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

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.
>>>>
>>>>
>>>> --
>>>> --
>>>> -----
>>>> You received this message because you are a member of the LisaList
>>>> group.
>>>> The group FAQ is at http://lowendmac.com/lists/lisa.html
>>>> To post to this group, send email to lisalist_at_email.domain.hidden
>>>> To leave this group, send email to lisalist+unsubscribe_at_googlegro
>>>> ups.com
>>>> For more options, visit this group at http://groups.google.com/group
>>>> /lisalist
>>>> --- You received this message because you are subscribed to a topic in
>>>> the Google Groups "LisaList" group.
>>>> To unsubscribe from this topic, visit https://groups.google.com/d/to
>>>> pic/lisalist/B_AWH9w7WeQ/unsubscribe.
>>>> To unsubscribe from this group and all its topics, send an email to
>>>> lisalist+unsubscribe_at_googlegroups.com.
>>>> For more options, visit https://groups.google.com/d/optout.
>>>>
>>>
>>>
>>
>

-- 
-- 
-----
You received this message because you are a member of the LisaList group.
The group FAQ is at http://lowendmac.com/lists/lisa.html
To post to this group, send email to lisalist_at_email.domain.hidden
To leave this group, send email to lisalist+unsubscribe_at_email.domain.hidden
For more options, visit this group at http://groups.google.com/group/lisalist
--- 
You received this message because you are subscribed to the Google Groups "LisaList" group.
To unsubscribe from this group and stop receiving emails from it, send an email to lisalist+unsubscribe_at_googlegroups.com.
For more options, visit https://groups.google.com/d/optout.



text/html attachment: alternate version of message