Notices by Tube🌞Time (tubetime@mastodon.social), page 4

so on my schematic, i've named all the previously nameless pins and the circuit makes sense now.

going to figure out what some of these custom chips do. this one is "10223-502."

the schematic PDF is in the repo as well as the KiCad source files. i'll be examining the firmware as well as the earlier board revisions.

OK, infodrop for you folks! https://github.com/schlae/HardDriveInfo

i've collected more information on the ASICs at https://github.com/schlae/HardDriveInfo.

hmm yes it sequences the stepper motor using the spindle motor's back EMF as the clock!

this one seems to be for retracting the heads using the stepper motor when power is lost to the drive.

i've moved on to the read/write LSI. the read channel is somewhat tricky because it uses four (!) one-shot timers to turn the raw analog signal coming from the read preamplifiers into an output pulse that represents a flux transition.

the "ring lsi" is interesting. it provides an adjustable linear regulator to set the stepper motor supply voltage (set through SPI bus) and it monitors the "ringing" on a winding after it has been driven and is settling.

collecting the info is a pretty manual process.

unfortunately we don't have a dump of the protected mask ROM, but it seems to be somewhat similar, and the 6502 puts *every* address it accesses on the bus, so you can see it accessing special function registers and the stack, allowing you to infer quite a bit about what is going on.

the firmware from the older board revision (which uses an external EPROM chip) has been dumped and (mostly) analyzed! see https://github.com/schlae/HardDriveInfo/blob/main/st251/firmware/ST251_commented.txt

this later revision of the st-251 uses a mask-ROM microcontroller. it's very similar to the R6518, at least in the pinout. it also latches its internal address bus on a bunch of the pins during the rising edge of phi2! this means i can spy on the program counter and any other memory addresses being accessed.

another day, another board revision!

this one has minor electrical changes from the previous one, mainly adding control over the spindle motor *power level* from the microcontroller. that way they were able to lower the average power consumption.

https://github.com/schlae/HardDriveInfo/tree/main/st251/21020

well that was epic! i've completely reverse engineered this older revision. details in the repo (https://github.com/schlae/HardDriveInfo)

here's where things get really interesting. i've pulled in the imagery of the early ST-251 board and overlaid my Kicad traces from the newer revision.

it's almost like a visual diff--now i can see exactly what changes were made between board versions, which means i can back out a schematic as well!

turns out there is another variation of the ST-225 logic board. looks like a cost reduction.

and just like that...the ST225 logic board is now in KiCad.

check it out: https://github.com/schlae/HardDriveInfo

this is the control board for the ST-225 hard drive. it's an earlier revision. so far it's not too dissimilar from the ST-251 but the stepper motor driver circuit is quite different.

the key observation is that the pin state depends on the frequency of the signal coming in. for DC, it is normally low. for frequencies *higher* than about 2.6MHz, it also stays low. but for a particular window of frequencies (1.4 to 2.6MHz) the pin goes *high*.