Testing a Southwest Technical Products Corp. 6800 System

How do you test that SWTPC 6800 Computer that you got from the garage sale or eBay? These notes are on how to bring up a SWTPC 6800 but the procedure is the similar for other SS-50 systems.

1. Record what you have.

Make a list of the all boards in the computer and their location (slot number). Include any connectors. If the cards have jumpers or DIP switches make a note of their setting. These notes can be very beneficial later. If you have documentation try to match the documents to the boards. You will find most of the SWTPC 6800 documents here. A typical system can be found here. If you are missing any ICs you can find most of them at Jameco or BGMicro. After you have recorded their locations remove all of the boards from the motherboard. Remove the motherboard from the case and see if it has any modifications. (The floppy disk system required a jumper on I/O slot 5.) Now is a good time to blow out all of the dust and dirt in the case.

2. Check out the power supply.

To check the power supply you need a voltmeter, any kind will do. The SWTPC 6800 system has exposed 110 volt AC, make a note of the fuse holder in the back and the terminal strip by the big capacitor. Stay away from these when the system is plugged in. The "System Checkout" procedure can be found with the CPU board documents. The MP-A board is here and the MP-A2 board is here.

3. Install CPU, Serial I/O and Memory.

The System Checkout procedure covers the stock boards. The CPU boards may have been modified. A common modification to the MP-A CPU was to disable or remove the MCM6810 RAM chip for FLEX 2.0 If this is the case you will have to install a memory board at $A000 or remove the modification. The MP-A2 has a DIP switch to disable the on board RAM. If your CPU board has a MIKBUG ROM (MCM6830) you will need a MP-C Interface card at I/O slot 1. The MP-C is limited to 110 and 300 baud. SWTBUG will work with a MP-C or a MP-S Interface card in slot 1.

4. Connect the SWTPC 6800 to a Terminal

You will need to make a cable to connect to the Molex connector on the Interface card. The best solution is to connect a 25 pin female connector (DB-25) and make the SWTPC look like a modem. Here is an example cable with a close up of the DB-25. The wiring instructions are here and the RS-232 definitions are here. You can now use a standard modem cable to connect to the SWTPC

The best terminal today is a PC with terminal emulation software. You can easily download programs (Motorola S19 format) from the PCs hard disk to the SWTPC 6800. I use a laptop with a 133 MHz Pentium but a lesser computer will do. The Terminal Emulation software I use is a DOS program called HiTerm. You can also the HyperTerminal program that comes with Windows.

5. Get the MIKBUG (*) or SWTBUG ($) prompt.

Set your terminal to the same speed as the MP-S or MP-C card (for example 300 baud, no parity, 8 bits and 1 stop bit.) Power on the SWTPC, if you get a "*" or a "$" prompt you are working. If you don't see anything press the front panel reset button. The SWTPC will then send a carriage-return, line-feed, and a prompt character. If you get a I/O over-run error from the terminal emulator you may have the baud rate wrong. Each time you press the reset the SWTPC will send this stream. You can measure this on the RO pin on the MP-S or MP-C board. (Use an oscilloscope or an AC voltmeter.)

6. Test the Memory

Plug in additional memory boards and run memory test on them. Monitor the +8 unregulated voltage, if it drops below 7.5 volts you may get memory and CPU errors. You can find memory test along with the S19 download files on the SWTBUG page.

Michael Holley's SWTPC Collection Home Page
This page was last edited August 14, 2013