It's been a while since I sent out an update, but I wanted to let everyone
know that the M100 is working! Turns out there were multiple issues.
Problem 1: A11 was stuck in a high state. Thanks to the advice from PeterN, I
used nippers to zero into the faulty IC (M21 Pin 14).Problem 2 and 2.5: Found
two bad ram modules (M8 and M9). This surprised me, I was only expecting one
to be bad. Thanks to Jeff Birt for his open source SDRAM tester
(https://www.youtube.com/watch?v=5fFRrfUjogs). I built it and it helped me
identify the faulty ram.
So, the M100 is currently running with 2 of 4 ram modules. I think it would be
fun to build a couple modules to replace the faulty ones. It looks like there
are a few designs on the forum so I think that will be the next project. Let
me know if anyone recommends a specific design to build.
Thanks again for everyone's assistance!
Eric
On Thursday, January 4, 2024 at 07:27:48 PM PST, Peter Noeth
<[email protected]> wrote:
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:
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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:
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