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.