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Interfacing an SWTPc 6800 using MP-S


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  Interfacing an SWTPc 6800 using MP-S by Bill Degnan - 08/07/2008 07:47
Bob Grieb and Dan Roganti came by for some vintage hardware hacking, and we decided to try and interface an SWTPc 6800 with a Teletype model ASR 33. The SWTPc has an MP-S card installed.

To start, the system was set to connect to a 300-baud terminal and it worked just fine. Why not take apart something perfectly good and *downgrade* the speed?

First take a look at the same MP-S card first wired for a terminal, and second for a TTY:

SWTPc MP-S wired for a display terminal running at 300 baud. Note the baud jumpers at the bottom of this picture.

Compare with the next two pictures depicting the same card with the addition of a second cable connected to a teletype:

SWTPc MP-S "bottom view" with R1 shorted to ground. This is necessary to allow the MP-S complete the 20 milliamp current loop circuit needed by a teletype. The 6850 in the bottom/middle of the front side of the MP-S card is used to complete the loop on the other end.

SWTPc MP-S wired for a teletype running at 110 baud. (NOTE: Yes there are two cables attached to the same connector. In order to return the card to "300 baud display terminal interface mode" easier, the original interface cable was left partially attached to the connector. The cable that connects to the TTY is the thicker of the two.

To return the card from 110 baud TTY mode to 300 baud display terminal mode:
1) remove the cable with wires attached to TI, TO and TC
2) Re-attach the wire connecting posts TI and TC.
3) Un-short R1 to ground
4) Move the baud jumper from 110 to 300.

For specifics on exactly HOW the interface communicates with a terminal or TTY consult with your SWTPc MP-S controller manual.

Hi-res SWTPc 6800 modification pictures

Regarding the ASR 33 teletype, Bob discovered that there is 4th fuse in the back, in addition to the three that are visible under the removable panel of the TTY chassis. This fuse needed to be replaced first, before we could power up the machine.

ASR 33 Teletype pictures and output


  current configuration of your SWTPC by Bill Degnan - 08/12/2008 21:47

You might want to mention some details about the current configuration of your SWTPC. It's important to mention that you want to match the serial card with the right Monitor Rom. The MP-S serial card --with the 6850 UART chip-- only works with the SWTBug Monitor Rom(which is what you have there). The earlier MP-C serial card (with the 6821 PIA chip) operates with a software UART and only works with the MIKBug Monitor Rom. Also, the MP-S card,much as the MP-C, is capable of operating a Teletype with a 20mA Current Loop interface using the existing power supply of the computer. The +12V and -12V power connections are available on the serial card and provide the necessary current for this interface. The Baudrate is limited to 110baud ASCII only(ASR-33/KSR-33)--as these serial cards don't support baudot code which is required on the ASR-32/KSR-32.

Dan Roganti



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