gedobbles wrote on 2021-04-30, 13:06:

haha, the original chip (almost for sure) is an UV-eraseable EPROM (I can feel the window through the sticker),
my plan is to eventually replace this by an EEPROM temporarily for testing/educational reasons.

However I don't have any idea about the pinout of this EPROM and whether compatible EEPROMs can be found.

I've got a feeling that 27 parts are all One-Time Programmable or UV EPROM, and the 28 series are EEPROMS. But the pinouts are only nearly the same...

There's a bit on this page https://theansweris27.com/bmw-e30-318i-engine … t-optimization/ about what they had to do to convert from a 27 EPROM to 28 EEPROM.

update: i figured out that the RTC Adress strobe and R/W lines are broken (but under the RTC chip so you can't see it)
I hope fixing these, revives the computer

update2: after I soldered these lines, it began beeping via the speaker!

update3: it also boots into BIOS now, I think this is resolved

I had to be a little creative with the soldering though:

Nicely done, it's really great that you got it working again 😀 Those wires definitely look a bit risky, maybe you could use some hot glue to hold them in place, or tape.

How did you work out that those two lines weren't working? Did they just show no activity when probing around with the oscilloscope?

since I guessed the problem was with the RTC, when reading the datasheet of the chipset, I came across the RTC address-strobe, data-strobe etc. lines,
so I checked them for continuity with a multimeter.
Yeah also before that, I measured the voltage on the corresponding RTC pins and they seemed floating (~60mV).

There's always a chance to do rework on those bodge wires, the important bit is you got it to boot.