Back in the 70's and 80's, when I used to do component level repair on Mini Computers (15" square PCBs in a card cage with an average of 150 TTL and CMOS ICs running at 5 volts), it was common practice to use a sharp, close cutting, pair of nippers to cut the lead of an DIP IC at the point where it enters the PCB to isolate it from the rest of the circuit to debug problems like this.
Mind you, this was on PCBs with 8 to 12 circuit layers, so protecting the PCB from trace damage was the top priority. We never removed an IC unless we were absolutely sure it was defective. If isolating a suspect IC lead this way proved not to be the cause, it was pressed back into place and a solder bridge connected it back to the pad. My experience is that it was extremely unlikely for the input pin of an IC to be the guilty culprit when a signal was stuck high (above 3 volts), but it sometimes happened when a signal was stuck low (below 2 volts). The most common cause was an output pin of an IC. I still use this method when I have to isolate PCB circuit problems. Always isolating and checking output pins first. Regards, PeterN On Thu, Jan 4, 2024 at 1:03 PM <[email protected]> wrote: > Send M100 mailing list submissions to > [email protected] > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.bitchin100.com/listinfo.cgi/m100-bitchin100.com > or, via email, send a message with subject or body 'help' to > [email protected] > > You can reach the person managing the list at > [email protected] > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of M100 digest..." > > > Today's Topics: > > 1. Re: Model 100 - LCD Shows Pixels Only ([email protected]) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 3 Jan 2024 15:04:44 -0600 > From: <[email protected]> > To: <[email protected]> > Subject: Re: [M100] Model 100 - LCD Shows Pixels Only > Message-ID: <[email protected]> > Content-Type: text/plain; charset="utf-8" > > Is it possible that your CTRL or BREAK key are not working, thus you are > not able to do a cold reset? It could also be the M2 data bus latch has the > bit you mentioned stuck. Sometimes you can look at a suspect bit on the > oscilloscope and see incorrect voltage levels, or what looks like multiple > signals at once. Note that with the multiplexed bus this can be difficult > to spot as there are some ?normal? goofy looking waveforms when the bus is > being multiplexed. > > The code for the test harness test ROM is here: > https://github.com/Jeff-Birt/TRS-80-M100-M102-Test-Harness . The test > harness ROM board has a small built in LCD for feedback. You won?t have > this but you can follow how far it gets though the test with your LA. > > > > Jeff Birt > > > > From: M100 <[email protected]> On Behalf Of Eric > Sent: Wednesday, January 3, 2024 10:23 AM > To: [email protected] > Subject: Re: [M100] Model 100 - LCD Shows Pixels Only > > > > Yes, I did hold down CTRL+BREAK+press reset. Repeated this three times > and the data is almost identical (repeatable results). I triggered off of > the reset signal and recorded 100ms pre-trigger and 2s post-trigger. > Performed data review just after the RESET occurred. > > > > Thanks again for everyone's help with this. I'm learning a lot about this > computer and feedback from the forum has been very inspiring! > > > > On Wednesday, January 3, 2024 at 05:12:15 AM PST, < <mailto: > [email protected]> [email protected]> wrote: > > > > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.bitchin100.com/private.cgi/m100-bitchin100.com/attachments/20240103/1cd45bc8/attachment-0001.htm > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > M100 mailing list > [email protected] > http://lists.bitchin100.com/listinfo.cgi/m100-bitchin100.com > > > ------------------------------ > > End of M100 Digest, Vol 157, Issue 3 > ************************************ >
