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Thread: DB37 Pinout for External IBM Floppy

  1. #1
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    Default DB37 Pinout for External IBM Floppy

    So a little while ago my IBM PS/2 4869 External 5.25 Floppy Drive "died". I was never sure as to why (either the MCA adapter card, the drive, or somewhere in-between).

    I had some time recently, so I did some digging with it on my IBM PC 5150. I opened up the case of the 4869, connected one of the 5150's internal card-edge connectors to it and it worked! I checked the 4869's power supply, and it's working fine.

    This means it must be that big, thick, honking DB37 cable that has a loose connection somewhere in it. So I'm probably going to have to build a new cable for it.

    Does anyone know the pinout for the DB37 external floppy so I can build a DB37-to-17pin card edge connector adapter?

    Or does anyone have any other thoughts or tips about this?

    I've tried googling it and can only get bits and pieces of info, but nothing definitive.

    Thanks!

  2. #2
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    Are you married to the idea of a round cable? Making an external drive cable from ribbon cable is a snap--basically a conventional internal drive cable with a DC37M crimped on and the ribbon cable (34 conductors) shifted toward the "high side" of the DC-37.

    Take a look at the techref for the 5150 diskette adapter--there's a chart of the internal vs. external connectors--it's pretty easy to see how they line up. The confusing thing is the convention used for conductor numbering. On the edge connector, it's odd-even (i.e. the first conductor in a ribbon cable is 1, then 2, etc.). On a DC37, it goes row-wise; i.e. 1-19 top row, 20-37, bottom row.

    http://www.pinouts.ru should have the information.

    Note that the PS/2 version for the Diskette Adapter/A does swap a couple of wires in the cable--see my blog in this forum.

  3. #3
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    Chuck,

    Thanks for the link. I didn't know about the swapped wires.

    Hmm. Now you've got me thinking. I took a look at your blog, and you talk about swapping some of the wires in the cable. I was testing this 4869 drive externally by simply connecting it to my 5150 by the external floppy DB37 port. Based on the swapped wires thing, though, this is wrong. You would NOT expect a vanilla 4869 external drive when connected to a 5150 to work this way (since the floppy is only wired to work in a PS/2 with an IBM external floppy adapter).

    So it seems that I need to decide if I want this floppy drive to work on an 5150 or a PS/2, and wire it accordingly, since no single wiring method will work with both machines. Is that right?

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    I think you're right. The best thing to do is to buzz the cable out and see what you've got.

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    Technical info (including pinouts) for the IBM 5 1/4" Diskette Drive Adapter is at http://members.dodo.com.au/~slappanel555/oa/oa.htm

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    Perfect! That's a great resource.

    Thanks.
    Last edited by Holmes; March 21st, 2011 at 07:17 AM.

  7. #7
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    Quote Originally Posted by Holmes View Post
    So a little while ago my IBM PS/2 4869 External 5.25 Floppy Drive "died". I was never sure as to why (either the MCA adapter card, the drive, or somewhere in-between).
    Sounds like you ruled out the drive and the power supply, but what about the adapter?

  8. #8

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    Quote Originally Posted by Holmes View Post
    ...You would NOT expect a vanilla 4869 external drive when connected to a 5150 to work this way (since the floppy is only wired to work in a PS/2 with an IBM external floppy adapter).

    So it seems that I need to decide if I want this floppy drive to work on an 5150 or a PS/2, and wire it accordingly, since no single wiring method will work with both machines. Is that right?
    Actually the 4869 cable goes straight to a standard 5-1/4" drive. Do you have the 360Kb (an asterisk on the faceplate of the drive) or 1.2Mb version? Which microchannel adapter do you have, the "pass-through" with a few chips, or fully populated? Use this as a reference.

  9. #9
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    OK, I'm getting a bit confused. Quick answer is I have NOT excluded the MCA adapter from being the problem.

    I just checked the model of my MCA adapter and it is a late-model FRU 15F7996 based on the link IBMMuseum provided. Also, my drive is the 360K model.

    Originally I had an IBM PS/2 Model 70 (8570) with the MCA Floppy Adapter (FRU 15F7996) connected with a 4869 external 5.25 floppy. Working fine 6 months ago and packed for a move. It was placed on the back seat of my SUV (to absorb vibration) and moved that way (along with most of my other vintage computer equipment).

    The move was completed, and I broke out the IBM 8570 with the 15F7996 MCA floppy adapter still in it. Connected the 4869 floppy to it, and it did NOT WORK. It would just blink the drive light and spin for about half a second on boot and then would not do anything else. I then shelved it (after a bit of cursing).

    I tested just the 4869 drive on my IBM PC 5150 by connecting it to the DB37 external drive port on my floppy card in the 5150. It did NOT WORK. I used this to conclude that there was something wrong with my 4869 drive. Based on the info provided by ChuckG, however, I am thinking this may be an incorrect conclusion, given the 4869's "flipped pins". Anyway . . .

    I opened up the 4869 drive, disconnected the floppy card-edge connector in the drive box. I then plugged into the exposed drive directly by using the internal card-edge connector from my 5150 (which was pulled out of the 5.25 floppy drive in the 5150's case). Note that the 4869 drive in this scenario is still being powered by the 4869's power supply. With this setup, it WORKED.

    So based on this I have concluded the physical floppy drive INSIDE the 4869 works. The power supply in the 4869 works. Since I am unable to get the 4869 working EXTERNALLY with either the PS/2 8570 or the 5150, I concluded that the DB37 connector/cable is faulty (since it is the common denominator).

    So I guess the question I still have is:
    Will a 4869 External 5.25 Floppy Drive work externally connected to an IBM 5150 Floppy Drive Adapter via the DB37 port? Or, does the PS/2's MCA Floppy Drive Adapter (15F7996) external floppy DB37 port work differently from the same port on the IBM 5150's Floppy Drive Adapter?

    Thanks!

    (please forgive my liberal use of CAPITALIZATION - I'm doing it for emphasis because this is getting confusing)
    Last edited by Holmes; March 21st, 2011 at 12:25 PM.

  10. #10
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    Yes, if you modify it per my instructions. Way back when IBM surplused their stock of these boxes I picked up a few cartons of them. Some I resold to customers as special 3-mode external drive boxes (used for machine tool floppies); others I converted for my own use. The IBM part number is 4869-002 (on the bottom of the box).

    My modification has not been a casual one-off affair. To do my blog, I pulled out a brand-new one and made the change.

    If you're convinced that it's the cable that's bad, drop me a line--I still have a spare (I modified a few of these boxes to hold SCSI devices) un-modified cable or two and would be happy to provide one for the cost of shipping.

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