TRS-80

For the Chicago-based electronica group, see TRS-80 (group).

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TRS-80 Model I.

TRS-80 was Tandy Corporation's desktop microcomputer model line, sold through Tandy's RadioShack stores in the late 1970s and early 1980s. The line won popularity with hobbyists, home users, and small-businesses. Tandy Corporation's leading position in the late 1970s was mostly due to retailing the computer through more than 3000 of its Radio Shack storefronts, (Tandy in the UK). Some features were its full-stroke QWERTY keyboard, small size, well-written Floating BASIC programming language, an included monitor, and a starting price of $599.

One major drawback with the original Model I was the massive RF interference it caused in surrounding electronics. This became a problem when it violated FCC regulations, leading to the Model I's phase out in favor of the new Model III.

By 1979, the TRS-80 had the largest available selection of software of the microcomputer market.

Early Z80-based home systems

History

Announced at a press conference on August 3, 1977 by Tandy Corporation, the Radio Shack TRS-80 Microcomputer (later redesignated the Model I) was Tandy's entry into the home computer market, meant to compete head-on against the Commodore PET 2001 and the Apple II. At $999 for a complete package including cassette storage, the computer was the most expensive single product Tandy's Radio Shack chain of electronics stores had ever offered. Company management was unsure of the computer's market appeal, and intentionally kept the initial production run to 3,000 units so that if the computer failed to sell, it could at least be used for accounting purposes within the chain's 3,000 stores.

Tandy ended up selling 10,000 TRS-80s in its first month of production, and 55,000 in its first year. Before its January 1981 discontinuation, Tandy sold more than 250,000 Model Is. By the end of its lifetime, the computer had become affectionately known by its users as the "Trash-80".

Hardware

The Model I combined the motherboard and keyboard into one unit, in what was to be a common case design trend throughout the 8-bit and 16-bit microcomputer eras, although it had a separate power supply unit. It used a Zilog Z80 processor clocked at 1.77 MHz (later models were shipped with a Z80A). The basic model originally shipped with 4 KB of RAM, and later 16 KB.

Keyboard

The transfer of information about what keys were being pressed was unusual, in that instead of transferring data via an I/O device or chip, the hardware mapped the keyboard to pre-defined locations in memory, i.e., there was no 'real' memory at this location, but performing a read from the keyboard area of the memory map would return the state of a particular set of keys.

A version of the computer was produced which replaced the nameplate with a numeric keypad.

Many users complained about the TRS-80 keyboards, which used mechanical switches and suffered from "Keyboard Bounce", resulting in multiple letters being typed accidentally. A Keyboard De-Bounce tape was distributed to compensate, which both ignored key contact closures if they were detected within a short time of a contact opening, and slowed down polling of the keyboard. Eventually, this was added to a later ROM revision. The keyboard was also changed to be less vulnerable to bounce.

Video

The TRS-80 was accompanied by a white-on-black display, which was a modified RCA XL-100 Black and White television. The actual color of the system was light bluish (the standard "P4" phosphor used in black-and white televisions), and green and amber filters or replacement tubes (to make the display easier on the eyes) were a common aftermarket item.

Later models came with a green-on-black display.

Because of bandwidth problems in the interface card that replaced the TV's tuner, the display would lose horizontal sync if large areas of white were displayed; a simple hardware fix (involving less than half an hour's work) could be applied to correct that.

Layout of characters and pixels on the TRS-80 display

The video hardware could only display text at 64 or 32 characters wide by 16 lines of resolution. This was because the video memory system used a single kilobyte of video memory. Seven of these bits were used to display ASCII characters, with the eighth bit used to differentiate between text and "semigraphics" characters.

Primitive graphics ("text semigraphics," rather than a true bitmap) could be displayed because the upper 64 characters of the 128 character set displayed as a grid of 2×3 blocks (very similar to Teletext). BASIC routines were provided which could write directly to this virtual 128×48 grid.

Although the original TRS-80 Model I could differentiate between upper and lower characters in memory, lower case characters were displayed without descenders. For example, the letters g, p, y, etc., did not display their tails below the base line of the uppercase characters. In order to display the descenders properly on the Model I, one had to solder or clip an eighth memory chip onto the back of one of the existing seven video RAM chips, and then bend up a pin to tap an address line off the system bus. This modification became a popular third-party add-on.

