Posted By
Richard42
on 2012-07-15
14:39:41
|  Re: Getting a pair of 1551s
Hans, that's correct. I also noticed that the working drive had 3 40-pin chips with yellow dots on them; presumably this was done in the commodore factory to indicate that the chip was tested/good. The non-working drive appeared to have 2 chips with yellow dots, but on closer inspection I saw that the 318008-01 PROM chip had a (presumably yellow) dot which had been painted over with black. So it seems that someone even tested the drive before me and found that the ROM was bad. That's unfortunate; I thought that both auctions stated that the drives were good.
For kicks, I dumped the data from the bad ROM chip as well as the good one. I ran each through 'hexdump -C' and diffed the output to see what the bad rom data looked like. The binary differences showed up only in the 5th and 6th bytes of each 16-byte block, and only in four 1k blocks, starting at 0x0000, 0x1000, 0x2000, and 0x3000. This eprom is 0x4000 bytes long. In these bytes, in the bad PROM chip, the 0x04 bit is always 0. This is an interesting failure mode for that chip. I presume that it's internally an NMOS eprom but with no quartz window.
|
