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5150 and IBM 64/256 RAM Expansion Board - Parity Error Whack-a-Mole

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First post, by pjw611

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Anyone have any hints on chasing down a randomly moving parity error on an original IBM RAM expansion board (the 64k-256k board)?

The symptoms are that when I populate the second bank of RAM, a random parity error will kick up indicating that bank, then either immediately or after a few minutes a PARITY CHECK 2 will occur.

I've replaced the RAM chips with known good ones. I've replaced a bad 74LS04. I've pulled and tested both 74LS158's (they passed). I've replaced the tantalum caps (but not yet the little myriad little 4.7's that flank every RAM slot as they're theoretically less likely to fail like tantalums do).

Appreciate any advice before I break out the technical documentation and run through every logic gate on the card...

They never warned me this hobby would be so addictive...
IBM XT to Pentium III
Anything newer's too new for me!

Reply 1 of 2, by BitWrangler

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In general, moving and flighty parity errors tend to indicate the voltage is low at the chips. So check for that and chase it back to source.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

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