cursor. It's easiest to look at this board in two parts. The input conditioning and sequencing is shown in Fig. B, while the actual character position counter is shown in Fig. 9. Both circuits are on the same board and internally connected via tests points A through D. Since virtuaJly any keyboard or encoder could be used with the TV typewriter, a relatively elaborate conditioni g circuit is provided. n The input conditioning eliminates contact bounce. It also waits after a contact is made for the encoder in the keyboard to catch up and put out valid data. After that, it delivers an update command that lasts exactly one frame_ Sometime during the next frame, the charactcr position counter decides where the new character gets put. If it gets entered at all depends on whether the pages are protected or not, and whether there is a CTRL or carriage return command present. Keypressed signals consist of the KP input (22) going to ground. This input is filtered by R 2 1 , C 1 6 to eliminate the worst of the keybounce and noise, particularly any noise on break or key release when you are using an elastomcric keyboard or something else without a snap action. Q l unloads the filter and drives a Schmitt trigger in leB that gives us a clean, snap action, and adds noise immunity to the input. The output of the Schmitt trigger trips a monostable lC9 that gives us around 10 milliseconds of delay. This makes sure the keyboard code is vaJid and everything is settled before we try entering any data. The output of the delay monostable is converted to a short pulse by C 12. The output pulse from ICB trips a one-and-only-one synchron izer IC7. This consists of a set-reset flip flop driving a synchronous type D flip flop, and provides an output that lasts for one whole verticaJ intervaJ, and only one whole vertical interval. The one frame output goes directly to the update gate back forth direction control in IC I and is inverted to handle the update and cursor gates in IC5. How this is done will become more obvious when we talk about the character position counter later. Finally, the one-and-only-one output goes through a CTRL detector in ICB that decides if a line fe .cd, carriage return or CTRL command is being received. If it is, a CTRL output is generated that prevents the character from being entered. At the same time, a " move up a

The cursor circuitry

10 RADIO-E LECTRONI CS

line" or a "move down a line" command is delivered to IC5. We can also optionally "force feed" the one-and-only-one in IC7 with the LlNESCAN input, which updates and moves us one character per frame. This option is handy for clocks and calculators but is not used for nonnal typewriter use. Also, we have set the circuit up so that any CTRL command gives us a carriage feed in order to save parts. If you want to you can add extra decoding and logic to independently bring out as many machine commands as you want to. A keypressed command is random with respect to the frame by frame system timing. So, something between a very small amount of time and an almost full frame has to go by before the one-and only-one can start with the next frame. The set-reset flip flop in IC7 absorbs this time difference. Up to the entire next frame may be needed for character entry, depending on where the character is. Thus, it takes two whole frames worst case to enter a character via the keypressed input. One to synchronize and one to actually enter. This gives us a 33 millisecond fastest possible update rate or about 30 characters per second. The normal computer teletypes run about 1 0 characters per second maximum ; thus the TV type writer can easily handle their data rates. Notice that, in interests of economy, the character information lines AI-A7 are unconditioned. This means that the selected character must be valid when the keypressed delay in lC9 ends and must stay valid for at least 33 milliseconds after that. For the vast majority of manually operated keyboards, this is no problem at all. For some special or faster systems, you might like to add latches to the input to store the valid data for the length of time it is needed. While you can, in theory update all 5 1 2 characters in a single frame, this takes a bunch o f more complicated circuitry, and if you can, run your system at less than 30 characters a second (CPS). This rate will take you about 17 seconds to fill the screen at 30 CPS and around 5 1 seco'nds at the 10 CPS typewriter rate. If you have two pages, you could fill one while using the other by changing the update and protect jumpers around.

