Thanks for the diagnostic tips!

On Wednesday, October 25, 2017 at 2:31:18 AM UTC+1, James MacPhail wrote:

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

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.

Sure, but this way is much more educational, and fun :-)

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.

These seem to be just fine.
 
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.

I'm using the Transfer program in the Workshop, at its lowest baud rate: 50 baud!
 
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.

When no data is transmitted, U9C pin 25 shows a suspiciously low 3.66v. Holding down ~ (0x7E, many bits on) sees the measured voltage oscillate between 1.9v and 3.1v with a period of roughly 11 seconds; holding down TAB (0x09, few bits on) sees an oscillation between 0.8v and 1.4v with roughly the same period. I'm not sure the oscillation is too meaningful; it could just be aliasing between the baud rate/key repeat rate and the sample frequency of my multimeter.

I am guessing that the low voltage is the problem right in front of us, but just in case, pin 3 of U8A is the same as above, and pin 1 of U8A: rests at .17v, oscillates between 0.6v and 1.9v with ~ and oscillates between 2.3V and 3.0V with TAB.

I'll stop there for now---I'm inclined to think that the SCC really is the culprit. I guess I'll work up the gumption to swap chips to be certain about it.

Thanks for the tips!
--Tom

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