Notices by Tube🌞Time (tubetime@mastodon.social), page 3

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

hmmm what's this

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.

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...

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

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

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.

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.

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.

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

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

this part takes a while.

whew. got the layout and schematic captured. there were a few differences compared with the official schematic.

it's actually 100 ohms. someone back in the day was making resistors of various values that were all marked 0. great...

ok, now what is THIS resistor value?

by looking at the ST-412 schematic, i figured out that the ST-251's SSI257 chip contains a bunch of steering diodes as well as an op amp (an NE592 equivalent).

the ST-225 drive uses a similar circuit but it is an older design and less integrated. the ST-251's SSI257.2 chip drives the center taps of the head windings. they just "garbage collected" the transistors, head select decoder, and resistors driving it along with the transistors that set the write current.

solved a few mystery pins on these Seagate custom chips. they are connected to the magnetic heads.

kinda horrifying but this is the best way to test SMD parts out of circuit.

I need to confirm some component values.