immediately at the beginning of a frame. This starts everything off on the right foot for a new sequence. The output of the character position counter drops immediately before the desired character position. 1t loads a winking cursor command into the proper slot on line 1 of this particular character group. And, if we are in an update cycle, and if we are on an unprotected page, it allows entry of the character by switching the memory from recirculate to update just this one character position. Turning to the actual circuitry of'Fig. 9. , the input clock is ANDed with an add character command in 'IC 1 . IC2 and half of IC3 form the character counter, while IC4 counts character lines. lC4's output is controlled and distributed by the cursor and memory updates by IC5, under control of the update command from the one-and-only-one and the cursor off-on switch. At the e,ld of an update, test point C suddenly drops pulses one of the AND gates in IC I . If pins 27 and 30 are open, this pulse is so brief that it gets added to the nonnal (/)1 clock pulses and we get an extra count � pulse, backing us up one character. Short pins 27 and 30, and the pulse is so long that it starts before the f ust normal clock pulse and lasts till after the second normal one goes away . Here, we add one pulse but block two, leavi g us in the hole by one pulse. The n character counter moves forward one character. Linefeed is controUed by the flip flop in IC5. To go up a line, the flip nop produces a very brief pulse. To go down a line, the flip flop is set and held exactly long enough to block two normal count pulses. Again, we add one and block two, moving us down a line. Meanwhile the character counter is reset to its maximum count, so that at the beginning of the next frame, we start at the lefthand side. The final flip flop in IC3 is set on a clear command and released on the beginning of the next field. This holds the counters in the upperleft position until the system clear is released and a new frame begins. The counters are always loaded or cleared to their maximum count. This way, the f ust system clock pulse at the beginning of the frame sets us to zero, instead of one, making sure everything ends up where it belongs.
Construction

Character position counter

The character poSition counter circuit is shown in Fig. 9. Many CUIsors use an add-subtract or up-down counter that's static and a big comparator to find out when the next character s to go in i its proper place. While this works, it's big, expensive, and takes a lot of fancy parts. It's also a bear to debug. We use a much simpler system here that gets the same job done without the need (or up-down counters or comparators. It's called a phase shi t counter. f AU we do is have a divideby-5 12 counter that goes around just like the system timing does, for it is driven by the 5 1 2 clock pulses that run the memory. The counter runs continuously in bursts just like the memory does. Once each frame, the output suddenly drops, indicating that this is the place to put a new character and that the cursor should also be shown at th time. If we don't tamper with is the inputs, the counter always drops on the same place in each frame. Now, the trick is to back the counter up or move it forward with respect to the system clock pulses. Add an extra pulse one frame, and the output drops one count earlier, backing us up one character. Hold back one pulse per 5 1 2 and the counter goes ahead one character, changing its reitJtive phase or character position with respect to the s ystem timing. To make things a little bit easier, we either throw in a short extra count to back up, or a very long extra count that's so long it overrides two system clock pulses to go forward. One extra minus two held back is the same as holding one back and a lot easier to do. So much for the normal character-to-character operation. To get a carriage return, you break the divide-by-5 1 2 counter into a

Because of the complexity of this project the construction MUST be done in progressive stages and should not be started until the complete story is on hand and thoroughly understood. If one step seems to present problems, DO NOT GO BEYOND ANY OPERA TION THAT DOES NOT SEEM CORRECT. STOP AND FIND OUT WHY! Here, very briefly is the suggested building and debugging sequence. 1. Mount the stack connectors. First, very carefully inspect the PC boards for any possible problems. Minute shorts will be extremely hard to find later. Noe the connectors are more or less alternated so that the stack fits together one and only one way. Be sure everything on each board foUows the same pattern. Be sure the notch on each board goes the same way. After the stack neatly snaps together, add all the jumpers and all the bypass capacitors on all the boards as well as the protection diodes on the memory boards. 2. Build the power supply. If the transformer has two secondaries, be sure to connect them aiding rather than bucking. The 5-V supply uses the 6-V transformer outputs, while the + and - l 2-V supplies use the 12.v taps. Be sure to watch the proper polarity on everything particularly the Zeners. Use a proper heatsink for lCI, All the switches can be mounted, being very careful not to short anything underneath the switches with the switch pads when they are bent and soldered in place. Check out the supply, looking for +5 on pins 58 and 59, -5 on 57 � 1 2 on 56 and + 12 at the optional keyboard power point. Mount the binding posts for ground and self-test. Do not proceed till all the voltages are correct. 3. Build and check the rf modulator. Wind the coil f irst, 6 turns on a 3/8" mandril, spaced out to I" long. Vertical mounting leads are then attached, making sure the tap is at precisely one turn. The tab on Q I is between the emitter and extra case connection. Eight nches of twinlead are attached to the output, and a new piece of i twinlead is taped or t>lastic bolted to that with a 2" overlap. forming as output attentuator and ending up with a suitable connector. To test the rf modulator, temporarily short the video output pin 20. This tells the modulator to put out maximum signal. Apply the output to a suitable TV. preferably a high-quality, small-screen