Later models came with the hardware allowing the lowercase character set to be displayed with descenders. The software, however, remained unchanged, and when using standard BASIC programming, no lower case characters could be displayed. A small keyboard driver written in machine language could overcome this shortcoming.

Any access to the screen memory, either by writing to it using the BASIC statement PRINT or accessing the screen memory directly, caused "flicker" on the screen. The bus arbitration logic would block video display while access was given to the CPU, causing a short black line. This had little effect on normal BASIC programs, but fast programs made in assembly language could be affected if the programmer didn't take it into consideration. Many software authors were able to minimize this effect. Notwithstanding this primitive display hardware, many arcade-style games were available for the Tandy TRS-80.

Cassette tape drive

User data was originally stored on cassette tape. A standard monaural audio cassette deck (CTR-41) was included with the machine. The cassette tape interface was extremely sensitive to audio volume changes, and the machine only gave the very crudest indication as to whether the correct volume was set, via a blinking character on screen when data was actually being loaded - to find the correct volume, one would sometimes have to attempt to load a program once adjusting volume until the machine picked up the data, then reset the machine, rewind the tape and attempt the load again. Users quickly learned to save a file three or more times in hopes that one copy would prove to be readable. Automatic gain control or indicator circuits could be constructed to compensate for this (fortunately the owner's manual provided complete circuit diagrams for the whole machine, including the peripheral interfaces, with notes on operation), and there was also an alternative tape interface that one could build in order to receive transmissions from the BBC's "Chip Shop" programme in the UK, an experiment in transmitting free software for several different BASIC home microcomputers, in a common tape format, over the radio. A special program (loaded using the conventional tape interface) was needed to access the custom interface over the expansion port and then load the recorded software. Tandy eventually replaced the CTR-41 unit with the CTR-80 which had built-in AGC circuitry (and no volume control). This helped the situation, but tape operation was still unreliable.

TRS-80s with Level I BASIC read and wrote tapes at 250 baud; however, "baud" was a misnomer in this application. The data rate was 25 bytes per second. Level II BASIC doubled this to 500 "baud", or 50 bytes per second.

Some programmers wrote machine language programs that would increase the speed to up to 1500 baud without loss in reliability.

For loading and storing data, no hardware controller existed. Instead, the processor created the sound itself by switching the output voltage from minus to plus and back, thus creating a click for every 1 and silence for every 0 in the bit stream.

Expansion interface

An optional (and expensive) Expansion Interface provided several important features - the ability to expand up to 48K of RAM, a floppy disk controller, a real-time clock, a second cassette port, a RS-232 port (as an option) and a Centronics parallel printer port.

Originally, one could not print from the model I without purchasing an Expansion Interface. However, Tandy Corp. soon sold a printer-only Interface for the Model I for approx. 300 Deutschmark in Germany.

The Expansion Interface was the most troublesome part of the TRS-80 system. It went through several revisions (a pre-production version is said to have looked completely different, and to have had a card cage) before on-board buffering of the bus connector lines cured its chronic problems with random lockups and crashes. Its edge card connectors tended to oxidise due to the use of two different metals in the contacts, and required periodic cleaning with a pencil eraser. The unit required a second power supply, identical to that of the TRS-80, and was designed with an interior recess which held both power supplies.

Since the cable connecting the expansion interface carried the system bus, it was kept short (about two inches). This meant that the user had no choice but to place it directly behind the computer with the monitor on top of it. This caused problems if one owned a monitor whose case did not fit the mounting holes. Also, the loose friction fit of the edge connector on the already short interconnect cable created the precarious possibility of disconnecting the system bus from the CPU if either unit happened to be moved during operation.

Floppy disk drives

To use the Model I with a disk operating system, one had to buy the "Expansion Interface" which included a "single density" floppy disk interface. This was based on a Western Digital 1771 single density floppy disk controller chip, but it lacked a separate external "data separator", and was thus very unreliable.

The Western Digital 1771 was "mis-programmed" by Radio Shack's early version(s) of TRS-DOS. Any time the OS issued a command to the 1771, it was required to wait for several instruction cycles before querying the 1771 for status. A common method of handling this was to issue a command to the 1771, perform several "NOP" instructions, then query the 1771 for command status. Early TRS-DOS neglected to use the required "wait" period, instead querying the chip immediately after issuing a command, and thus false status was often returned to the OS, causing various errors and crashes. If the 1771 was handled "correctly" by the OS, it was actually fairly reliable.

