The Tandy TRS-80


The TRS-80 Model 1 is a Basic oriented computer. The screen is 64 characters wide and 16 characters high. There is no startup beep. The welcome message is:

When ENTER is pressed, the reply is:
The rare Model 1 Level I is quite different and the Japanese Model 1 is very similar to this description. The character set is upper case only, lower case required a hardware modification and a driver from cassette or build in DOS.

With an Expansion Interface connected, but without boot disk, the Model 1 will not start and show a screen of garbage chararacters. If BREAK is pressed during power up or reset the MEMORY SIZE? message is displayed. The reset button is at the left back side, next to the expansion connector.

The Model 1 came with its own monochrome screen, actually a modified television. With an adapter it can be connected to a standard monitor.

Most Model 1 have 16 kByte build in (type "?MEM" at the prompt), another 32 kByte in the Expansion Interface.

Power supply

The Model 1 has three 5 pin DIN connectors at the back for Power Supply, Video Out and Cassette port. The Power connector is the one next to the power switch. Connecting a live power supply to the Video port will damage the Model 1.

The power supply is a black brick-on-a-rope transformer.


Basic is the original programming language of the Model 1. This is a standard Microsoft Basic 12 kBtye in size.

Via DOS several languages are available like Assembler, Fortran, Pascal Cobol from both Tandy and third parties.


The Expansion Interface contains more memory 32 kByte max., a Floppy Disk Controller, a cassette port switch and the optional serial port. The power to the EI is supplied with a brick of the same type as for the Model 1. The EI has room for two such transformers, so both could be build in.

The original disk drives were units with their own power supply build in. A special cable supporting four drives max. was supplied with drive 0. The connector used on the cable determined the drive number. Drive 0 was always at the end (similar but different from the IBM PC solution). The drives were 35-40 tracks, single sided, single density.

For the Model 1 is a great variety of DOSses available, all more or less based on TRSDOS 2.x, the original from TANDY. The original disk controller supported only single density.

Numerous third parties and hobbyists made their own extensions to the Model 1. Most were compatible with the Tandy original, but not always.

Due to space restrictions, several parts of DOS share mamory space and are only loaded when needed. So a proper boot disk should always be available in drive 0.

Possible problems

There are numerous reasons why a Model 1 will not boot.


F.J. Kraan, 2004-05-02