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

Hi everyone. I've gone ahead and ordered replacement 26LS30s, and I'll be sure to update this thread when I know whether they've solved the problem.

In the meantime, here is a special *bonus mystery*. Perhaps it's related. It's so strange that I thought I'd share it.

Making use of my one good serial port, I've been using the Pascal Workshop's "Transfer" program to copy some source code from Clascal/ToolKit disk images on Bitsavers
<http://bitsavers.informatik.uni-stuttgart.de/bits/Apple/Lisa/toolkit_3.0/> over to a modern computer. It makes the code a bit more convenient to read. You can find everything I've transferred over here: http://stepleton.com/ Lisa/clascal/

As I transferred all of these files at a stately 9600 BPS, I noticed that vertical lines on the screen became a bit wavy! This was a bit of a surprise---ever since replacing the caps and pots in my PSU and video card, the video has looked great. But when you send data over the serial port, and only when you do that, you can see ripples on the screen.

Here is a video of the phenomenon. It's hard to see---you will need to view the video in full-screen mode, and even then it's tough. Look at the edge of the Transfer program's window, though, and you might be able to make it out.

https://photos.app.goo.gl/hwwRyZWfb0DY7aiU2

(Ignore the "ehehello again" text---that was just me making sure the serial link was up.)

Why on earth would sending data over the serial port cause the screen to go slightly wavy? Bizarre. It only seems to happen on my 2/10---my other Lisa isn't affected.

--Tom

On Mon, Nov 27, 2017 at 12:35 AM, Tom Stepleton <stepleton_at_email.domain.hidden> wrote:

> My friend with the scope came by this evening. What he found leads us to
> suspect that U10E, the 26LS30, has gone bad. It's receiving a healthy TTL
> signal from U8A, but the signal it emits varies between -3.85V and -4.2V or
> so only. Channel 1 in this screenshot (yellow) tells the story:
>
> https://photos.app.goo.gl/ulNyJN32gExRLXE13
>
> It looks like replacing U10E would be a good idea. Even though it's an
> obsolete part, it's not too hard to find on eBay. Still, I thought I'd seek
> the wisdom of experts to be certain---does this sound like the right thing
> to do? For what it's worth, there's no sign that U10E's output pin is
> getting shorted somehow...
>
> Thanks,
> --Tom
>
> On Mon, Oct 30, 2017 at 9:31 PM, Tom Stepleton <stepleton_at_email.domain.hidden>
> wrote:
>
>> Hello again,
>>
>> Since I don't know for TTL... :-)
>>
>> ...I tried a different strategy. Unless there's a difference between the
>> Z8530PSH that came on the Lisa and the Z8530PS (no H) that I found for
>> cheap somewhere, the SCC was not the culprit after all.
>>
>> Back to the original strategy: probing! I'll update if I have any
>> discoveries...
>>
>> --Tom
>>
>> On Thu, Oct 26, 2017 at 1:37 AM, James MacPhail <
>> gg__at_email.domain.hidden> wrote:
>>
>>>
>>> When no data is transmitted, U9C pin 25 shows 3.66v
>>>>
>>>
>>> For TTL signals, the logic "high" threshold is 2.0V, and "low" is 0.8V.
>>>
>>> So, 3.66V is within the specification for a high level TTL signal.
>>>
>>> Anything between 0.8 and 2.0V is not high nor low... which is to be
>>> expected for a signal in transition but may indicate a problem for a steady
>>> state circuit.
>>>
>>> High signals are often much closer to 5V, and low signals much closer to
>>> 0V; it depends on the parts involved.
>>>
>>>
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