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

Does your display still get the wobbles when transmitting?

On Fri, Dec 1, 2017 at 4:32 PM Tom Stepleton <stepleton_at_email.domain.hidden> wrote:

> Hi again,
>
> Not too surprised that there aren't any bites on my bonus mystery. I
> promise I'm not making it up!
>
> But the main bit of news is that the 26LS30 was the culprit. I've put in a
> (socketed) replacement and now serial port B is back.
>
> Case closed,
> --Tom
>
> On Mon, Nov 27, 2017 at 9:51 PM, Tom Stepleton <stepleton_at_email.domain.hidden>
> wrote:
>
>> 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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>>>>
>>>>
>>>
>>
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-- 
Jason Perkins 313 355 0085 Sent from my iPhone

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