VERIFIED · 01.10.2026 UPDATED · 01.10.2026

TRS-80 Model I: the third big player

On 3 August 1977 Tandy announced the TRS-80 in New York: a Z80 processor, keyboard, monitor and cassette recorder for $599.95, sold through Radio Shack stores. Byte magazine put it in the "1977 Trinity" alongside Apple and Commodore. Here you see what is inside, what is true and what is not, and how to run one today.

⏱ 20–25 min Beginner Minitech retro hardware · history · BASIC

trs80gp (emulator)🔒 local SDLTRS (emulator)🔒 local TRS-80.com (community)🌐 global

🔄

UPDATED · 01.10.2026 — what changed

Every claim has been checked against sources (see "Sources"). Corrected: the $599 price is for the bundle with monitor and cassette recorder ($399 is only the CPU unit with keyboard); "10,000 units in the first month" is "over 10,000 in the first six weeks or so"; "7,000 stores" is Tandy's own claim ("7,000 stores in 40 countries"), while in the US there were more than 3,000; "the first mass software industry" and "democratised computing" were removed as unsupported; cassette speed is 250 baud with Level I and 500 baud with Level II (not 500 for both); Electric Pencil dates from December 1976 and was written for the Altair, not "written specifically for the TRS-80" (the TRS-80 is one of its 78 ports); VisiCalc shipped first for the Apple II (17 Oct 1979) and did not "find its first home on the TRS-80"; Dancing Demon is by Leo Christopherson, not Radio Shack, and we cannot confirm its year; the "Profile" and "Star Trek" cards were removed — we found no source; in the BASIC example POKE 16526 is not video memory (it is the USR address), video memory starts at 15360, and OUT 255,1 is not "the relay" — the relay is switched on with OUT 255,4; the model table was corrected (Model III — 2.03 MHz; Color Computer — 6809E and 4/16/32 KB); the dead Internet Archive link was removed. Added: Level I versus Level II, the Expansion Interface, the reason the Model I was discontinued (FCC), the trs80gp and SDLTRS emulators (as of 2026) and a "Try it yourself" section. 1977–1984 prices are in $ (historical prices), without conversion.

01What you'll learn

What is inside the TRS-80 Model I: processor, memory, screen, cassette — with exact numbers.
Why Radio Shack mattered: a computer you could buy from a store around the corner.
The difference between Level I and Level II BASIC and who wrote the second.
How sound works without a speaker and why the cassette was a pain.
Which software is documented and what we cannot claim.
Why the Model I was discontinued and how the line leads to the Model 4 and the Tandy 1000.
How to run a TRS-80 today: an emulator.

02Before you start

No prior knowledge needed — knowing what a processor and RAM are is enough.
Optional: a computer for the trs80gp or SDLTRS emulator 🔒 local.
TRS-80 ROM files (BASIC and the operating system) are protected by copyright. Use ones you are entitled to use and follow the emulator's documentation.

03Steps

The machine at a glance

The TRS-80 (the name comes from "Tandy Radio Shack" and the Z80 processor) is a desktop microcomputer from Tandy Corporation. The keyboard and mainboard share one case, the power supply is separate, and the monitor is a 12-inch black-and-white RCA unit. Why it matters: it was sold through more than 3,000 Radio Shack stores in the US — Wikipedia describes it as one of the earliest mass-produced and mass-marketed retail home computers. Its nickname is "Trash-80"; Tandy's executives disliked it.

Parameter	TRS-80 Model I
Announced	3 August 1977, New York
Price (historical)	$399 CPU unit with keyboard · $599.95 with 12-inch monitor and cassette recorder
Processor	Zilog Z80 · 1.774 MHz
RAM	4 KB standard (Level I); up to 48 KB with the Expansion Interface
ROM	4 KB (Level I BASIC) · 12 KB (Level II BASIC)
Screen	64 × 16 characters; each character is a 2 × 3 dot block → block graphics of 128 × 48; uppercase only
Sound	no speaker; tones through the cassette port
Storage	cassette: 250 baud (Level I) · 500 baud (Level II); floppy later, via the Expansion Interface
Sales	over 10,000 in the first ~6 weeks · 55,000 in the first year · over 200,000 in total (per Wikipedia)

⚠️

The Model I's weak spots

The mechanical keys "bounce" and type several characters at once (with Level II); the cassette is sensitive to volume; the Expansion Interface connects through a short unshielded cable whose contacts oxidise, making the machine reboot on its own. Without a modification there are no lowercase letters.

