TV Typewriter eBook

TV Typewriter eBook

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Description: Partial eBook reprint of classic text widely regarded as the opening shot fired in the personal computer revolution. The rest of the text may be freely uploaded from < >.

Author: Don Lancaster (Fellow) | Visits: 2728 | Page Views: 3045
Domain:  High Tech Category: Displays Subcategory: Engineering History 
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TV Typewriter Cookbook
by Don Lancaster



3860 West First Street, Thatcher, AZ 85552 USA (928) 428-4073

Copyright � 2010 by Synergetics Press Thatcher, Arizona 95552


All rights reserved. Reproduction or use, without express permission of editorial or pictorial content, in any manner, is prohibited. No patent liability is assumed with respect to the use of the information contained herein. While every precaution has been taken in the preperation of this book, the publisher assumes no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use of the information contained herein. International Standard Book Number: 1-882193-13-4

Created in the United States of America.


This book shows you how to put your own words and pictures on ordinary tv sets. It's also a book on cheap things that can be connected to a microprocessor to get it to do genuinely useful tasks. If you are a computer hobbyist with a home-brew lashup, a serious professional data processor concerned with low-cost small machine systems, a video games freak, or a ham working with rttv. vou will find in depth information on tv typewriter technology, which today represents the only truly low-cost ( $30 to $150) microcomputer and small-systems display interface. If you are a software specialist, we >-ill be showing you the hardware that makes your software work, giving you the depth of background you will need for effec tive and efficient small-systems coding. If you are teaching micro processors, you will find this book useful as a primary or supple mental text on the high school through university levels. If you are into video recording, cable tv, or studio broadcasting, you will find techniques here for low-cost titling and annotation of existing pro gram material, as well as the means for video art synthesis. And, if you are an electronics technician, herein lies the answers on the standard ASCII code; serial transmission formats; keyboards and encoders; Teletype, cassette, and modem techniques; along with many of the integrated circuits and systems concepts that back them up. We start in Chapter 1 with some basics-what a tv typewriter is and what are its uses, configurations, and operating principles. After this, we take a quick look at the fundamentals of television raster scanning, followed in tum by the standard computer codes and their formats. A collection of essential terms involved with tvt's and microcomputer interface then follows. Chapter 2 is a catalog of integrated circuits you will find useful in tvt and interface work. It is organized in much the same way as the similar device chapter of the TTL Cookbook, also published by Howard W. Sams & Co., Inc. Memory is the topic of the next chapter. We consider three basic types: you-program PROMs, including a seven-segment calculator interface and a Selectric converter; factory-programmed ROMs, including particularly the dot-matrix character generator; and read-

write or RAM technology that lets you selectively store and retrieve information. The 2102-style devices and their improved offspring are emphasized because of their ultralow cost, extreme simplicity of use, and microprocessor compatibility. Basic concepts of tvt system design appear in Chapter 4, including such things as timing-chain design and line-lock and EIA sync tech niques. Cursor and update circuitry, which is usually the hardest-to design part of a stand-alone tvt, is covered in Chapter 5. Cursor techniques range from the traditional count and compare and the ultrasimple McFadden system, to dedicated "super front panel" tvt's that work directly with microcomputers. Update systems include both frame-rate and direct memory access (DMA) systems. Keyboards and encoders follow in Chapter 6, with a number of basic encoding techniques, key arrangements, and circuits shown. An extensive review of serial interface centered around the UART, or universal asynchronous receiver transmitter, is presented in Chapter 7. Here we also look at the bit boffer software-indepen dent, speed-tolerant cassette storage system, along with Teletype and standard interfaces and various types of modems. The care and feeding of the tv set are next in Chapter 8, which shows us both the rf and direct-video methods, along with several ways of extending video bandwidth for long line-length displays. We end t}_le chapter by looking at color subcarrier techniques-simple ways of adding one or more colors to your system for video games or art synthesis, or for other applications where you want to use color for added emphasis or value. Our last chapter is on hard copy (printed records) and color graphics. The full gamut of hard copy is covered, from computer Teletypes, Baudot Teletypes, and economical Selectric office type writer conversions, to ink squirters and special paper systems that include the electrosensitive, thermal, impact, and photographic systems. Our color graphics section shows you both ultracheap ways of converting an ordinary tvt into a limited-performance graphics mode and into a sophisticated 96 X 96 color matrix system that in cludes "chess" and "pong" interchangeable software. Finally, we take a quick look at where you can go from here-what is needed now in the way of further tvt-related development in which you can play an important part. DoN LANCASTER

This book is dedicated to an amplifier with nothing to amplify.







Organization-Some TV Fundamentals-Dot-Matrix Scanning-The ASCII Computer Code-Baudot and Selectric Codes-Some Terms


Baud-Rate Generators-Character Generators-Keyboard Encoders -Line Drivers and Receivers-Programmable Read-Only Memories -Random-Access Memories-Serial Interface UARTs



Read-Only Memories-Dot-Matrix Character Generators-Which Character Generator-Read-Write Memory-Bus Organization



Timing Restrictions-Timing Circuits-Blanking-Sync, Position, and Video Combination




Cursor-Dirct Memory Access


Techniques-A Microprocessor Cursor-A Double Cursor-A Stored Cursor System-The "Uncursor"-Captive TVTs-Updating-Repeat Actions-Control Detection-Clearing-Screen-Read Circuits-Out put Options-A Complete Cursor System


Keyboard Design Factors-Encoder Circuits-Extra Keyboard Fea tures-Mounting and Interconnect Techniques-Nonstandard Codes -Keyboard Design Examples



How Fast-A 110-Band Standard-Tolerances-Using DARTs Teletype Interface-Industrial Interface-Cassette Interface-The Bit Boffer System-Radio Data Links-Modems-Phone Characteristics





Direct-Video Methods-Extending Video and Display Bandwidth Direct RF Entry-Color Techniques-RGB Color System


Hard Copy-Computer

. 208


Typewriters-Electrosensitive Paper-Thermal Paper-Impact Sys tems-Other Hard-Copy Methods-Color Graphics-Some Television Limitations-Some Simple Graphic Displays-A 96 X 96 Full-Color Display-Where to From Here?


