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

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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