Level I and Level II BASIC

Level I is 4 KB in ROM, created by Steve Leininger on top of Li-Chen Wang's Tiny BASIC with floating-point arithmetic added. It has only two string variables (A$, B$) and its only error messages are "WHAT?", "HOW?" and "SORRY". Level II is 12 KB in ROM and was written by Microsoft; according to Bill Gates the whole development took about four weeks. It arrived around mid-1978 together with the Expansion Interface; earlier machines were upgraded for a fee (originally $199). Level II has double precision and a much wider command set, but no user-defined functions and no descriptive error messages.
TRS-80 Level II BASIC · graphics and direct memory
```
5 REM Level II BASIC: block graphics and POKE/PEEK
10 CLS
20 PRINT @ 660, "TRS-80 MODEL I"
30 FOR X = 0 TO 127
40 SET(X, 20)
50 SET(X, 25)
60 NEXT X
70 POKE 15360, 191
80 PRINT @ 900, PEEK(15360)
```
What it does: clears the screen, prints text at position 660 (the screen is 0–1023), draws two block-graphics lines (X from 0 to 127, Y from 0 to 47), writes a block-graphics character with the value 191 into the first cell of video memory (which starts at address 15360) and prints the value read back — we expect 191. The command OUT 255,4 switches the cassette recorder's relay on; port 255 is the cassette port.

⚠️
Not tested live
We did not run the code in an emulator for this lesson — so there is no TESTED label. Addresses and commands were checked against the documentation on trs-80.com and the port description; if something does not work, do not change addresses at random (addresses 16526–16527 hold the USR routine address, not video memory).

🔊
Sound without a speaker
The Model I has no built-in speaker. Square-wave tones are produced by writing to the cassette port and connecting an amplifier to the recorder's microphone input; most games use exactly this. The cassette is sensitive to volume, so users set the level by ear and made several copies of a program.

Software and games

By 1979 the TRS-80 had the largest selection of software in the microcomputer market (per Wikipedia). Here are six documented examples.

Program	Year · author	What it is
Electric Pencil	December 1976 · Michael Shrayer	The first word processor for home computers; it appeared first for the Altair and was later ported to 78 versions, including the TRS-80.
VisiCalc	17 Oct 1979 (Apple II) · VisiCorp	The first spreadsheet for personal computers; it shipped first for the Apple II and later for the TRS-80 Model I, II, III and 4.
Adventureland	1978 · Scott Adams	A text adventure written first for the TRS-80 and then ported to many other systems; its success led to the founding of Adventure International.
Dancing Demon	author Leo Christopherson	A strange experimental program: you compose a song for a devil and choreograph his dance steps.
Microchess	1976 (KIM-1) · Peter Jennings	A chess program that fits in the Model I's standard 4 KB; a TRS-80 port was offered by Radio Shack.
Zork I	1980 · Infocom	A text adventure ported to the Model I/III and sold by Radio Shack.

📊

What we do not claim

We could not confirm "the first mass software industry", the year of Dancing Demon, or entries such as "Star Trek" and "Profile" from the old version — so they are gone. What is known: hundreds of small companies made software and accessories for the TRS-80, while Radio Shack for a long time did not encourage outside developers.

The TRS-80 family

Model	Processor	RAM	Year	Feature
Model I	Z80 · 1.774 MHz	4–48 KB	1977	The original · Level I/II BASIC
Model II	Z80A · 4 MHz	32 or 64 KB	Oct 1979	Business machine with an 8-inch floppy; not compatible with the Model I; $3,450 base price
Model III	Z80 · 2.03 MHz	4–48 KB	26 Jul 1980	All in one case; broadly compatible with the Model I (Tandy claimed 80% of the software)
Color Computer	Motorola 6809E · 0.895 MHz	4 / 16 / 32 KB (CoCo 1)	announced Jul 1980	An entirely different design, colour graphics; $399
Model 4	Z80A · 4 MHz	16–128 KB	26 Apr 1983	80 × 24 screen; compatible with Model III and CP/M applications

Prices are historical, in $.

