Finally - Success! I really cannot believe it!! the explanation is below - I am not posting pictures, because after so many expe […]
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Finally - Success! I really cannot believe it!! the explanation is below - I am not posting pictures, because after so many experiments my 34-pin regular PC floppy drive cable is an ugly mess that will confuse you and that's why I will explain in details with words only.
It was entirely brute-force approach, i.e. doing countless experiments and observe the results from them, because as you will see, when you read below what solved it - that all information online including PS2FF adapted made by June Tate-Gans that I mentioned in the few previous posts are wrong! (at least wrong for my PS/2 model 8556, which I believe is also called Model 56SX).
So, it's amazing how simple it is when you figure it out - no any resistors or circuitry is involved - hence making adapter PCB makes even less sense. What is most confusing of all is that IBM own "PS/2 3.5-inch Diskette Drives Technical Reference" document (the "ps2_fdd_trm_s42g2194_00.pdf" attached to my 1st post) is wrong - my best guess is that document is made before those models like 56, 76, etc were released and hence the information there doesn't apply for them. (I say IBM docu is wrong, because if we use the information in that docu then setting the motherboard to 1.2MB 5.25" drive made my 1.44MB 3.5" drive work - clearly that documentation cannot be right for those newer PS/2 models like 56).
Initially, when I made it work I was thinking it's a fluke and maybe it will work just with that particular regular PC 1.44MB drive I was using at the time, but luckily - it wasn't - I've already tested with 3 different regular PC 1.44MB drives - Sony, Teac and Chinon - they are all working!
Here is the minimum working configuration - one more time no resistors, no circuitry, all modifications are done on a regular PC 34-pin floppy drive cable:
1) cut pin3 and pin6 wires (see my first post pictures of those cuts and why those pins corresponding wires need to be cut). So, I cut pin3 wire after the middle connector on the cable (for considerations I explained in my first post, which are no longer relevant, but anyway - it needs to be cut - where is the best place on the cable to cut it - you decide). pin6 wire I cut close to the end of the cable that goes to the motherboard and that is the best place as it's +12V that will most likely fry your PC 1.44MB drive if it's not cut there. Nothing new here - those cuts are done for all PS/2 models when you want to mod them with regular PC 1.44MB floppy drive.
2) cut pin4 wire - again see my first post about that
3) the New finding and Critical for the Solution: cut pin9 corresponding wire - it's the wire just before the "cable twist" part of the cable starts - that's why I cut it there, i.e. after the middle connector on the cable. That pin9 is shorted to GND via the floppy drive connector - that's why it needs to be cut, because when it's shorted to GND makes 1.44MB drive not possible to work.
4) on the middle connector of you regular 34-pin floppy cable for PC, use jumper wire and short pin4 to pin5, i.e. short pin4 directly to GND (pulling it down via resistor - I used 2.2kOhms one - is not working, needs to be directly shorted to GND).
That's it - now my recently acquired IBM PS/2 is free of IBM custom parts - I removed their custom (and now dead) power supply and modded it to work with modern regular PC ATX power supply (if anyone needs guide/howto for that - let me know, it's super easy when you acquire proper parts and materials for doing the ATX mod - I did it with all off-the-shelf parts). now it's free from IBM very special floppy drive as well (dead in my case as well). In other words now it uses all standard parts - my RAM is also regular PC one that I modded to be compatible with PS/2, using this:
https://ardent-tool.com/memory/Mods_02.html
the HDD is SCSI, but standard one without any IBM "customization" and thus no changes are needed there in order to acquire cheap available replacements (if needed).
I hope the above will help other PS/2 users - as far as I can tell based on PS/2 Floppy Drive Interface Pinouts here:
https://ardent-tool.com/floppy/Pinouts.html
the solution should work with PS/2 models 56, 57, 76, 77, 90 and 95, etc, i.e. all models that have 40 or 44 pin floppy drive connector and are "media-sense" type.