divide-by-32 and a divide-by-16, the former for characters across a line and the latter for lines. For carriage return, you reset the character counter to its highest count and hold back or add one line count pulse from the line counter. This returns you �to the left and up or down a Line at the beginning of the next frame. We either add a brief pulse to move us up a line or a long pulse that overlaps two normal ones to move us down a line. Finally, to home or get to the upper lefthand corner, we reset both counters to their hi hest state, and release the counters g

RAD I O - E LECTRONICS

11

black and white receiver. When power is applied, you should be able to tunc the trimmer capacitor to channels 2,3,4, or 5. You should get a completely blank screen and complete audio quieting with proper tuning. There is no sync yet. Do not go beyond this point till you are sure the modulator and oscillator are working properly. 4. Build the timing board and the video output. Add all parts to the timing board, picking three position jumpers at random. Add only ICIO and ICI I and their related parts to the memory A board. Apply power and check for any shorts on +5V. If all is well, you should have 16 rows of 32 white boxes on the screen, and the position jumpers should move you into 3 possible vertical and 4 possible horizontal locations. Pick the best one for the TV you are using. If you have any problems, check first to make sure the main timing chain is ending up with 60 Hz (not nothing or 54 Hz!). Then look for it I,;umposite sync output on pin 38, followed by composite video on pin 20. If you have a raster that is blank, look for output load 45, video clock 47, or ICIO problems. The boxes come about since you are loading open-circuit " 1 ''' 5 onto ICIO's inputs. Don't forget to remove the pin 20 ground from step 3 ! This i s the first plateau o f the construction. Beyond this point, things are more or less sclf-checking. At this time it is a good idea to go through and check ellery terminal in the system with the self-check input, carefully noting everything, learning as much as you can about the timing, and looking for potential problems. (The boxes may be erased by temporarily shorting two output reg isters to ground. If you can't get to this point, an oscilloscope . with a triggered sweep is almost essential for servicing. Beyond thiS point, the circuit more or less services itself. DO NOT GO BEYOND THIS POINT UNLESS YOU HAVE 5 1 2 WHITE BOXES STABLY AND CLEARLY DISPLAYED WHERE YOU HAVE SELECTED ON THE TV SCREEN. 5. Add the character generator. First check the place where 1C9 is to go for -12V on pin 1, +5V on pin 24, and -5V on pin 1 2 . Add the puUup and pulldown resistors and solder IC9 i n place. A s with all the MOS IC's i n this project, leave them in their protective foil or foam and quickly solder them in place with a small soldering iron. Always be sure all related circuitry is n place be/ore adding i any MOS IC. Very briefly apply power. You should get a screen rull orV RI!>SETS STROlE DELAY TINE

,

."

."

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'" '"

'"

'" '" ". "on
G

CIS POSITIONS �....00�� COMM... NO WITH RESPECT TO (> I .

LEFT FIG .

8

--

CURSOR INPUT conditioning

and sequencer circuit.

INPUT CLOCK

F

E

'

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.0012

"

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CHAR

STAIIT " L NE

I

-5V

A CLOCK

'-.

