Gregory's Serverwell now i feel a bit stupid. this thread from a few years ago includes links to disassembled drivers and ROMs, as well as a sample disk image.
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 not all the surfaces have a good amplitude. this could explain some of the issues i was having. (channel 2 has a pulse whenever the head changes.) anyway this analog signal comes off the drive's analog test points (circled). this signal comes after the differentiator (IC 9E1) and the first stage analog filter, but before the second stage filter. i'm trying to correlate the drive's data output (valid on the rising edge, each pulse has a fixed width) with the analog data. it seems to have a delay of about 180ns caused by the analog filter and the propagation through the comparator. going to dump the drive firmware for the ST-225N's 8051 microcontroller. this has never been dumped before. this firmware reads additional firmware off the platter, but i've already dumped that data. i've got another ST-225N to try and extract firmware from. the board was apparently dead, but eventually i might try it out with my other ST-225N logic board (i hope it still works) just have to move this hacked up ST-225 logic board over. it's cabled to an Arduino that runs the stepper motor so i can get to the hidden tracks. it spins up and i've captured the negative tracks! (the Marker bit is a signal pulsed by the Arduino when it changes tracks or heads. it just cycles through all the heads in order, then goes to the next track) looks like this came out of an Apple system. this is on track 0 which is user data. track -1, head 0, sector 19 contains the drive serial number as ASCII text. interesting. track -2 contains the same 32 sectors on all heads. it seems to be 8051 code (02 is the LJMP instruction). i'm guessing this is the main drive operating code.   @tubetime Lot of drives had the firmware on hidden tracks. I am entertained that it's an 8051 though. The newer drives got much better CPU  |
i decided to try and get a better read of the firmware tracks (-1 and -2 on all the heads). this time i'm capturing analog data from the drive's test points.