Double density floppy disks

A Data Separator and/or a Double Density disk controller (based on the WD 1791 chip) were made available by Percom (a Texas Peripheral Vendor), LNW, Tandy and others. The Percom Doubler added the ability to boot and use Double Density Floppies (they provided their own modified TRSDOS called DoubleDOS), and included the Data Separator. The LNDoubler added the ability to read and write from 8" Diskette Drives for over 1.2mb of Storage.

Double sided floppy disks

All TRS-80 disk formats were soft-sectored with index-sync (as opposed to the Apple II formats, which were soft-sectored without index sync, with many Apple drives lacking even an index hole detector), and except for some very early Shugart drives (recognizable by their spiral-cam head positioner), all TRS-80 floppy drives were 40-track double-density models. The combination of 40 tracks, double-density, and index-sync gave a maximum capacity of 180 kilobytes per single-sided floppy disk, considerably higher than most other systems of the era. On the other hand, the use of index-sync meant that in order to turn a floppy disk into a "flippy," it was necessary not only to cut a second write-enable notch, but also to punch a second index hole window in the jacket (at great risk to the disk inside). Or one could purchase factory-made "flippies," or use the back side for Apple Computer systems (as some software publishers of the era did).

The drives sold by Radio Shack were 35-track models with a 160K capacity.

Printers

One unusual peripheral offered was a "screen printer": an electrostatic rotary printer that scanned the video memory through the same bus connector used for the E/I, and printed an image of the screen onto aluminum-coated paper in about a second. Unfortunately, it was incompatible with both the final, buffered version of the E/I, and with the "heartbeat" interrupt used for the real-time clock under Disk BASIC. This could be overcome by using special cabling, and by doing a "dummy" write to the cassette port while triggering the printer.

For Printing, there were offered another two printers: one for 57 mm metal coated paper, selling at approx. 600 Deutschmark in Germany, and one Centronics (the name does not only stand for its interface, but for the manufacturer, Centronics Company) for normal paper, costing at first 3000 Deutschmark, later sold at approx. 1500 Deutschmark in some stores. It had 7 pins so letters like "g" did not reach under the baseline, but were elevated within the normal line.

BASIC

Two versions of the BASIC programming language were produced for the Model I. Level I BASIC fit in 4 KB of ROM, and Level II BASIC fit into 12 KB of ROM. Level I was single precision only and had a smaller set of commands. Level II introduced double precision floating point support and had a much wider set of commands. Level II was further enhanced when a disk system was added, and the Disk Based BASIC was loaded.

Level I Basic was Li-Chen Wang's free Tiny BASIC, hacked by Radio Shack to add functionality. It achieved a measure of noteworthiness due in large part to its outstanding manual, written by David Lien, which presented lessons on programming with text and humorous graphics, making the subjects very easy to understand. The basic had only two string variables (A$ and B$), 26 variables (A - Z) and one array, A(). Code for functions like SIN(), COS() and TAN() was not included in ROM but printed at the end of the book. The error messages were: "WHAT?" for syntax errors, "HOW?" for arithmetical errors (like division by zero), and "SORRY" for out of memory errors.

Level II BASIC was licensed from Microsoft. It was a cut-down version of the 16 KB Extended BASIC, since the Model I had 12 KB of ROM space. The accompanying manual was not nearly as colorful and suited for beginning programmers as the Level I Basic manual.

The Disk Based BASIC added the ability to perform disk I/O, and in some cases (NewDos/80, MultiDOS, DosPlus, LDOS) added powerful sorting, searching, full screen editing, and other features. Level II BASIC recognized some of these commands and issued a "?L3 ERROR", suggesting that a behind-the-scenes change of direction intervened between the recording of the Level II ROMs and the introduction of Disk BASIC, which Radio Shack didn't call Level III.

Microsoft also marketed a tape-cassette based enhanced BASIC called Level III BASIC. This added most of the functions in the full 16 KB version of Basic.

The first models of the Model I also had problems reading from the cassette drives. Tandy eventually offered a small board which was installed in a service center to correct earlier models. The ROMs in later models were modified to correct this.

