Electronics > Repair

Solartron 7081 Earthy Processor Board Reverse Engineering

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Eleclu:
A few months ago I purchased a Solartron 7081 on Ebay and the Earthy Processor Board was missing. So I took the schematics and designed a new pcb :box:
The modernized design features a super cap for data storage, only two SMD SRAM-Chips and the original firmware on a single larger EPROM. The whole PCB is  routed on 2 layers and can be manufactured bei JLCPCB or PCBWay. Maybe someone may find this useful :D
If you have questions, feel free to ask.

Kleinstein:
The super capacitor should likely only power part of the RAM. In the plan it looks like it would power the whole 5 V part.

At the EPROM  /OE is wired to GND. Not sure this is a good idea, as it could cause issues with a  DMA or if for some reason a write to a EPROM is address is done.

Eleclu:
The negative terminal of the super cap is connected to the SRAM VSS-pins and Q2. When there isnt any 5V supply, Q2 will prohibit the currentflow to the rest of the circuit.
/OE is the output enable signal and must be pulled low (Take a look at the public available 7081 schematics).  The EPROM is UV erasable and this circuit will never try to write to it, nor the firmware. It can not provide the programming voltage.
This circuit runs in my multimeter for three weeks now without any issues.

perdrix:
A fine piece of work!

My only concern would be that you made it two layer board as that's more likely to emit EMI that will interfere with the floating boards esp. the analogue board.

What software did you use for the design?   Was it KiCad or Multisim/Ultiboard?  If so would you be prepared to share the files?
Do you have a BOM as well?

David

Eleclu:
Is the original earthy processor board really more then two layers? The earthy logic board is also only two layers and this board is original.
But maybe a four layer layout reduces the noise of the meter. This pcb be would not be as cheap. The size needs to be 16cm bei 15cm, so the original mounting hardware can be used.

This pcb was created with KiCAD. I included the whole project folder with BOM. Feel free to use it :)

Edit: Attached corrected files

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