On 3 August 1977 Radio Shack, the electronics chain owned by Tandy, held a press party in New York to announce a computer. Its first newsletter to owners thanked “you modern day pioneers who are willing to buy this exciting product sight unseen in most cases”. The TRS-80 came fully built and tested, with a screen, a cassette recorder and a 300-page manual, for $599.95, and you bought it over the counter in the same shops that sold Radio Shack’s own stereo receivers and radio scanners, which the newsletter mentioned in the same breath. By Radio Shack’s own advertising in BYTE that October, that meant “over 6000 locations in nine countries”.
Radio Shack never said how many it sold, and the guesses show how quickly it took off. In the autumn of 1978 Creative Computing estimated 8,000 to 20,000. In August 1979 SoftSide put the figure at “more than 150,000”. In June 1980 BYTE reported a Radio Shack executive as saying the company had manufactured 370,000 TRS-80s by 1 March 1980, at a rate of 600 to 700 a day. Its users called it the “Trash-80”, though Electronic Fun with Computer Games noted in 1982 that they said it about their “beloved machine”.
Fast facts
- Manufacturer: Radio Shack, a division of Tandy Corporation.
- Announced: 3 August 1977, New York.
- CPU: Zilog Z80 at about 1.77 MHz: a 10.6445 MHz crystal divided by six.
- RAM: 4K as standard, 16K inside the case, up to 48K with the Expansion Interface.
- ROM: Level I BASIC in 4K; Level II BASIC, written by Microsoft, in 12K.
- Display: 64×16 characters (Level II adds 32×16), with block graphics at 128×48.
- Sound: none.
- Storage: cassette; floppy disks through the Expansion Interface.
- Price: $599.95 with screen and cassette recorder.
Everything is memory
Tandy’s technical handbook states the design principle plainly: “the TRS-80 microcomputer system is Memory Mapped”. The Level I memory map shows what that means in practice.
| Address | Contents |
|---|---|
0000–0FFF |
Level I ROMs |
3800–38FF |
Keyboard |
3C00–3FFF |
Video Display |
4000–41FF |
RAM used by BASIC |
4200– |
“Useable RAM starts here” |
The keyboard is not a port. It is 256 bytes of address space you read, and the screen is 1,024 bytes you write, so a program can put a character on screen with a single store instruction. The handbook draws the distinction itself: “In memory mapping, the CPU knows where the data is. In a port, the CPU does not know and couldn’t care where the data is located.” The Level I machine used exactly one port, for the cassette recorder.
A keyboard that doesn’t know its own letters
The 53 keys are “single-pole, single-throw normally open”, wired as a matrix, and the handbook is clear about what they do not do:
this Keyboard does not output ASCII. It is scanned, like calculator-type keyboards. Each key represents a switch across a matrix node. When closed, the switch will short out a horizontal line to a vertical line. ROM software will detect the node short and generate ASCII equivalents for that particular key.
There is no keyboard encoder chip. The machine reads a grid of shorted wires and works out in software which letter that was. Owners met the other side of that bargain. In November 1978 Radio Shack’s newsletter described “what we call ‘Keybounce’ — multiple letters from one keystroke”, blamed dirty key contacts, and promised a cassette program that would extend the ROM’s “debounce” routine.
Six RAMs, seven bits
The best example of building to a price is in the video section. A character code needs seven bits. The TRS-80 stores six.
There are seven RAMs. Six are used for ASCII storage, and the seventh is used as a graphic/alphanumeric definition bit. There are eight data lines. Notice the line labeled Bit 6. It is sourced by NOR gate Z30 at pin 13. If Bit 5 and Bit 7 are low, then Bit 6 will be high. Z30 is a sneaky way of squeezing a seventh ASCII bit out of six RAMs.
Bit 6 of every character on screen is not stored anywhere. It is computed, at display time, from the state of two other bits. The seventh RAM is spent not on the code but on a flag saying whether this cell is a letter or a graphic. Saving one memory chip per machine was worth designing the character set around, and it is why a stock Model I cannot show lower-case letters.
The graphics that flag selects are equally frugal. Each character cell is split into six smaller rectangles, two across and three down, so the 64×16 text screen becomes the 128×48 block-graphics grid the machine is remembered for. The “graphics generator” chip, in the handbook’s words, “does not generate anything. Rather, it steers the ASCII addresses around to simulate a graphics generator.” There is no bitmap mode: every picture on a TRS-80 is built from those blocks.
Level I and Level II
The first machines shipped with Level I BASIC in 4K of ROM. In January 1978 Radio Shack wrote to owners announcing the next step: Level II BASIC, a 16K memory upgrade, and an Expansion Interface that took the machine to 32K or 48K and added a printer port and up to four mini-floppy drives. A CPU with 4K and Level II cost $499; with 16K, $789.
The handbook explains the difference in one line: “Level I uses 4K of ROM. In Level II, the system uses 12K of ROM.” The extra 8K, it says, was “enough to give text editing; transcendental functions; giant numeric and string arrays”. Level II could also use the display’s 32-character mode, which Level I could not reach. The code came from Microsoft; a 1980 guide to the ROM names it as “Level II ROM BASIC written by Microsoft’s Paul Allen and Bill Gates”.