. 251



Some Basics

A tvt, or tv typewriter, is any low-cost way of displaying lots of words, numbers, or graphics on an ordinary tv set. The single most important use fqr a tvt is that of an input/ output device for a micro processor or microcomputer, but the concept of using a low-cost, mass-produced display to present locally generated and used infor mation is incredibly broad. It is so broad that tvt techniques repre sent a totally new communications media-a decentralized media that is based on the fast and easy transfer of information rather than the difficult and energy-intensive moving of physical ob j ects, par ticularly cars and people. So, what can we do with a $30 to $150 machine that puts words and pictures on a tv set? Some of the uses we know of today include: " Computer terminals " " " " " " " " " " " Video games Microprocessor "super front panels" Video titling Deaf communications Calculator readouts Advertising displays Touch typing learning aids Teaching machines Electronic notebooks Remote message centers Cable tv response units

" Mailing list generators " Word processing and editing systems

" Color graphics displays " Political polling " Ham rtty " Phototypesetting " News, weather, and stock displays " Community direct-information access systems There are several overlapping ways we can classify the different types of tvt that are available: A stand-alone tvt does just that. It can interface with any source of information, with or without a microprocessor, and contains its own cursor, update, storage, and modification circuitry. A dedicated tvt is usually intimately associated with a micro processor or other devices that make the tvt a component part of a larger system. Typical dedicated applications include tvt's used as microprocessor super front panels, on which the tvt can display every memory location in the entire computer system on a page-at-a time basis; word-processing applications in which extensive editing, rearranging, or format changes are needed; and "pong" and chess style games in which move computation is needed. A utility tvt usually displays 16 lines of 32 characters at a time, most often in an uppercase-only 5 X 7 dot-matrix character format. This type of tvt has very simple timing requirements, a minimum memory cost, and is cheap to build, interface, and use. Often a second page of memory is internally provided to increase the total display to 1024 characters on a selected basis. A premium tvt can display more information and uses fancier character presentations that include lowercase and that have better character definition. These presentations can display as many as 4000 characters simultaneously, and a quarter of a million or more resolvable elements in graphic displays. Premium tvt's almost always need a modified tv set that lias additional display bandwidth. They are usually associated with larger memories, faster components, higher cost, and more critical timing. Since a television set has no internal way to store information for more than a tiny fraction of a second, a memory, often called a refresh memory, is needed and must somehow be associated with the tvt. A tvt with dedicated internal memory has its own internal storage available only for its own use. A shared internal memory tvt has its own memory but makes it available to other circuits on a DMA, or direct memory access, basis. This is handy for games and calculator displays. An external memory tvt simply borrows the information it needs from a microcomputer, microprocessor, or other system memory any time a display is needed.

An input/output, or i/ o tvt has ways of both accepting and re turning information. If we have a cassette storage interface, a modem, or a hard copy (printer) associated with the tvt, or if we have the ability to retransmit the tvt screen or to sense a portion of it with a light pen, we have an i/o tvt. A read only, or RO, tvt can only accept and display information without electronically returning any of it. Typical examples are a microprocessor memory readout and a game graphics display.

Courtesy Radio-Electronics

Fig. 1-1. The tvt-1 using nL and shift-register memory.

Figs. 1-1 through 1-3 show some typical early stand-alone tvt's representing three generations of design. Sources of tvt's include Polymorphic Systems, Processor Technology, and Southwest Techni cal Products. Fig. 1-4 shows the typical display of a 16 X 32 utility tvt.

The arrangement of our tvt system is very much a dynamic and changing process, owing to the recent (at this writing) dramatic cuts in memory components and microprocessor circuits. Fig. 1-5 shows one possible block diagram of a tvt.

Courtesy Southwest Technical Products Co.

Fig. 1�2. The tvt�2 system.

Fig. 1-3. The tvt�3 uses CMOS and:RAMs; it is B�bit microprocessor DMA compatible.


Fig. 1-4. Typical page display of utility tvt.

A memory is an essential part of a tvt. The memory has to store the characters for us, delivering them to the tv as needed to generate the desired display. As we have seen, this memory can be a dedicated internal memory, an internal memory accessible from the outside, or an external microprocessor or minicomputer memory. The mem ory is often arranged as 1024 words of 8 bits each, with one memory word associated with each displayed character or a close grouping of graphic display dots. The 2102-style MOS random-access memory or one of its improved offspring is often a good choice for tvt memory. We will be looking at these devices briefly in the next chapter and will take a detailed look at them in Chapter 3. Our tvt has to have some system timing that converts and presents information in such a way that a television set is able to use it. The system timing is usually faster than can be handled by early micro processors and is often done by a half dozen or so dedicated inte grated circuits, which are often stock TTL or CMOS chips. System timing also provides for line locking to eliminate weaving or "sea sick" displays, and optionally allows us to lock onto an existing pro gram for titling annotation, or adding the scores to a graphic