Electronics > Repair
Fluke 8840A Faulty CPU
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essele:
Looking at the various CPU options I'm finding it really difficult to understand how you would attach an external PROM to replace the internal one without using an extra address line ... I'm sure it's possible as I've found a few comments about it, but I just can't seem to get my head around it.

So, I'm liking the Z86E40 ... it seems to be a modern version and is reasonably priced and available (Mouser and Digikey) it has a different pinout, and a few more capabilities, but it seems to be backwards compatible from a register perspective, and it has a fairly standard method for programming once you translate the pins, therefore using it in a PLCC or LQFP format you could probably build a simple board that includes the pinout for programming and then use a normal TL866 type programmer.

I'll look at this in some more detail once I get a bit more time.
essele:
You only seem to be able to get the PLCC or DIP40 version of the Z86E40, so I've pulled together a couple of board designs based on the PLCC44, one is the actual board that would go into the 8840A and the second is the one you'd use to program it (the schematic is mostly taken from the Z86E40 data sheet.)

They were both a real pain to route, but I think they are ok.

I'm just going to check a few things (like will it actually fit in the 8840A) and go through the data sheet in a bit more detail to make sure that it stands a chance of working.


Kleinstein:
For such an adapter board I would also include a decoupling capacitor close to the chip. One can still not populate it, but it can help to have the option.
essele:
Thanks Kleinstein ... I've added a decoupling cap as close to the socket as I could sensibly get it.

The boards are being made now, so hopefully I should have then in about a week.
essele:
I've just found an old 8840A/AF which also wasn't working ... and I've learnt the hard way that the bloody newer TL866II Plus doesn't actually have the capability to burn older EPROMS requiring >18V, so I had to buy a new programmer. Grrrr.

So I now have two different "working" CPU's ... one from the second 8840A and one from the 8840A/AF.

The prom from the AF (v2.5) matched one I found online, the label EPROM sticker was floating around in the device, so I assumed it was not going to work, but it seemed fine (I've burnt a new copy anyway.)

I've also found two different v2.3 images online, including one with a matching image for the Z8.

The v2.3 images didn't work with either CPU ... but that's to be expected as I understand you need a match between Z8 firmware and the EPROM.

The 8840A/AF cpu in the first 8840A didn't work with the v2.5 original image, however it did work with the newly burnt v2.5 which matched the one actually in the 8840A/AF ... which was strange since that combination didn't work in the AF, so that's either a chip seating problem, or there is another issue with that unit (one for another day.)

So now I have one complete 8840A, with a working RMS board, and it seems to be really nicely calibrated.

The second unit now has the prior "working" CPU with the original v2.5 image and it's still showing the problem with the RMS board ... so I'm no nearer to really understanding this, it could still be the CPU or the PROM since they are paired.

And I have the 8840A/AF which is now missing a CPU and PROM.

So ... I'm now fairly dependent on my adapter and programming boards. I have the v2.3 paired versions which I'm hoping I can get onto the Z8. The boards are due to arrive on Friday, so I can see the weekend being fun ;-)

The other option is to use the 'test mode' that Paul suggested to see if I can read the image from the working CPU I have, but that's going to mean rigging up a test bed with a clock, an octal latch for the address/data demultiplex, and writing some code to read the ROM and spit it out the serial port ... sounds like a great project, but a real time burner!
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