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Thread: IBM PS/2 Floppy converter development

  1. #1
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    Default IBM PS/2 Floppy converter development

    Hi all,

    A few months ago I purchased an IBM PS/2 model 50 for $20 CAD. The floppy drive didn't work, so I've been developing a PCB to connect a standard floppy drive to the computer. At the moment, it consists of an edge connector and floppy connector, as well as some pullup resistors and a power connector(output for the floppy drive, so I don't have to modify the power supply).

    It kind of works - when the computer boots, the floppy head moves back and forth, just like on the original drive. However, I get a floppy error and the system refuses to boot. Doing some research, I found one or two people saying that a floppy drive with jumpers is required. This is to change the logic level between CMOS and TTL(I'm not sure which one is the correct one for the PS/2 systems). Since the header requirement is rarely mentioned, I have a feeling a lot of drives work with both logic levels by default(but clearly mine don't!)

    If anyone has a floppy drive with headers they are willing to give away(or maybe sell), that would be great. I need to confirm the board works before I start designing a revision with logic level shifting built in. Eventually, I would like to sell the boards alongside a 3D printed floppy drive mount. (I am also designing an MCA card, but that's another project entirely.)

    I am also interested if anyone knows more about this. I was unable to find very much information on the specifics.

  2. #2
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    AFAIK, all SA400 floppy drives, no matter their technology work on TTL levels. Signaling is done via open-collector (or in the case of CMOS, open-drain). Since it's OC signalling, some sort of pull-up termination is usually required.

    You might want to have a look at this chestnut from 2013 Adapting a cheap 1.44M drive to a Model 50

    Sadly, I can't find a mirror for the charter.net page--maybe there's one out there. Sigh. The old problem of writing our web content in sand...

  3. #3
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    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  4. #4
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    Quote Originally Posted by Chuck(G) View Post
    AFAIK, all SA400 floppy drives, no matter their technology work on TTL levels. Signaling is done via open-collector (or in the case of CMOS, open-drain). Since it's OC signalling, some sort of pull-up termination is usually required.

    You might want to have a look at this chestnut from 2013 Adapting a cheap 1.44M drive to a Model 50

    Sadly, I can't find a mirror for the charter.net page--maybe there's one out there. Sigh. The old problem of writing our web content in sand...
    Yeah, I've seen that video along with other posts mentioning the same thing. I already have those pullup resistors included - they're the ones I mentioned in my post.

  5. #5
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    Quote Originally Posted by eeguru View Post
    I have not. Interesting that they have chips on board. Guessing they're logic level converters - I can't quite read the model numbers on top.

  6. Default

    Quote Originally Posted by eeguru View Post
    I think that's actually for the External floppy drive option. The MCA card just gives you the external port to hook up the drive, and then you use that connector to connect the card to the Drive B slot in the Floppy Riser inside the system. You can see it originally came together as a kit in this listing:

    https://www.ebay.com/itm/IBM-6450245...frcectupt=true

    Now if it will work for any floppy drive though I don't know. At the very least I'd imagine you're still going to be missing the power pins.

  7. #7
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    Just curious--what reference did you use for the PS/2 floppy pinout--and how did you accommodate it to a SA400-interface drive?

  8. #8
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    Quote Originally Posted by Chuck(G) View Post
    Just curious--what reference did you use for the PS/2 floppy pinout--and how did you accommodate it to a SA400-interface drive?
    The PS/2 pinout is identical so a modern floppy drive. There are a few places on the internet which mention this. I also found a PDF detail the floppy protocol(with pinout) used in the PS/2.

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    Quote Originally Posted by steve314 View Post
    The PS/2 pinout is identical so a modern floppy drive. There are a few places on the internet which mention this. I also found a PDF detail the floppy protocol(with pinout) used in the PS/2.
    There's also this page:
    http://ohlandl.ipv7.net/floppy/Floppy_Pinouts.html

  10. #10
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    Quote Originally Posted by Chuck(G) View Post
    AFAIK, all SA400 floppy drives, no matter their technology work on TTL levels. Signaling is done via open-collector (or in the case of CMOS, open-drain). Since it's OC signalling, some sort of pull-up termination is usually required.

    You might want to have a look at this chestnut from 2013 Adapting a cheap 1.44M drive to a Model 50

    Sadly, I can't find a mirror for the charter.net page--maybe there's one out there. Sigh. The old problem of writing our web content in sand...
    Chuck,
    I know from past experience with external ATARI drives that in 5.25" drives all are not equal and the Atari ST will not work with some, usually older 5.25" drives. So perhaps not CMOS but LS TTL or something that requires a lower drive level
    Dave
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

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