TRS-DOS--Radio Shack's operating system for its TRS-80 computers--was so ineffectual that most discerning TRS-80 owners spurned it in favor of NewDOS, a third-party rival sold by a company called Apparat. (Apparat Personal Computers, Denver CO went out of business in 1987) (Eventually, it was one of multiple TRS-80 alternatives--others included LDOS, DOSPLUS, and VTOS.)

Software applications

Quite a few popular software applications were available for the TRS-80. Many leading developers, and independent software companies such as Big Five, ported over popular arcade hits like Namco's Pac-Man and Galaxian, Atari's Centipede, Sega's Zaxxon and Stern electronics Berzerk (with digitized speech). Some ported games from other home computers of the area, such as the original Zork adventure game. There were also many games unique to the TRS-80, including shooters like Cosmic Fighter and Defence Command and strange experimental programs such as Dancing Demon, which was not strictly considered a game but did have a big entertainment value.

The TRS-80 also had a full suite of office applications, including the VisiCalc and As-Easy-As spreadsheets and the Lazy Writer and Electric Pencil and Scripsit word processors.

Clones

TRS-80 Model I clone the DGT-100 by DIGITUS Ind. Com. Serv. de Eletrônica Ltda.

Many clones of the TRS-80 Model I came on the market: the Lobo Max-80 (Lobo also produced their own version of the Expansion Interface), the LNW-80 Models I/II and Team Computers (LNW also produced an alternate version of the Expansion Interface), and the Dutch Aster CT-80, a computer that could run both TRS-80 and CP/M software, plus it had all the improvements of the later Model III.

EACA in Hong Kong made a Model I clone that was marketed around the world under different names with modifications. In Australia and New Zealand it was the Dick Smith System-80, in North America it was PMC-80 and PMC-81, in Hungary the HT-1080Z, in South Africa the TRZ-80, and in Western Europe it was Video Genie. The expansion bus was different and EACA also made its own Expansion Interface to fit it. There were several versions and it was later split into a 'home' and a 'business' version, Genie I and II, and System-80 Mark I and II, where the II would have a numeric keypad instead of the inbuilt cassette player. EACA's Colour Genie was also based on TRS-80 Model I but with improved graphics and other changes, therefore it was not very compatible.

In Brazil there were several manufacturers of different Model I/III/IV clones. Digitus made the DGT-100 and DGT-1000, Prologica made the highly-successful CP300 and CP500 series, Sysdata Eletrônica Ltda. made the Sysdata Jr. Dismac made the D8000/D8001/D8002 series. Prologica also made the CP400 / CP 400II which were copies of the TRS80-Color, with the external case being almost a copy of the Timex 2068.

In Germany, S.C.S. GmbH in Mörfelden- Waldorf offered the Komtek-I Model I clone. Noteworthy were the four relay switching outputs.

Model III

As a follow on to the Model I, in July 1980 Tandy released the Model III, a more integrated and much improved Model I. The improvements of the Model III included built-in lower case, a better keyboard, and a faster (2.03 MHz) Z-80 processor. With the introduction of the Model III, Model I production was eventually discontinued as the Model I's did not comply with new FCC regulations regarding radio interference. In fact, the Model I's radiated so much RFI (Radio Frequency Interference) that many game companies made their games so you could put an AM radio next to the computer and use the interference to get sounds. The TRS-80 Model III also came with the option of integrated disk drives.

Model 4

The successor to the Model III was the Model 4 (April 1983, with "4" written as an Arabic numeral), which included the capability to run CP/M.

Running CP/M had previously only been possible via a hardware modification that remapped the BASIC ROMs away from memory address zero, such as the third-party add-on sold as the Omikron Mapper board, or by running a version of CP/M modified to run at a starting address other than zero. However, this also required modified applications, since the area of memory at zero contined the vectors for applications to access CP/M itself.

The Model 4 also had the ability to display high-resolution graphics with an optional board. The Model 4 also came in a "luggable" version known as the Model 4P (1983) which was portable. It was a self-contained unit that looked like a small sewing machine.

Tandy 10

Prior to the introduction of the Model II, Tandy briefly sold a desk based system known as the Tandy 10.