Software
A machine in thousands of shops drew programmers. Scott Adams wrote Adventureland for his own 16K TRS-80, the first of the adventure games that brought the text adventure to home computers, and by early 1979 his games were advertised in the TRS-80 magazine SoftSide. Radio Shack sold its own programs, including the Scripsit word processor, and Michael Shrayer’s Electric Pencil was an independent word processor for the machine. Radio Shack’s own newsletter and, from 1980, 80 Microcomputing printed listings and hardware projects every month.
The FCC and the Model III
The Model I’s open design, with a keyboard unit, separate screen and a ribbon cable to the Expansion Interface, leaked radio interference. New US rules required “home computers” made after 1 January 1981 to meet limits on interference with televisions and radios. Jon Shirley, a Radio Shack vice president, explained the decision to owners in March 1981: meeting the rules took “new circuit board layouts, metal shields, added capacitors, shielded ribbon cable (very expensive) plus a little black magic”, and “there was no way to make the Model I meet the requirements without a total redesign. Since the Model III was, in effect, a totally redesigned Model I, our decision became obvious also.” Model III production with FCC certification began in January 1981.
The rest of the family
- Model II (1979): a business machine announced at the National Computer Conference in June 1979, with a 4 MHz Z80, 32K and an 8-inch floppy drive, from $3,450.
- Model III (1980): the Model I rebuilt as one unit with the screen and room for disk drives, from $699 for 4K and Level I to $2,495 for 32K with two drives. Radio Shack said it would run “appropriate Model I software”.
- Color Computer (1980): announced alongside the Model III at $399, but built around Motorola’s 6809 and not compatible with the Z80 machines. See Tandy Color Computer.
- Model 4 (1983): the Z80 line’s successor to the Model III, on sale by mid-1983.
After the IBM PC, Tandy moved its mainstream computers to PC compatibles such as the Tandy 1000. The public sources used here do not establish when the last Z80 TRS-80 was made.
Sources
- Radio Shack, TRS-80 Microcomputer Newsletter, volume 1 (1977): the 3 August 1977 press party in New York and the “modern day pioneers” who ordered sight unseen.
- BYTE, vol. 2 no. 10, October 1977, p. 43: Radio Shack advertisement, the $599.95 system and “over 6000 locations in nine countries”.
- Radio Shack, TRS-80 Microcomputer Newsletter, special edition, January 1978: Level II BASIC, the 16K kit and the Expansion Interface; CPU prices of $499 (4K, Level II) and $789 (16K, Level II).
- Radio Shack, TRS-80 Microcomputer Newsletter, edition no. 2, November 1978: “Keybounce” and the debounce program.
- TRS-80 Micro Computer Technical Reference Handbook, 2nd edition (Radio Shack, 1982): the memory map (p. 13); the 10.6445 MHz crystal and 1.774 MHz CPU clock (p. 14); the six video RAMs and NOR gate Z30 (p. 34); the graphics cell and graphics generator (pp. 36-37); the keyboard and the memory-mapped design (p. 44); Level I and Level II ROMs (p. 55).
- Robert M. Richardson, Disassembled Handbook for TRS-80, volume 2 (Richcraft Engineering, 1980), introduction: Level II ROM BASIC written by Paul Allen and Bill Gates of Microsoft.
- MICRO, no. 9, February 1979, p. 29: bibliography summary of David Ahl, “Personal Computing: The Size of the Market”, Creative Computing, September/October 1978, estimating 8,000 to 20,000 TRS-80s.
- SoftSide, August 1979: “more than 150,000 TRS-80’s have been sold”.
- BYTE, vol. 5 no. 6, June 1980, p. 176, “BYTE News”: Radio Shack “very secretive about the sales volume”, 370,000 manufactured by 1 March 1980, 600 to 700 a day.
- Electronic Fun with Computer Games, vol. 1 no. 1, November 1982: users calling their “beloved machine the Trash-80”.
- SoftSide, January 1979: advertisements for Scott Adams’s adventures.
- Radio Shack, TRS-80 Microcomputer News, vol. 3 no. 3, March 1981, p. 3: Jon Shirley, “View From the 7th Floor”, on the FCC rules and dropping the Model I.
- 80 Microcomputing, October 1980, p. 8: Wayne Green on the Model III paving “a way toward eventually stopping production on the Model I, which FCC noise requirements would have dictated anyway”.
- Radio Shack, TRS-80 Microcomputer Newsletter, June 1979: the Model II shown at the National Computer Conference, orders from June, $3,450 for 32K with one disk. TCS newsletter, June/July 1979: the Model II’s 4 MHz Z80 and 8-inch drive.
- Radio Shack, TRS-80 Microcomputer News, vol. 2 no. 7, September 1980, pp. 1, 8: the Model III range from $699 to $2,495, and the $399 Color Computer.
- 80 Micro, July 1983: dealer advertisements for the Model 4.
- Tandy, Tandy 1000 Computer Service Manual (1985): Tandy’s PC-compatible line.