Interesting situation. So, firstly, Can you take a pic of your motherboard and post it here? Let's confirm that it is indeed the later variant that uses a ROM socket that is compatible with 27C256. (there is an outside chance that your M100 is an early one and has the oddball main ROM pinout. not likely but possible)
Or, what is the silkscreened PCB number on your main board? see https://bitchin100.com/wiki/index.php?title=Model_and_ROM_information Secondly, it would be rare that the ROM is broken but sure it can happen. Luckily you have a programmer. get a 27C256 and put the default M100 or T102 ROM image in there. See if it tests clean on your programmer and then drop it in. Of course if your motherboard is killing roms, you might want a spare 27C256 around just in case you blow the first one. I have had to replace the bus buffers before on an M100. So they can fail. Especially since those signals go to the expansion socket. Is there anything plugged in to the Expansion socket? Is there an option ROM installed? What is your RAM configuration? I guess you have a fully socketed board now, so you can use any and only one of your RAM modules as needed. I would suggest that you verify that all of your address and data lines to the main ROM get signal. repeatedly press reset while observing A0-A14, and D0-D7. Heck check all signals at the main ROM. If your bus drivers are working you should see valid logic levels and sharp edges on all signals to the main ROM. If any signals look wierd, go back to the 80C85 and check the corresponding signals to see if they are wierd at the processor. Often signals look different relative to their other peers; Ex. all data line signals should look pretty similar. All Address lines should be toggling cleanly. You are definitely doing the right things to isolate the issue. Keep at it, you will find the fault. Good luck! Steve On Sat, Apr 12, 2025 at 5:30 PM Jerry Gibson <[email protected]> wrote: > Picked up a non-working M100. I've opened it and attached bench power > supply, verified all ICs get proper voltage (5v.), I see display darken and > all pixels seem to be on (or maybe off state). > > I picked up an recap kit and installed that, but no change. > > Thinking the ram might be bad, I unsoldered the RAM module, and installed > a socket, and tested Ram optional module #1 and #2 in it, but no change. > > I then got my scope, and verified crystal is working on pin 2 of the > 80c85. On first power up, I do it see it try to talk to the bus but then it > stops after X clock cycles. Probing address lines at M1, I noticed that > AD7 goes low and stays low (unbuffered side), A7 stays low the entire > time. Verified MR2A, MR1A resistors are not shorted or open (correct > values). Maybe M1 is bad? Though M1 still might be good, as M2 could be > pulling it down, or the 80c85 itself could have a bad pin. At this point I > not sure how to dig deeper unless I buy replacement ICs. > > Thinking maybe the ROM has gone bad, I tried to read it. The ROM it has > the 3256C07 in it (verified VCC is at pin 28), I then pulled it out and > tried to read it using my TL866 programmer. But it fails on Pin detect on > same pins, I've selected every variant of 27c256, and still these pins fail. > I've unchecked Pin detect, and forced a read but data looks incorrect, and > when I try to read it again, it will not match. > > At this point, I'm going to pause and think about what I could do next. > > Any help be greatly welcomed! > > Thanks, > Jerry >