Model II

TRS-80 Model II

In October 1979, Tandy began shipping the Model II, which was targeted to the small-business market. It was not an upgrade of the Model I, but an entirely different system, built using the faster Zilog Z80A chip running at 4 MHz, with the computer, 8" floppy disk drive, and monochrome 80x24 monitor built into a single cabinet, DMA and vectored interrupts that the Model I lacked, and a detached keyboard. It was available with 32 KB or 64 KB of RAM; two RS-232 serial ports and a Centronics printer port were standard. Unlike the Model I, the video and keyboard were not memory-mapped, leaving the entire memory space available for programs. Hard disk drives and additional floppy drives were available as options. The Model II ran TRSDOS-II and BASIC. TRSDOS-II was not very compatible with TRSDOS for the Model I, thus the Model II never had the same breadth of available software as the Model I. This was somewhat mitigated by the availability of the CP/M operating system from third parties such as Pickles & Trout.

Tandy offered a desk custom-designed for the Model II for US$370. It could hold an additional three 8" disk drives or up to four 8.4MB hard drives.

Tandy eventually offered a 16-bit upgrade to the Model II, the Model 16. The Model II was later replaced by a cost-reduced Model 12, which added half-height 8" floppy drives, a green phosphor screen, and a white case, but dropped the expansion card cage (though one was available as an option).

Model 16, Model 16B, and Tandy 6000

Tandy later released the TRS-80 model 16, which was a follow on to the Model II; an upgrade was available to Model II owners. The Model 16 added a 6 MHz, 16-bit Motorola 68000 processor and memory card, keeping the original Z-80 as an I/O processor. It could run either TRSDOS-16 or Xenix, Microsoft's version of UNIX. Of the two operating systems, Xenix was far more popular. TRSDOS-16 was essentially Model II TRSDOS, with no additional features and little compatible software. 68000 functionality was added as an extension, loading 68000 code into the 68000 memory via a shared memory window with the Z80.

Xenix, on the other hand, offered the full power of UNIX System III including multi-user support. The Model 16 family with Xenix became a popular system for small business, with a relatively large library of business and office automation software for its day. Tandy offered multi-user word processing (Scripsit 16), spreadsheet (Multiplan), and a 3GL "database" (Profile 16, later upgraded to filePro 16+), as well as an accounting suite with optional COBOL source for customization. RM-COBOL, Basic, and C were available for programming, with Unify and Informix offered as relational databases. A kernel modification kit was also available.

TRS-Xenix was notable for being a master/slave implementation, with all IO being performed by the Z80 while all processing was done within the otherwise io free 68000 subsystem.

The Model 16 evolved into the Model 16B, and then the Model 6000, gaining an internal hard drive along the way and switching to an 8 MHz 68000 and half-height, 8-inch floppy drives (double-sided, double density, 1.2 MB). Tandy offered 8.4MB, 15 MB, 35 MB, and 70 MB external hard drives, up to 768 KB of RAM, and up to six additional RS-232 serial ports supporting multi-user terminals. Additional memory and serial port expansion options were available from aftermarket companies.

Internal variants of the Model 16 architecture were built running at speeds in excess of 10Mhz, 68010 processors, up to 8Mb of RAM, SCSI disk interfaces, and up to 12 RS-232 ports.

Other systems

TRS-80 Color Computers

Tandy also produced the TRS-80 Color Computer (Coco) using a Motorola 6809 processor. This machine was clearly aimed at the home market, where the Model 2 and above were sold as business machines. It competed directly with the Commodore 64. OS-9, a multitasking, multi-user operating system was supplied for this machine.

TRS-80 Model 100 line

In addition to the above, Tandy produced the TRS-80 Model 100 series of "laptop" computers. This series comprised the TRS-80 Model 100, 102 and 200. The Model 100 was designed by the Japanese company Kyocera with software written by Microsoft. It is reported that the Model 100 featured the last code that Bill Gates ever wrote.

The Model 100 had an internal 300 baud modem, built-in BASIC, and a limited text editor. It was possible to use the Model 100 on essentially any phone in the world with the use of an optional acoustic coupler that fit over a standard telephone handset. The combination of the acoustic coupler, the machine's outstanding battery life (it could be used for days on a set of 4 AA batteries), and its simple text editor made the Model 100/102 popular with journalists in the early 1980s. The Model 100 line also had an optional bar code reader, serial/RS-232 floppy drive and a Cassette interface.

