While I was dumping EPROMS from my 486 there were instances where I got different checksums from the same chip. Reseating the chips in the programmer's zif socket didn't fix the issue. Are these chips going bad? And what's the right image/checksum? Repositioning the chip in the zif didn't do much except yield yet another different checkum.
Turns out that dual-wipe sockets read from 2 surfaces of the pins. That's fine. And these surfaces are the ones you'd typically clean. But most all programmers read from the 2 sides NOT EVER touched by the socket contacts. The edges are 90 degrees apart. Perfect "hiding place" for long-term oxide buildup. Especially over 25+ years. These edges never see and friction/cleaning action from insertion and removal.
So. Solution is simple, you must clean the sides of the pins which are never touched by the socket contacts. After doing that each BIOS chip I was dumping read reliably 10x in a row, with or without a little wiggling and jiggling and repositioning.
IC sockets contact one area of the chip's pins, but 'prommers contact a different area.
Therefore ALL 4 sides of each pin must be clean in order for your system to work and for your 'prommer to grab data.
I'm willing to bet that many bad dumps over the years happened because this issue wasn't addressed. Not so much with soldered-in chips, those are clean on all 4 sides after removal usually. But I'd imagine arcade cabs are ripe for letting muck and dirt build up. If just pulling the chip and ripping it with the 'prommer, you're reading through decades of oxide accumulation, and dust. Unless you clean all 4 sides of each pin.
TRIVIA: Reading an EPROM with a bright light shining into the window will result in a massively bad read.
Edited by Keatah, Mon Apr 15, 2019 4:08 AM.