Recent content by MacOSMonkey

Yes - they are sanded on the original (probably earlier) board. But, now that I think of it, there is no guarantee that the original part is the same vendor as the newer-looking board with Lattice parts. I think the original design included National Semi (possibly PALs) that were then switched...

Yes - damage to the part is a possible outcome. Sorry to hear that happened. Maybe you should send the good one and only try to read that specific part. Then, if you can read it, program a new device, replace it into the old board and see if it works again. If so, then proceed using the old...

OK - sounds good. The full waveform for U19 should be helpful. Testing every resistor value is not necessarily the same as testing the resistor connections to the board. The resistors could be OK, but there could be a dry/bad solder joint. You don't have to remove them from the board. For...

Send the bad one! :D But, if it worked, that would be pretty awesome! :love:

I just woke up and had another thought. There might not be anything wrong. The problem might be a missing or broken resistor. You said that you thought the board took an impact, and in that kind of scenario, it's possible to lose or fracture ceramic discretes -- especially in the case of weak...

I don't think the reading trick is guaranteed -- but maybe Bolle could comment. .I think there's also a chance you might damage the part, whether or not it worked.

The other thing to remember is that, in 1-bit mode, the SQD01 shouldn't be doing anything (at least as far as I know), since there is no graphics acceleration - not using accelerated transfers. So, if there is an issue with U19.23, it could be that it is related to the SMT02 instead. Also, the...

No shame there - for either of us - hard to know anything about a 35-year-old de-identified build. ;) But great that you now have a spare...and hopefully it works! You can retire the other one. :D

The MT42C4256 VRAM pins you are looking at have different functions. Some are serial data and others are just data. Also, SDQ is a Serial Data pin. SQD is Squid - careful with those acronyms, buddy. ^_^ Maybe write out Squid instead or use a little squid icon. :D When you're comparing U19.26...

Well...I told you they were GALs. Alas, I did my best. :P ^_^

Yep - GALs. That board was probably produced later after nobody cared about secrecy. You can compare the serial numbers to check. But, same hardware, later build.

Did you hook up the entire VRAM to the logic analyzer? Try to look at the entire transfer waveform to see what might be wrong with the timing, strobes, rise/fall times, etc. Maybe there is something hanging on the line or a bad discrete. It could also just be a single failure in one of the ICs...

Good job on the debug schematic capture! I would really suggest hooking up a logic analyzer to all the pins on U19 and U21 to see what the RAM waveform/cycle timing looks like. You don't have the functional spec for the SuperMac devices, but you do have specs for the RAM. If there is bad data...

Good job!! You should probably just reflow all the RAM pins and maybe the SMT02 -- but carefully and only after thorough cleaning -- you could have microscopic debris shorts that have worked their way behind the pins. Blast it out with compressed air. Check all of the SMT02 pins for shorts --...

If the traces are damaged, then they will be resistive and/or intermittent. The problem you are seeing with the broken column is 100% reproducible, so not intermittent. Try not to flex the board too much -- you could do more damage. Just carefully check resistance on the VRAM traces and look for...