CHAIIACTEII

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SHf I(5T SCRHN � !iii I MH, !GIlAINV VERT BLUII. MOST TWI !96VfIlT lINESI (.aVERT lI"1ESI

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CCLOC'; OCtOCK



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STOPS AT 32 22000SEC

Il2 . 16 00TSI

('8 VERT LI"IfSI
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LINE CLOC'; OUTPUT LOA0

TY1'ICAl UPO

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:

STOPS AT n STOPS "T:n SlOPS "T lJ ONCE ONLY

132 VERT LI"IESI
(BRIEF fLASH_ OVERLAPS C ' 021 '" " LI"Ie VERT LINESI VERT LINESI VERT LINESI VERT LINESI VERT LINE) I]2VERT LINESI

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F CLOCK G CLOCK H CLOCK

r' r---



7

7
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POSSI8LE HSYNC ITIONS

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8291

--IBl lI"I( RATE TIMING I

-==

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----

ovRING RETRACEI

" " " " "
' '

t1 VEIIT Ll"IE

" ADD CHARACTER

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7402 lOP VIEW

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0 "" " S CLOCK

0.001 UPO"TE LINE GOES HIGH FOR 1 FRAM{ FOR uPDAU

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.

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.. UP P CLOCK -00.'

K CLOCO: LCLOCO: M CLOCK LINE 1 TRA SF R N CLOCO:

LINE DIR

o LlNE�� I'OS.
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UPDATE

,

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LINE GOES LOW FOR 1 FR"ME FOR UPDATE

UPDATE LINE GOES LOW OR 1 FRAME Q!i!:.!. I F CTRL

-, - _ ====== t-;.====:::"==::;:=---.r--.., - - L ..r r---- ..
_ _ _ _ _ _ _

...,

_ _ ( 22 HORIZ LINESI
I 22 HORIZ LINESI
I

-;;- "' c.: :L ;::: '''' ----I-j
r
12CHARACTERS PAES�NT�Q IN THIS TIME 112 HOAIZ SCANS' ICI CHAAACHA ROW liMING '

16 HOROZ lINESI

I 18 HORIZ LINESI

FROM CURSOR SEOUENCER IFIGURE 16)

1 FRAME 60 Hl 16 1"SEC
o CLOCK

SELF TEST SCREEN

position, counter schematic.

FIG.

9

--

CURSOR BOARD

--

character

.p CLOCK a CLOCK
R CLOCK S CLOCK T CLOCK

r-' ,..-, r-> ..., ..., ..., ..., ..., ..., ,..-, ,..-,


1 1 1 HORIZ LINE Sf

;:::

1 5 HORIZ LINES)

1 3 HORIZ L1NESI 1 1 HORIZ LINE I

1 I HORI, LINE )

l'
VERTICAL ACTIVE SCA

=tC""" NOT VIEWABLE 4DJISEC

POSSIBLE VSVNC POSITIONS

I

BLANKING AND RETRACE lNOT VIEWABLE PRESENT I F STABLE PICTURE)

10) FRAME RATE TIMING. 1

KEVPRESSEO INPUT BASE 01 CURSOR TPH


"

Ir...r----l

NOISV INPUT

F I LTEREO SOUARED

I

CURSOR TPG S CLOCK CURSOR fPC

r-1q1f.iEC

r

U I-

DELAYED

l F RAME

-j

ONEAND ONLV ONE OUTPUT

lEI INPUT CONDITIONING I

KEY WAVEFORMS at various points o f TV Typewriter circui t .

GO FORWARD ONE SPACE WAVEFORM STARTS BEF"bRE fIRST 01. LAST THRU SECOND 01. BUT STOPS BEFORE THIRO 1 CLOCK ON FRAME FOLLOWING UPOATE GO BACK ONE SPACE; B R I E F PULSE BEFORE fiRST 0 1 ON FRAME FOLLOWING UPDATE IF) CuRSOR MOTION

GO DOWN ONE LINE P CLOCK RESETS CURSOR ICS AFTER TWO LINE CARRIES HAVE PASSED BUT BEFORE T H I R D ONE ON FRAME FOLLOWING UPOATE GO UP ONE LINE BRIEF PULSE BEFORE fiRST LINE CARRV ON FRAME FOLLOWING UPDATE

I

EACH KEY SHORTS W I R E S UNDER IT. " RO U N D�� KEYS A R E EXTRA A N D OPTIONAl.

