SWTPC 6800


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The Story

In spring 2025, I inadvertently came across a SWTPC 6800 in a North Texas antique radio auction and recognized it as a significant early computer that was worth bidding on. Fortunately during the auction, I did not encounter much competition and easily bought the computer alongside with a Lear-Siegler ADM3 terminal. The previous owner had lived in Arlington, Texas (between Dallas and Ft. Worth) so this computer and components likely have remained in Texas since they were first produced. Fortunately, there are plenty of resources and documentation posted online about the SWTPC 6800 that I was able to figure out exactly what I had on my hands and tinker with it.


SWTPC 6800 system configuration and board pictures

This is a picture of my SWTPC 6800 as found in the auction listing.

The SWTPC 6800 as found was configured with the following boards and memory map:

Board Memory/ROM Memory Range Notes
SWTPC MP-16 16KB DRAM 0x0000-0x3FFF Original configuration
SSB M-16-A 16KB SRAM 0x4000-0x7FFF DIP switches S1-S5 configured to 00100 (no write protect, start address x4000)
SWTPC MP-LA Parallel peripheral interface x8018-x801B Original configuration plugged in port 7 of MP-B. Peripheral ports are memory mapped in 4 byte increments starting at x8000.
SWTPC MP-8M2 8KB SRAM xC000-xDFFF DIP switches S1-S4 configured to 0000, address decoder wiring modification to use 7th 8KB block
Percom LFD-400 ROM #1 xE000-xE3FF Empty slot (normally MiniDOS). The disk controller board had address decoder wiring modification to change base address of disk controller to xE000.
ROM #2 xE400-xE7FF Empty slot (normally MPX/9 DOS)
MiniDOS ROM (custom) xE800-xEFFF Custom MiniDOS firmware (normally ROM #3 is for MiniDisk)
Percom SBC/9 1K RAM and I/O xF000 - xFBFF
Psymon ROM xFC00 - xEFFF Original 1979 version 1.2 firmware matching documented binary in Psymon User Manual.
SWTPC MP-B SS-50 motherboard N/A Original configuration


Percom SBC/9

The SBC/9 is a complete Single Board Computer with a 6809 microprocessor, RAM, serial and parallel ports, and a simple monitor ROM firmware. This board was marketed as an upgrade to replace the original SWTPC 6800 CPU board. The board was produced in 1980 by Percom and the ROM contains Percom's system monitor called Psymon.

Documentation


Percom LFD-400

The LFD-400 is a disk controller produced by Percom. This controller board was made in 1977 and was designed to work with the newly introduced Shugart SA-400 single 5.25" mini floppy drive. Prior to the introduction of the mini floppy, the standard size for a floppy disk was 8 inches. Shugart Associates had introduced the SA-400 drive in late 1976 and released in early 1977.

Documentation


SWTPC MP-8M2

The MP-8M2 is a 8KB RAM card consisting of sixteen 2102 static RAM chips produced and sold by SWTPC. Each 2102 is 4K by 1-bit. The board was made in circa 1977-78. This was a manually assembled kit, the factory assembled version was MP-8A. The kit was priced at $200 in 1978.


SSB M-16-A

The M-16-A is a 16KB RAM card consisting of thirty-two Texas Instruments TMS4044 static RAM chips. Each TMS4044 is 4K by 1-bit and drop-in equivalent to the 2102 that the board was originally designed for. The board was produced in 1977 and sold by Smoke Signal Broadcasting (SSB). This board was the first non-SWTPC product made for the SWTPC 6800 SS-50 bus and was originally priced at $529 in early 1977 and later reduced to $379 in 1978.

Documentation


SWTPC MP-16

The MP-16 appears to be a rebranded Motorola SMS3506 board. It is a 16KB dynamic RAM card consisting of sixteen 4116-equivalent 16K by 1-bit dynamic RAM chips produced by Semiconductor Memory Systems (SMS) division of Motorola, the photo shows only half of the sockets populated. The user manual for the board mentions it was designed for Motorola MCM6616 chips (equivalent to the 4116 according to this source), but the obscure and undocumented DO8385W124 labeling on the Motorola chips themselves do not match. The 7801 is likely the date code for week 1 of 1978. I believe this may be intentional mislabeling to avoid patent infringement or licensing fees.

The board contains all the DRAM refresh control logic so memory could be accessed from the SS-50 bus in the same basic manner as if it were a static RAM card. In 1978, this board was priced at $400 and for an additional $250 could be expanded to 32KB by ordering the M16-X expansion kit for 16 more DRAM chips to plug into the empty memory sockets on this board.

Documentation


SWTPC MP-LA

The MP-LA is a parallel peripheral/printer interface produced and sold by SWTPC using the 6820 PIA. This board was produced in 1976 at an introduction price of $35.

Documentation


SWTPC MP-B

The MP-B is the main SWTPC 6800 SS-50 bus motherboard for power distribution, data and address line sharing. Address decoding was sometimes modified on this board, but this example is original. The board was priced at $40 when introduced in 1975.

Documentation


SWTPC MP-68 Mini Floppy Disk System

The MP-68 is a dual 5.25" floppy drive assembled and sold by SWTPC for the 6800 computer. It requires a SWTPC DC2 floppy drive controller board that did not come with this computer when I bought it. I'm uncertain if it was ever used and the SWTPC 6800 setup may have relied entirely on the LFD-400 and the single Shugart drive.

Documentation


Using this SWTPC 6800

As this SWTPC 6800 system includes the Percom SBC/9 and LFD-400, its operation follows the user manual instructions for Percom hardware and software, rather than instructions for SWTPC's own disk controllers and the MIKBUG or SWTBUG monitors.

The SBC/9 has a serial port that connects to a serial terminal that in my case is either a laptop running PuTTY or a period-correct Lear-Siegler ADM3 terminal. The board has configurable baud rates and my board happened to be configured for 9600 baud, 8 data bits, 2 stop bits, and no parity. The ADM3 terminal has configurable DIP switches to match the SBC/9 settings for proper communication.

At first power up of the SWTPC 6800, the SBC/9 6809 processor starts a read at the reset vector address xFFFE in the Psymon ROM space that is programmed to xFC00 instructing the processor to jump to xFC00 where the Psymon firmware starts. The first thing that prints out on the terminal is a "CMD?" prompt. The Psymon User Manual describes the supported commands in this monitor:


M <address>     - Memory examine/change (e.g. peek/poke)
G <address>     - Go to address
R <register>    - Register examine/change
L               - Load program from tape
S <start> <end> - Save program to tape
B <address>     - Set/list breakpoints
U <address>     - Unset breakpoints
Z               - Jump to address xC000 
                  (shortcut to default LFD-400 MiniDOS ROM #1)

A simple 6809 program

The simplest way to interact with the 6809 processor via the monitor firmware is manually entering or examining memory contents.



Using the LFD-400

Stay tuned (last updated June 2025).

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