The board came as-is to me, without history. So don't know how long it hasn't worked correctly
And we don't know if someone attempted a repair, and 'broke' things in the process.
As an example, someone came on these forums with a faulty 5150/5160 motherboard, and it turned out that someone had fitted the wrong type of RAM chip in one of the sockets. We can rule that out in your case because you "
removed all RAM from banks 1, 2, and 3, only leaving bank 0" and "
replaced with known good RAM and still have same issue."
Presumably, those known-good chips are 4164 class, being that you have a '64 - 256KB' motherboard.
I've actually been looking at the various options of the mem test:
- I removed all RAM and the error was same as the initial picture, *EXCEPT* that the error was on 0000:0000
- I removed a single chip and then the error was like the picture you reference, see below. The error is on 0000:0000 instead of 0000:0001.
- The rest accurate documents the missing chip. I can remove / move the missing chip and the error follows the location.
That informs me that when Ruud's Diagnostic ROM (RDR) tests motherboard address 00000, that an address
somewhere in RAM bank 0 is being successfully tested.
When RDR goes to test motherboard address 00001, something is very wrong. For example, maybe (repeat: maybe) a ROM address or an 'empty' address is somehow being hit.
3. Do you have one of the devices shown at [
here] ?
I'll see if can borrow such a device locally, but I don't.
You should look to acquire one anyway. They are cheap, and used as the output device for some of the test code for U18 at [
here]. When I write custom test code for U18, the device is my preferred output device, but alternately, I can send beep patterns to the speaker, assuming that the associated circuitry is working. Do you hear the short-long-short beep sequence from the speaker when RDR starts ?
I can also output to a serial terminal (or terminal program) via a serial port at 3F8h (the standard I/O address for the first serial port). I will be, very shorty, releasing a new version of RDR that does that.
And a
logic probe is something else you should get. Based on the information that I have presently, a logic probe will probably be required down the track, i.e. I send custom ROM code to you (via DM) and get you to report what the probe shows on certain IC pins.