NOTE E X T E R N A L DEBOUNCE & N K R O LOCKOUT SHOU LD BE ADDEO I N MOST SYSTEMS PC BOARD

PROGRAM JUMPE R 'SV - MECH CONTACT > 1 2V - E lAST CONTACT �12V� NEEDED O N L Y FOR lO00n CONTACTS '5V

.5V

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AI

x

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NOTE 0 1 - 04 MAY OPTIONALLY BE PLACED ON KEYBOARD. SAVING TWO I N T E RCONNECT LEADS

MC789A
TOP VIEW
� R l R2 M I:. C H CONTACT-IK lSV) ELAST CONTACT�3.3K ( 1 2 V !

ICJ

IMPROVED ASCII ENCODER schematic .

o
o N

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o '"

1111111111 '

~
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FIG.

ill .

-j

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o

10

--

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FIG .

12

--

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memory board.

PIG .

13

--

PARTS PLACEMENT on memory

circuit board .

FIG .

14

--

FULL SIZE FOIL PATTERN

for timing board.

FIG .

15

--

PARTS LAYOUT on t iming board.

FIG .

cursor board .

16

-- FULL SIZE FOIL PATTERN for

-

. 0. +

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17

-- PARTS PLACEMENT on cursor

Memory board parts l i s t ,

page 6 ;

02 ,

03 ,

07 should be lN4001 .

The power transformer is ava i l able from Signal Transformer Company , Brooklyn , N.Y. 1 1 21 2 , a t $ 6 . 00 plus postag e .

One Junius

Stree t ,

Molex sockets are available from Force Electronics , 90301 . The sockets ( 0 9- 5 2 - 3 1 0 3 ) are 3 4 � each. per 10-pin assembly.

3 4 3 Hindry Avenue ,

Inglewood ,

Calif.

The matching pins

(09-64- 1 1 0 1 )

are 3 9 �

Minimum order $ 10 . 0 0 plus 5 0 � postag e .

Here are some additional comments from Do n Lancaster wh i ch may help answer other reader questions : A color TV set has a mor e . video bandwidth of only 2 . 5 MHz; a black and wh ite set has Commerc ial s l ightly

This limits the number of characters per line to 3 2 or possibly 4 0 , particularly an economy one.

i f an un terminal

modified stock TV is to be used, almost 1 0 MHz . More memory can be added , ups the cost. modified TV ,

systems of 7 2 or 80 characters per l ine use special video systems with bandwidths o f

but since the memory is the mo s t expensive part,

it very much O f cours e ,

Considering the limitations on video bandwidth and overscan on an un it would be difficult to do more than 5 1 2 characters per pag e . you can get denser d i splays . use a read-onl y-memory or a data selector in and

if you want to modify the TV ,

I f you want an all -the-t ime fixed memory , stead o f the shift registers still keeps its t o program them. o f any. used. enough to use with the TV typewr iter , complex and hard to use on a small Next year, Most terminal

There is no memory device I know of

that is cheap but i s

can be written into very rapidly and s imply , Mag core comes clo s e , right now ,

information when powe r is d i s connected. system.

S o do erasable ROM ' s , but i t takes a wh i l e I don ' t know

we ' l l probably s e e better devices ;

appl ications don ' t need memory through power-down times anyway ,

and those that do can run on standby power. At least 1/3 and preferably 1/2 the scan i n each direction must b e saved for retrace and blanking, particularly on economy TV ' s . interlace (the only time you either need or want full interlace i s when

To obtain full

you must super impose your message on top o f an existing , you get hori zontal and vertical characters on. TTL compatible. lock loop styl e . control

uncontrollable program sour c e ) , They also must be

signals from the system you are going to superimpose the

These must be separate and not combined as EIA sync .

