TRS 80 Model III

The Tandy TRS 80 Model III was the third member of one of the most famous computer family preceded by Model I in 1977 and Model II in 1979. Tandy TRS 80 family was one of the first home computers and was launched at the same time as famous computers like the Apple II or the Commodore PET. TRS 80s were nicknamed "Trash-80" by competitors.

The TRS-80 was developed within the Radio Shack engineering group, based

upon several processor chips, SC/MP, PACE, 8008, 8080 and finally the Z80.

It used a black & white TV set, made by RCA, without tuner as monitor. The earlier models use a poor basic called Basic Level 1 (the Basic and the OS fit in the 4 KB ROM!). It was replaced later with the Basic Level 2 which needed a 12 KB ROM.

To offset its poor characteristics, Tandy developed a device called Expansion Interface which brought a lot of new features: additional 16 or 32kb RAM, two tape unit connectors, a printer port, a floppy disk controller, a serial port and a real time clock.

When it was connected to a floppy disk unit, the TRS-80 uses the TRS DOS operating system, which was pretty bugged and most of the TRS-80 users preferred NEW DOS. NEW DOS was an operating system done by a third-party company called Apparat. This OS was the real TRS-80 operating system.

The Model III is generally regarded as the successor to the Model I. Its two 5.25" floppy disk drives could convert Model I disks.

Initially Radio Shack wanted to sell both the Model I and III at the same time, but the FCC forced them to stop selling Model I. Is so they were discontinued because of the excessive radio noise that they put out.

However, the Model III wasn't FULLY compatible with the Model I. There were differences in ROM, which meant some programs had to be converted, especially those machine language ones that made ROM calls.