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Thanks Tez!

There is a pull-up already on the 600 board, you should not need another. But yes, it would be easiest to take the GPIO of the RPi trough a 2n3904 transistor or small mosfet, like a 2n7000. If you wanted to get deeper into the RPi, there may be a way to turn off the active pull-up on it's GPIO lines...

The device operating the DD line must be open collector or open drain. The DD, when used with the 32K RAM board is pulled high by a resistor on the 600 board so you cannot use a device with an active pull-up.

Hey Mike, do you have more of an explanation of what we are seeing here?

I wonder if it could be re-located to run in ROM, or does it mix data and code?

That's the 2nd best use I've seen for a PC case.

Call it 5 amps to account for any other logic on the board.

I'd say, in order to keep things reasonably cool you would need 7 amps reserve on that power supply to add another 527 board.

What are the specs on the transformers?
What are the pass elements in the regulators?
How much current does a fully populated 527 board draw?

We should be able to figure this out.

Wow

Welcome...?

For some reason I cannot find them. Did they sell already? Can someone post a link?

Never mind. Found them...