You compare the two horizontal outputs with a phase detector such as The crystal is removed and replaced with a capacitor and the voltage Cost of this mod involved for each application.

the Motorola MC4044 and derive an error signal to correct he 4 . 5 6 MHz o s c i ll ator phase input is driven by the error output of the phase detector. but custom engineering is

i f i cation is under $ 1 0 ,

Baudot and EBOIC and SELECTRIC codes are generated on the keyboard s im ly by redefining p the key matri x , and poss ibly adding a flip flop or two . At the TV TYPEWRITER end , you have to add a read-only-memory such as the Harris PROM 0 5 1 2 and another convert to ASCI I , or you can sometimes use commercial code converters . but I am swamped with work right now , and they won ' t flip flop to

We are trying to work up add-on ' s , be immediately available . orbitant rates , responding to this proj ect .

CUstom engineering at this time ,

even at our incredibly ex

s imply i sn ' t ava ilable .

My thanks to the incredible fiurnber of readers

Here ' s a 1.

few corrections

to

the TV

Typewriter

supplement : and the negative supply diodes are

Diode 06

is backwards on

the power supply overlay ,

shown backwards on the 2. On is 3. figure 3 correct . schematic ,

schematic . NCLR pin 25 should a l so go to keyboard "c" input B and the diodes The PC board

010- 1 4 .

The connection between diodes 010-14 and

should be deleted .

Callouts are missing o n the keyboard

edge

connector.

"A"

is nearest the RF

twi n l e ad j

" L"
4.

i s nearest J l . and R12 lK, from the mainframe figure 3 schematic . Add R5 , R6 to the mainframe

Delete Rll parts list,

1/4 watt carbon . The overlays on the

5.

There are several printing problems on the supp lement over l ays . kit PC boards are correct and complete .

6.

On the 4 and

improved ASCII 5 of IC4 .

encoder

schemat ic ,

I C 2 should be

74 0 2 .

TP

tie points go

to pins

7. 8.

Tab l e V . Timing E,

For normal use ,

the

switch should be l e ft i n the FULL position. 1 0 m i l l i seconds , not microseconds .

cursor TPH should be

9.

An additional the TOP of study of one short

0 . 0 5 ufd disc capacitor with min imum lead lengths might be needed across ( 7 40 B ) fron pin 7 to 1 4 . Counter IC substitutions You can in one i n the extra cursor dot in pulse widths and position s . cursor board . tell by a careful

cursor ICl

might require s l ight shifts test point F on the

I n the SUBTRACT position , 512 timing pul s e s .

should appear before the 5 1 2 timing pulses every keypr e s s e d . line should e l iminate two o f 11 and 1 2 the normal of ICB , is of

the ADD position ,

10.

An inverter formed from pins pin 1 and 16)

cursor board must B.

be placed between f o i l pattern

IC6

" A " on the cursor board .

This

shown correctly on the figure

( f igure

but shoudl be added to the schematic

11.

The dot to the foil is

l e f t o f C14 o n figure B cursor should b e a no connection.

Once again,

the

correc t . stack pins 1 5 and 1 6 are correct a s shown on the foil patter n s . III . of C5 should go to R3 . The �l and � 2

12.

Connector

notation o n l y a r e apparently backwards i n figure 7 and Table 13. On the main timing chain schematic , RIGHT end of C 6 14. I n figure 3 , should go to R2 . schematic , figure six , the LEFT end

The

foil pattern

i s correc t . not 5 5 . Unless things change with

mainframe

CURSOR OFF-ON should b e S 7 , foil patterns .

In gener a l ,

s o far ,

we have

found no errors o n the are correct .

more correction s ,

always assume the

foil pattern and the printed overlay

(with the exception

o f power supply diode 06 over l a y ) Errors are

almost inevitab l e on a proj ect this

complicated. in

My thanks to the readers who so we can keep others up to date.

have sent i n the correc tion s .

Please keep sending them