Comments on: Hammond XB-2 ROMs?

By: Laci Biro

Laci Biro — Tue, 01 Mar 2011 09:39:25 +0000

I have a similar problem: the ROMs in my old XB-2 are defective and I’m unable to find V2 images in order to burn a new set.

By: Merlin Skinner

Merlin Skinner — Wed, 23 Feb 2011 13:26:16 +0000

I have seen EPROMs with “flaky” bits. Presumably the charge has decayed to a point near the threshold of the sense circuitry. I can’t think of any mechanism that would make the bit latch in the flipped state – it would just more more and more likely to be read incorrectly. Light falling on the chip can also cause this.

Joe Koberg’s idea of varying the supply voltage is likely to be a good strategy, as the EPROMS I had were sensitive to this. At least some programmers (e.g. DataIO) allow you to select the voltage (4.5/5.0/5.5V, perhaps). Heating as well as cooling (hairdryer/freezer) is very likely to affect this threshold as well.

Good luck!

By: Joe Koberg

Joe Koberg — Sun, 13 Feb 2011 15:38:49 +0000

You may also try reading with different voltage levels supplied to the chip; and even perhaps with the chip cold. These are just shots in the dark. It’s my understanding that once the cell has flipped back (lost its stored charge), it’s flipped for good.

By: Keith Neufeld

Keith Neufeld — Sun, 13 Feb 2011 15:23:43 +0000

Joe, unfortunately one of my keyboards has the original ROMs and the other has the v2 upgrade, so I can’t compare.

I’m still thinking about taking the ROMs out, reading them slowly lots of times, and “merging” the different readings that I get from each chip, on the theory that perhaps a rotting bit won’t have fully blanked yet and will still sometimes give me a 0.

By: Joe Koberg

Joe Koberg — Sun, 13 Feb 2011 15:20:48 +0000

Err, the other way around. Keep the 0′s….

By: Joe Koberg

Joe Koberg — Sun, 13 Feb 2011 15:19:58 +0000

If you have two of the units maybe you can read and compare the two ROMs.

EPROM bit rot turns 0′s into 1′s, so everywhere one ROM has a 1 but the other a 0, keep the 1.

Joe