Electronics > Repair

Fluke 8840A Faulty CPU

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coromonadalix:
Im "mad"ly impressed by your tought process

I hope it will work for you, i would like to have this kind of knowledge,  i have a crazy idea for a new controller board ( arduino based / mega 2560 ) for an meter Ie: keithley 196 or an tektronix dm5120, the analog board is controlled by shift registers and the analog board values are read by somekind of a pwm signal ...

fixitanonymous354:
Hi i have recently acquired an 8842A with the Z8613RS and have dumped the 2732 roms for the CPU and IEEE board using an Enhanced Willem.
I had to use an VCC jumper fix described on the Willem forums to get the Willem to read the ROMS but the U222 ROM matches to whats available on xDevs so i think the dumps are reliable.

U202 is rom on top of main CPU
U901 is rom on the GPIB board
U222 was the same as available on xDevs

essele:
Thanks for these ... I’ll have a look at them in the next couple of days ... I believe there are quite a few differences between the 8840a and the 8842a, but I can’t see a reason why the same approach couldn’t be adopted.

essele:
Making some progress...

First stab at a PSoC based board ... only done some very simple testing so far, it powers up, can be seen and programmed by the MiniProg3, and I can generate an 8Mhz clock, unfortunately the pins for the external crystal don't allow routing, so if the native 8840A crystal doesn't work I'm stuck with a PWM generated clock, but that all seems ok based on this first test.

Step 2 will be putting it on some breadboard with a crystal, then step 3 will be in the 8840A. Hopefully the native crystal will be fine. Not sure if I'll be able to power it from the MiniProg and leave the 8840 powered off for now, I suspect not.

Oh ... and yes, I will populate the decoupling caps - I was just keen to see if the MiniProg would see it!

UPDATE: just tried it in some breadboard with a random 8Mhz external crystal in the 8840A configuration (i.e. no caps, and 10M in parallel) works fine as a source for the PSoC, I can generate a much more accurate 1Mhz test signal from the ECO rather than the IMO, however pin 2 doesn't swing much (looks like 1V max) so I am doubtful if this will be sufficient to drive the ADC, but I'll try it over the weekend as it may be different with the particular crystal, or may well be sufficient.

FURTHER UPDATE: doesn't seem to work from the onboard crystal, it's oscillates but only at just under 1V, I can see the 8MHz signal getting to the ADC, but no sign of the 1MHz output. Programming whilst installed works fine, and a quick test of the 8MHz PWM output seems to work, I see a 1MHz output from the ADC and that reaches the keyboard/screen controller. So this should be sufficient to start working on the bus access bit in Verilog. I should be able to get a test program to read the EPROM or maybe even attempt to update the screen. I may consider a separate crystal for the next version, although I'm still not sure how important accuracy (or matching the onboard one) is.


essele:
Well it's getting a bit exciting ... I've now got a working bus read capability, and I can read bytes from the U222 EPROM using my Verilog bus access module, this should also work for reading from the ADC, the Cal Memory, and the keyboard/display controller (assuming I've got the timing right.)

Next step is the write side of it .. testing that is going to be slightly harder, I guess I'm going to need to get the display to output something!

One slight worry at this stage ... because the XTAL2 pin (chosen because of the potential use of the external crystal) is not routable inside the PSoC then my only output option for an 8MHz clock is a PWM. This is then not synced with the UDB 8MHz clock that's used for the read/write logic ... not sure if it's really a problem, but it feels like something I should fix. I may just bodge another pin over to it for now so I can directly output a clock.

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