Gregory's Serverit's actually 100 ohms. someone back in the day was making resistors of various values that were all marked 0. great...
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 @tubetime The “0” stands for “this marking gives you zero clues as to what the resistance of this resistor might be.”  @tubetime A manufacture marking what looks like a 0 on SMT parts that are not zero-ohm is horrible. 😬 whew. got the layout and schematic captured. there were a few differences compared with the official schematic. solved a few mystery pins on these Seagate custom chips. they are connected to the magnetic heads. 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. 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). so on my schematic, i've named all the previously nameless pins and the circuit makes sense now.  @tubetime Where is that betamax pin! congrats on the great reversing. kinda horrifying but this is the best way to test SMD parts out of circuit.  @tubetime I have to imagine this is what Tombstone, AZ looks like   @tubetime that's a slightly unsettling image, idk why. random thought: if you're testing precise values, don't forget that MLCC capacitance rises significantly during soldering and takes around 48h to return to nominal, which may still be much higher than it was in-circuit due to aging. capacitor manufacturers tend to physically cut them out of the circuit when they're doing failure analysis. OK, infodrop for you folks! https://github.com/schlae/HardDriveInfo 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. going to figure out what some of these custom chips do. this one is "10223-502." this one seems to be for retracting the heads using the stepper motor when power is lost to the drive. hmm yes it sequences the stepper motor using the spindle motor's back EMF as the clock! i've collected more information on the ASICs at https://github.com/schlae/HardDriveInfo. 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. 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. 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. 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 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. 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! well that was epic! i've completely reverse engineered this older revision. details in the repo (https://github.com/schlae/HardDriveInfo) 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 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. and just like that...the ST225 logic board is now in KiCad. check it out: https://github.com/schlae/HardDriveInfo @tubetime @tubetime I've never wanted to play a DOOM map this badly in my life. turns out there is another variation of the ST-225 logic board. looks like a cost reduction. OK, this one is done! amazingly, it also helped me solve a mystery on the ST-251. on this later drive, the board has a custom chip that captures data from the current track and outputs a "processed" version of it to the MCU through the PA2 GPIO pin. how does it process it? i didn't know until today. so the 20527 board in the ST-225 uses a discrete circuit for doing the same task. it has two 74123 one-shot pulse generators connected in series with each other and going out to the MCU (on PA7 because this board uses a variant). 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*. why is this useful? it turns out that these drives have a 1.75MHz signal recorded onto specific tracks, letting the MCU (which controls the stepper motor) know if it has gone outside of the data area. so basically, you step around, and if you see this pin go high (and stay high) then you know you're out of bounds. there is a special signal recorded at track -2. it is the same 1.75MHz but it has short bursts of twice the frequency (2F). this lets the MCU know that (1) it is at track -2, and (2) it lets it synchronize the hall effect sensors. you get multiple pulses per revolution, so you need a way to sync up the divider chain so you get an index signal that is repeatable.  @tubetime The ST-412 has an NCR 6500/1. This was also used in the Amiga keyboard controller. There's a schematic floating around from the Amiga ppl for a circuit which dumps out a 6500/1's internal ROM. I still have the 6500/1 off my dead ST-412, but haven't gotten around to making the dumper yet. Wonder if a similar circuit could be used for R6518... @cr1901 i found this: https://e4aws.silverdr.com/hacks/6500_1/ but it seems to be for the Commodore version of the 6500/1 @cr1901 also i think the st-412 (at least one version) also uses an external EPROM which has been dumped and is on bitsavers.  @tubetime whatever happened to the capsoff people who used to dump arcade maskroms/maskrom microcontrollers? are they still around anywhere? @tubetime I can actually help with the 11695-502. It's a speed control chip, also used on the ST-225. It looks like the Seagate custom motor driver the later-production ST-251 uses combines the Hitachi chip, the 11695-502, and the glue op amps from the early production board (as seen on that dubiously-correct M.I.T. schematic on BItsavers). @lee4hmz ahh very interesting, I'll have to use that to fill in details when i get to the other board revs. @tubetime at least if any parts are dead theyre already in the cemetary!  @tubetime this might be the most cursed electronics thing I've ever seen. whyyyyyy would they do that >_< |
@tubetime no that's the Omega resistor company's logo