Dumping DEC RD53-A (Micropolis 1325) Firmware
Channel: Mark's Tech Channel
Date: 2020-05-10

Dumping the 8051 microcontroller firmware from a DEC RD53-A (Micropolis 1325) hard disk drive!

I am trying to fix an ailing Micropolis 1325 hard disk drive from a Cadnetix workstation so that I can extract its contents. After looking for the usual problems such as the head positioner being stuck to the rubber end stop, I bought this nearly-identical DEC RD53-A drive (because it's what I could easily find) to try swapping boards and performing more dangerous experiments. I decided to dump the drive firmware and reverse engineer it to figure out why the 1325 is not trying to un-park its heads. The problem seems to be isolated to the mechanical subassembly, and I suspect that the drive is giving up after failing to regulate spindle speed. The 8051 chips on the RD53-A and 1325 have the same markings, so I think that their internal mask ROM contents should be the same.

None of my *PROM programmers can extract internal ROM code from 8051 mask ROMs, so I made this board to do the job. It boots the 8051 from an external EEPROM, which then uses a GPIO to drive the EA# pin in order to re-enable the internal mask ROM. The special dumper firmware then uses MOVC instructions to read from the internal mask ROM, and dumps the contents out the serial port in Intel hex format.

I bought this RD53-A from my friendly independent surplus dealer: https://www.youtube.com/user/uniservo

You should buy stuff from him, too.

I put the design files for this board up here:

https://gitlab.com/NF6X_Retrocomputing/8051dumper

I do not plan to sell these boards at this time.