DRAM 128K system has stopped working

[someone]

:-)
I just tried all the inputs and outputs, (Pin 3 is NC as far as I can tell from my reading of the schematics, thus floating) but only listed the inputs as the outputs are just inverted.

Yeah, I don't know what I'm doing :-D

Chesh

The suspect data bit 3 is on pin 5 (according to the 1248 schematic).
To see if the data bit transceiver bit is working correctly you'll need to compare both sides of the transceiver simultaneously when transceiver is enabled and the direction pin is set for reading.  There should be no inversions of the signals - the should match.

I would have thought that as the "Universal RAM Test for microbee v2.12" is displayed correctly that would show IC 11 as functioning correctly.
I would think that IC14 would be a more likely candidate.

IC11 is the transceiver between the CHAR RAM and system bus.  The schematic has its bus connector slanted backwards giving a false impression of the source of the signal.
IC14 is the transceiver between the FONTROM & PCG RAM to the system bus.

From the photo and videos IC11 seems to be working for writing but may be broken for reading.


As for writing a test ROM, it's pretty easy.
In this case , just use the beginning CRTC init routine followed by whatever test you want.
Keep your first test very simple and specific at first even if it is just a confidence test to see if you've written the ROM correctly.
Even just getting the CRTC initialised correctly is a great first attempt.
Usually, to learn, someone would init the PIO and turn a connected LED on or off.
In this case we know the CRTC initialisation is working so we can adventure a little further.

If you can, write the code so there is no reliance upon any RAM/STACK whatsoever. There's enough CPU registers to do that.
It's recommend to terminate the test code with a JR 0FEH to terminate the test program in a perpetual loop.