*** M E M O R U M ***

Surely board connected into ROM B socket with a couple of flying leads to ROM A and ROM C would give all the lines needed to run a 32K ROM? Put the three ROM select lines through an inverter and use the ROM B and ROM C select output as two address lines. Then put all three inverted ROM select lines...

Still only very rare ROM select pulses, so that is possibly the issue, which would point towards the PAL. IF the memory is misreading, from either faulty decode, or duff rom, or address/data bus shorts, the MTX could be running random code from ram, which could account for the lack of rom select si...

Device programmers: The first one I had was an ART Epp-3 that came from Maplins (remember them ?) but that's parallel memory only, and been obsolete for 10 years or more, the software is designed for windows 98 I think, but ran on XP. For the MTXplus project we used the MiniPro TL-866A, that's got a...

Neither of my boards has the original 14L4 PAL fitted, as they're both been upgraded from 32 to 64k. However, this is the waveform for the /CEA rom select output while sitting at "Ready" from the replacement ATF18V8B-15 doing the memory decode. pin17.jpg The MTX had a tendency to crash eac...

I probed the same pin on the MTX here, very similar looking waveform. 500ns between peaks, I wonder if I've captured an opcode fetch. So spike when the opcode read starts, then another spike before the refresh starts. Then the same with the next opcode. The bit before the trigger could be a longer o...

Monitor code completed. The test rom will now list Z80 code as well as dump it. There's a couple of hundred spare bytes free in the rom, which is about a 50-50 mix of code and data tables. Short routines can be input in Hex using the E command and then listed back, and for the really adventurous the...

I went with a small LED connected to the spare pin and 5v supply line. That way the GAL is sinking current rather than sourcing it. The default for the pin is to output low so if the CPU doesn't start the LED will stay on, to confirm there's power to the board. heartbeat.jpg The LED will flash at ap...

the LS138's at 7C and 7D decode the lowest 8 I/O ports 7C is the IN ports, 7D for OUT. So A0 is connected to both devices. If they're shorting, one or both of those could be holding A0 low.

Looks amazing! I’ve just had a thought though... What if the graphics chip has a problem? Would there be a way to maybe send diagnostic data out of the parallel port if the diagnostics find a graphics chip fault? I imagine that this is the first thing that you initialize. Maybe sending a “booting d...

I reached the attachment limit on the last post. So for anyone that's interested, here's the source code, and the listing output from the assembler.