Re: 2/10 serial port B won't transmit

>Serial port B can receive data, but it can't transmit.

If the SCC will receive properly (and presumably Serial A works since you didn't report otherwise), then the data bus is working.

Swapping with a known good SCC is likely the quickest troubleshooting step after checking to see if the Lisa is turned on, but for the sake of discussion, I'll pretend you aren't able to.

I'd guess you can troubleshoot this with a voltmeter (perhaps more easily than with a 'scope).

Since the negative supply voltages aren't used for much in the Lisa (so you might not notice symptoms of them being bad), I'd check those first. Pin 1 of U10F should be around -12V and pin 8 of U10E should be -5V. U10E is the driver for Serial B.

In some cases, you can observe the existence of serial data using a voltmeter. With a low baud rate, the pulses are long enough for many voltmeters to detect on the AC setting, and if the signal is TTL, the DC setting is likely to average the voltage enough to see if pulses are there or not.

Try setting the baud rate as low as possible (300 baud or even 110), then sending a intermittent stream of characters (good candidates are ? and U due to their ascii values of $3F and $55). By observing the AC and/or DC voltage at various points in the circuit, starting and stopping the character stream might reveal how far the signal gets.

The transmit signal for Serial B originates at pin 25 of the SCC U9C, which drives pin 3 of U8A with a TTL signal. Pin 1 of U8A drives pin 2 of U10E, also with a TTL signal.

The RS-232 signal (which is bipolar, so you might find the AC setting of your voltmeter works best) coming from Pin 15 of U10E leaves the I/O board at pin 112 of the card edge. Once on the motherboard, the signal goes to pin 13 of RP2, and from pin 8 of RP2 it goes to the DB25 connector.

Good luck!

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