SWTPC

Southwest Technical Products Corp

SWTPC 6800 Computer

Southwest Technical Products Corp (SWTPC) was an electronics design and manufacturing firm that opened its doors in 1967. In late 1975, they began shipping the SWTPC 6800, a computer designed around Motorola's new 6800 microprocessor. These were the earliest days of the microcomputer revolution with just a couple of other well known names like Altair and IMSAI in the market.

The SWTPC 6800 was an exceptional value compared to the Altair and IMSAI, while at the same time being easier to use and offering equivalent performance. Because of this, the SWTPC 6800 established a good market share in its early years.

See this playlist of videos to see the SWTPC 6800 computer in action.

The pictures below are of a SWTPC 6800 computer I restored. This computer is typical of a system around 1978 with 20K of RAM, a floppy disk controller, and a cassette interface. The CPU board has mods to support the improved SWTBUG monitor ROM versus the original MIKBUG monitor. Additional mods visible on the CPU board move the on-board scratchpad RAM at $A000-$A07F to off board RAM (to support the FLEX 2.0 operating system) and place a 9600 baud serial clock on the bus.

Percom LFD-400 Floppy

In late 1977, a third-party company, Percom, introduced the LFD-400 floppy disk system for the SWTPC 6800. About this same time, SWTPC also introduced their MF-68 floppy disk system. These were the first floppy disk options available for the SWTPC 6800. Unfortunately for SWTPC, similar floppy disk options had been available for S-100 computers for over a year prior to these releases, resulting in a loss of market share for SWTPC during this time.

The Percom design directly paralleled the popular North Star floppy disk system that was introduced about one year earlier. They both used the new 5.25" mini-disk drive from Shugart, they both used a hard sector controller, and both used 10 hard-sector media with 256 byte sectors and 35 tracks. This provided 89,600 bytes of formatted storage  per diskette.

Even the disk operating systems provided with the Percom and North Star floppy disk systems were similar. North Star DOS and MPX/MiniDOS from Percom were primitive operating systems that provided little more than the ability to load and save programs to and from memory. However, there was one significant difference between the two DOS implementations:  Percom chose to put its MPX/MiniDOS in EPROM on the floppy disk controller rather than waste space on the small capacity diskettes. This also meant RAM wasn't used for the DOS and the DOS was protected from runaway code in EPROM. Finally, it also meant a working boot disk was not a requirement for operation - a disk was never anything more than a data disk.

The SWTPC 6800 and Percom LFD-400 in action can be seen in a few of the videos in this video playlist.

FLEX Disk Operating System

Very early disk operating systems, like MiniDOS/MPX from Percom (see previous section) and North Star DOS for S-100 machines, were a good fit in the earliest days of the floppy disk. They used very little storage on the original, low capacity, 5.25" disks, and also required very little RAM which was still an expensive luxury. But with increasing disk capacity and rapidly declining RAM prices, it wasn't long until more full-featured operating systems like CP/M became the norm. CP/M became a defacto standard in the S-100 market for 8080 and Z80 machines. On the 6800 side of the market, the FLEX operating system filled the same role as CP/M.

FLEX was created by Technical System Consultants (TSC) at the request of SWTPC for their new MF-68 floppy disk system. FLEX was easily adapted to run on other 6800 hardware and was soon running on a variety of 6800, and later, 6809 machines. In order to use FLEX on my SWTPC 6800, I created FLEX disk drivers for the Percom controller. Here's a video of FLEX running on the SWTPC 6800 with a Percom controller.

CT-1024 Terminal

The CT-1024 was a very low cost terminal ($275) that generated 16 lines of 32 characters per line on a standard video monitor or modified TV. It appeared as the "TV Typewriter II" project in a series of articles that started in the February 1975 issue of "Radio Electronics" magazine. Due to its limited display area (32 x 16) and the fact the CT-1024 did not support scrolling, its usefulness as a computer terminal was limited.

In May of 1977, SWTPC introduced the CT-64 terminal for $325 that provided several improvements over the CT-1024 including scrolling and a 64 x 16 display. Later, in November of 1978, the CT-82 (82 x 16) was released.

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Mike Douglas

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