Also available as an option to the Model 100 was an external expansion unit supporting video and a 5 1/4" disk drive. It is connected via the 40 pin expansion port in the bottom of the unit.

TRS-80 Model 200 line

File:TRS-80 Model 200 and Vaio.jpg

TRS-80 Model 200 beside a Sony Vaio laptop.

The Model 200 was introduced in 1985 as the successor to the Model 102, a Model 100 variant. The Model 200 had 24 KB RAM expandable to 72 KB, a flip-up 16 line by 40 column display, and a spreadsheet (Multiplan) included. The Model 200 also included DTMF tone-dialling for the internal modem. Although less popular than the Model 100, the Model 200 was also particularly popular with journalists in the late 1980s and early 1990s.

TRS-80 MC-10

The MC-10 was a short-lived and little-known Tandy computer, similar in appearance to the Sinclair ZX81.

It was a small system based on the Motorola 6803 processor and featured 4 KB of RAM. A 16 KB RAM expansion pack that connected on the back of the unit was offered as an option as was a thermal paper printer. A modified version of the MC-10 was sold in France as the Matra Alice.

Programs loaded using a cassette which worked much better than those for the Sinclair. A magazine published offered programs for both the COCO and MC-10 but very few programs were available for purchase. Programs for the MC-10 were not compatible with the COCO.

TRS-80 Pocket Computers

TRS-80 was also used for a line of Pocket Computers which were manufactured by Sharp or Casio, depending on the model.

TRS-80 PC-Compatible Computers

In the early-1980s, Tandy began producing a line of computers that were more or less PC compatible. Two of these systems were referred to as TRS-80 Model 2000 and Tandy 1000. As margins decreased in PC clones, Tandy was unable to compete and stopped marketing their own systems.

Originally, Tandy offered computers manufactured by Tandon Corporation, and then started producing their own line of systems.

The TRS-80 Model 2000 system was similar to the Texas Instruments Professional Computer in that it offered better graphics, a faster processor (80186) and higher capacity disk drives (80 track double sided 800k 5.25 drives). The industry was moving away from MS-DOS compatible computers (like the Sanyo MBC-550 and the TIPC) and towards fully compatible clones (like the Compaq, Eagle, Columbia MPC and others).

The later Tandy 1000 systems and follow-ons were also marketed by DEC, as Tandy and DEC had a joint manufacturing agreement.

TRS-80 in Popular Culture

• In the 1980s, Tandy Corporation and Radio Shack distributed a series of free comic books [1] in which schoolchildren and their TRS-80 computers foil various evil schemes. The kids acted mostly on their own, but Superman appeared in two issues, accompanied by Supergirl in one and Wonder Woman in the other.
• In the Futurama episode "The Honking" Bender's cousin is named Tandy and has euroTRaSh-80 printed on his front, which has the characters TRS-80 written in capitals.

Emulators

• Windows: HT1080Z EACA Emulator for Windows XP
• Windows: TRS32 Model 1/3/4 Emulator for Win XP (shareware)
• Windows: Windows (shareware) Several TRS-80 Emulators; emulated in 80486+ Assembly
• Linux/Unix: xtrs (open-source)
• Java: Java applet
• Mac OS X: M.E.S.S. Emulator includes TRS-80 emulator
• Mac Classic: TRS-80 Emulator for Mac OS (no sound support) for Mac OS 7.5.5 or higher

See also

• List of TRS-80 games
• List of TRS-80 software
• List of TRS-80 clones
• Scripsit, the Tandy-branded word processor software
• 80 Micro, magazine devoted to the TRS-80
• Creative Computing, pioneering Microcomputer magazine that catered to Atari, Apple, PET and TRS-80 users during their heyday
• SoftSide, magazine with BASIC programs for the TRS-80 and other microcomputers of the same era
• The Alternate Source Programmer's Journal, magazine with deeply technical programming articles, most of which were at the Assembly Language level, for the TRS-80.
• Tandy 10 Business Computer System

External links

TRS-80 at Curlie

• Radio Shack Model I and II catalog RSC-3
• Ira Goldklang's TRS-80 Revived Site
• TRS-80 Model 1 at www.old-computers.com