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the only plausible case for me is that the chip somehow enters "programming mode" and D0..D7 are inputs which allows the external (microdisc) EPROM to be read fine and because we don't have high voltage (12.5/13V) for programming the data is not changed, additionally the /ROMDIS pulse is much short...

Hopefully some replacements will turn up this week. If it's still no go, then I think we're back to first principles -- get out the continuity tester, checking all 8 "row/col" addresses from the ULA are correctly connected to the correct pins on the multiplexer, that the other 8 address lines from ...

Dbug wrote: ↑

Sun Dec 01, 2019 8:38 am

That's when this happens you are glad you spent the time installing sockets instead of soldering directly the new chips

100% agree. That goes double when you venture into removing the PSG, VIA and CPU!

A0 & A1 are like this (Pulsing): (Pulsing) A2 is like this: (Low) and A3 is like this: (Hi) A4-A7 are low and A8=A15 are all high. So it looks like it completed the first test and failed on the full test. A0/A1 are pulsing because somewhere the CPU is still running round 3 consecutive addresses, an...

Are the letters being "typed" (with a keyclick, and you can delete them?) or are they just appearing on the screen (not really there, memory corruption, silent arrival) ? Only if it's memory corruption, rather than phantom typing, does this apply :- On the Cumana interface board, there is a preset t...

the newly programmed 27c128 eproms (I tried several) will not allow the computer to boot when the Microdisk controller is attached, though it boots fine with the old 27c128 inserted?!? Assuming your EPROMS aren't broken/fake/mislabelled etc :- EPROMs for system ROMS and external devices (the disc i...

I got nothing when running Mikes ROM, Can't answer for ISS's ROM, but when you say "got nothing", you must have a VERY early failure in the tests that are so bad I couldn't get a message up on screen. See the documentation -- error codes are put onto the address bus. "A RAM test will be performed a...

So you think it’s still a ram issue? Does the pattern suggest the CPU might be functioning? TL;DR -- if you ripped out the CPU, VIA, PSG and ROM you'd still get a pattern :) The problem is that the pattern comes from the DRAM's imagination, it wasn't put there by anything, so it only shows that the...

You're right, there is definitely a (good) pattern there. The cyan-red bars are about where I would expect the "white" bars in a black screen. If you count carefully, the pattern repeats 6 lines down and 16 characters across, which is 256 bytes later (6*40+16), which is normal. I just checked the AT...

About the address bus, doesn't the ULA also read the ram to get image data, thus changing the address bus lines? Yes, ULA sets the address lines ... To be clear: NO it doesn't, not in the context I'm talking about. The system address bus, A0-15, is ONLY driven by the CPU. The ULA does NOT put anyth...

Adding: Just realised, the only SOURCE on the address bus is the CPU. So If you really do have #FFFF (all address high) then the CPU is either crashed (given up asserting addresses) or never bothered starting. The bus floats high when idle. Tip: Pin 40 of the CPU -- once you've powered up, dab (brie...

Mmmm I am not sure that pin 3 signal is correct. Isn't that the WRITE signal? If it is, then it goes out directly from the ULA. Maybe the ULA is not working or there is something in that track. Also the address lines should be jumping from 0s to 1s. They seem to be held high, as that pin 3. You sai...

The first of your two pictures above makes me think you must have used Chinese DRAM, it's come preloaded with pictograms ! The flashing cursor-like thing is probably just a byproduct of an accidental "flashing attribute" found in junk in memory, and doesn't mean anything significant (other than the ...

Dessicator is great for keeping already-dry stuff dry, but it wouldn't be much good for the disks I'm thinking about. Aside from my spelling error, a desiccator may not be the right thing then, I was thinking of the sort of unit used to dry "wet" (raw) fruit and veg for preservation. On Googling, I...

Has anyone ever tried micro-waving 3.5" disks, to see if drying them out helps any? That ferrite-like layer used, seems to absorb moisture over time. No: But I wouldn't expect your data to be intact afterwards! :) Are you thinking of techniques used to bake old reel to reel tapes, to stop them endi...