✅

Rule: "TRS-80" is not one computer

After the Model I the name became generic for several different lines. The Model I, III and 4 are compatible with each other; the Model II and its successors are another line, and the Color Computer has nothing to do with the Z80 — Byte wrote that "the only similarity is the name".

Timeline: from the announcement to the Tandy 1000

Date	Event
3 Aug 1977	The TRS-80 is announced in New York — $399 without a monitor, $599.95 with monitor and cassette recorder. The first customer deliveries were in autumn 1977 (sources give different months).
1978	Around mid-year Level II BASIC and the Expansion Interface appear; floppy drives from July.
October 1979	The Model II starts shipping (announced in May 1979).
26 Jul 1980	The Model III. The Model I is dropped by the end of 1980 — redesigning it for the new FCC radio-interference rules was too costly.
July 1980	Tandy announces the Color Computer.
26 Apr 1983	The Model 4 — Radio Shack's last Z80 family; sold until late 1991.
November 1984	The Tandy 1000 — IBM PC compatible, $1,200; by early 1985 it is Tandy's best-selling computer.
Today	The community is still active — TRS-80.com posted new software and news in September 2026; emulators support the Model I.

📊

Numbers with a caveat

Until 1982 the TRS-80 was the best-selling line of personal computers, outselling the Apple II by roughly five times — this is one analysis cited by Wikipedia, not an undisputed fact. For all TRS-80 models together, 2.4 million units are given (1978–1982, US).

Try it yourself: the Model I in an emulator

The easiest route is an emulator 🔒 local. trs80gp (version 2.5.8) runs on Windows and ships with builds for macOS, Linux and Raspberry Pi; it emulates the Model I, II, III, 4 and the Color Computer. SDLTRS (version 1.2.35 of 17 May 2026) is an alternative. We describe the steps below in principle; we have not checked the menus live.
1. Download the emulator from its official page (see "Sources").
2. Choose the Model I and a ROM you are entitled to use (see the emulator's documentation).
3. Start it, press Enter at the memory prompt and try the example from step 2.
⚠️
Not tested live
We did not run the emulator for this lesson — so there is no TESTED label. Follow the trs80gp or SDLTRS documentation for exact settings.

⛔
Copyright
Software and ROM files have owners. Do not upload or distribute other people's images.

05What's next

Next lesson · Minitech

Apple II · 1977

The third of the "1977 Trinity": compare Apple's approach with Tandy's.

Continue → The Ladder

Quick experiment

We try one of your tasks in front of you and leave you a ready helper for it. You leave with a result.

See the Ladder

06Sources

TRS-80 (Wikipedia) — date, prices, sales, hardware, BASIC, cassette, Models III and 4, FCC.
TRS-80 Model II (Wikipedia) — 1979, hardware, price.
TRS-80 Model 4 (Wikipedia) — 26 Apr 1983, 80 × 24, CP/M.
TRS-80 Color Computer (Wikipedia) — 6809E, memory, price.
Tandy 1000 (Wikipedia) — November 1984, $1,200.
Software (Wikipedia): Electric Pencil · VisiCalc · Adventureland · Microchess · Zork.
Level II BASIC (TRS-80.org, Matthew Reed) — Microsoft, Level I versus Level II.
TRS-80 Tips and Tricks — Level II 🌐 global — video memory from 15360, OUT 255,4.
Model 1 Internals (archive mirror) — I/O ports, cassette port 255.
trs80gp 🔒 local — emulator, version 2.5.8.
SDLTRS (GitLab) 🔒 local — emulator, version 1.2.35.
TRS-80.com 🌐 global — community news and archive.
Vintage Computer Federation — forum 🌐 global — Tandy/Radio Shack section.