I'm tinkering with some old hard drives, like this ST-251.

it would be nice to be able to repair the logic boards. a schematic exists, which is quite helpful.

what's interesting is that the board hosts a microcontroller. the schematic lists it as a "6510" which makes me think it is 6502-based.

on my board this is the part marked 80118-501
R1512-11

and it has the Rockwell logo, making me think it is something from their R6500 series of 6502-compatible microcontrollers.

another version of the board has the EPROM footprint populated, and the chip is marked R6518AJ, which is definitely a Rockwell microcontroller. the firmware has already been dumped.

you can find good info here: https://bitsavers.org/pdf/seagate/mfm/ST-251/

this definitely looks like 6502 code. time to pull it into Ghidra.

oh yeah. i've mapped it starting at address 0x3000 but the hardware maps the ROM chip in starting at 0x0100, where it just gets aliased all the way up to 0xFFFF. the absolute jump addresses located in the vectors at the top of the ROM refer to addresses starting at 0x3000, so i'm just using that.

hmm, looks like there are a few instructions that aren't 6502. apparently Rockwell defined a few new ones. it would be nice if Ghidra could handle those.

hmmm what's this

ahhhhh much better. thanks to whomever added R6518 support to Ghidra just now. oh wait that was me

ok so funny story, the Ghidra target of "65C02" already supports these instructions. at least i know how to customize Ghidra...

some interesting routines. this one waits for the spindle speed (as measured by the index pulses) to exceed 3593 rpm (or so).

they have a clever delay-by-x routine. the entry point is delay_entry. and yes, the routine calls its own rts as a way to insert cycles...

so one of these drives blew a tantalum capacitor. i think i will take this board apart for reverse engineering purposes. there are multiple revisions but this will be a good starting point.

with a combination of hot air and a desoldering gun, i've taken it down to just a few surface mount parts. i'm going to leave those in place for now since many of them are not marked. there is also no silkscreen.

they scanned well. i've taken the top and bottom images and lined them up precisely. these reference images are 600dpi but i may go for a lower res version for KiCad since it stores the images in the pcb file, which slows it down a lot.

this part takes a while.

what resistor value do you think the circled chip resistor has?

all of you are wrong. it is a 3K resistor.