First post, by mattw
I was not able to find online any information for the aforementioned in the subject IBM PS/2 model floppy drive interface pinout. It's 40pin connector, but here:
https://ardent-tool.com/floppy/Pinouts.html
model 8556 is not even mentioned. So, I can rule out this one:
https://ardent-tool.com/floppy/Pinouts.html#40p_Planar
because I measured and there are no +5V on pin38 and +12V on pin40, that's why I presume it's more similar (or even exactly the same) to this one:
https://ardent-tool.com/floppy/Pinouts.html#40p_Planar_MS
So, here is what I did - attaching pictures on which I also added some comments:
A. confirmed there is +12V on pin6 and cut the corresponding wire on my regular 34pin floppy cable (see "picture 1")
B. confirmed there is +5V on pin3 and cut the corresponding wire on my regular 34pin floppy cable after the middle connector(see "picture 2") . this way +5V doesn't go to the connector after the "cable twist" at the end of the cable where my floppy drive will be connected, but I still have +5V on pin3 of the middle connector, which I will use to pull-up all necessary signals (that are all open-drain and need to be pulled-up on PS/2).
after A. and B. I tested the floppy cable on PC and it is still working, because those wires are not used on PC with 1.44MB floppy drive.
C. further "picture 2" shows exactly how I pulled-up the necessary signals - essentially using the middle connector as a breadboard. plus it shows that I cut pin4 wire, which is "Drive Select" in order to be able to pull-up/pull-down that signal, which is supposed to select that way between 720KB and 1.44MB floppy drive type. IBM document "ps2_fdd_trm_s42g2194_00.pdf", which I am attaching to this post, says "Drive type signal, when Active 1.44MB drive has been selected" - I guess that means pull-up.
D. "picture 3" additionally shows just how I pull-down the pin4. I can in similar way pull it up, but in both cases I cannot get proper work.
Finally, the problem and my question:
on startup the drive sounds very healthy - in fact watching videos it sounds exactly like original working IBM drive sounds when PS/2 is powered on, making the exact same series of sounds. However, when it's time to boot from the drive, it makes few sounds (I presume reads) and it gives up. I was thinking that is indication the "Drive Type" is not properly recognized, because on PC I can kind of simulate similar behavior when select in the BIOS 1.2MB drive and try to boot from that same 1.44MB drive. However, as I mentioned in point D. - no matter if I pull-up or pull-down pin4 "Drive Select" signal - nothing changes.
So, any ideas what I am missing and/or doing wrong? Any information how to make regular PC 1.44MB floppy drive work in IBM PS/2 model 8556? thanks!