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Thread: FDD Cable Twist - necessary?

  1. #11

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    Quote Originally Posted by Stone View Post
    Keep in mind that many 5" floppy drives do have drive select jumpers, e.g., DS0, DS1, DS2 and DS3.
    This one doesn't. The PCB area has nothing installed (i.e. no headers) so can't even jumper it if you wanted to.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  2. #12

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    On later Compaq machines, the floppy drive cable twist is built into the motherboard, allowing the use of a flat (straight-through) cable to the A: drive. Therefore if you use a cable with a twist in it so you can add a B: drive, then the A: drive goes first and the B: drive goes after the twist, opposite of the normal convention for PC clones.

  3. #13
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    Isn't that what I said?

  4. #14

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    Quote Originally Posted by vwestlife View Post
    On later Compaq machines, the floppy drive cable twist is built into the motherboard, allowing the use of a flat (straight-through) cable to the A: drive. Therefore if you use a cable with a twist in it so you can add a B: drive, then the A: drive goes first and the B: drive goes after the twist, opposite of the normal convention for PC clones.
    Can you define later? Are you talking Deskpro era, proliant, prosignia?

    I tried a regular floppy cable and connected the drive after the twist - no go both floppy drive access lights came on and stayed on. I am going to try the floppy drive in a known good machine and make sure the drive is ok. On last chekc it was fine but who knows...
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  5. #15
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    Hmmm, drive lights that come on with power indicates that your cable is installed "upside down". Don't insert any write-enabled floppies in that setup--they'll be clobbered.

  6. #16

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    Quote Originally Posted by Shadow Lord View Post
    I am going to try the floppy drive in a known good machine and make sure the drive is ok. On last chekc it was fine but who knows...
    Good idea. Check it both with and without the twist to exhaust all possibilities.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  7. #17

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    Quote Originally Posted by Shadow Lord View Post
    Can you define later? Are you talking Deskpro era, proliant, prosignia?
    "Later" as in the Pentium II and III era Deskpros are that way, and probably until Compaq stopped including floppy drive support in their machines. I'm not sure exactly when the practice began.

    I'm not sure how it affects the terminating resistor that most 5.25" drives include. Normally you're supposed to install the terminating resistor when it's the last drive on the cable (normally the A: drive) or leave it off when it's on the middle of the cable (the B: drive). But with Compaq's strange cabling you may need to experiment with either including or leaving out the terminating resistor.

  8. #18

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    Ok after long hiatus I am back to beating my head against the wall with this system. So here is what I have found so far:

    1. The Motherboard and cable are fine. I connected a Teac FD-55gfr drive to the system and it works as expected. Install the drive after the twist and it over rides the BIOS makes the Teac "A" drive and is set to 1.2MB in BIOS automagically. Installing it after the twist sets it to B and 360KB. However, switching it back 1.2MB in the setup is fine. The system does not complain on POST and it boots/works fine (I was able to access both SD and HD disks on the Teac).

    2. Installing the drive on another computer also caused issues. This leads me to think that this could be a drive problem. So I got my hands on another drive for a whopping $7.99. That drive exhibits the same behavior. Interestingly this is not the first time the same exact issue has come up on the forums:

    Compaq slim floppy drive problem

    So unless all of these drives are failing world wide in the same way we are missing something. The closest thing I have found to a manual are the following thanks to, who else, Minus Zero Degrees :

    MF501B and MF503B and MF504B - Specifications and Application Notes

    MF501C and MF504C - Installation Guide

    Note that this drive is a MF504S. I am not sure what the S is supposed to signify. However, this PCB only has a SS and SB jumpers which are responsible for the drive speed (before anyone suggest it I have already switched the jumper with no change in behavior).

    Any suggestions? Any ideas? Any hope?
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  9. #19
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    Quote Originally Posted by Chuck(G) View Post
    Compaq could be strange wrt the floppy cabling. For example, on my Deskpro ENP, a flat cable from the mezzanine defines the "A" drive (it comes from the factory this way). If you want a B: drive, you use a cable with a twist to get it. In other words, the cable would be <motherboard>->(flat)->Drive A->(twist)->drive B. There is no "reverse the A and B drives" in the BIOS setup that I can recall.
    I may beg to differ, in my experience, all FDs should be jumpered Drive 1 (respond to DS1 not 0), the twist between connectors 2 and 1 swaps the DS1 and related signals to the DS0-related positions. So connector 2 (nearest the mobo) is the B: drive and connector 1 (at the other end) the A:.

  10. #20
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    I didn't say that the drive selects weren't the second (DS1 or DS2, depending on the manufacturer's naming) drive in the chain. After all, all the DSx pins wind up in the same place, right? The only difference is where that common point ends up on the drive connector.

    No, what I was saying was that a straight cable on a Compaq ENP to a drive jumpered for the second drive select will be addressed as BIOS unit 0--and that the similarly-configured second drive, placed after the twist, will be BIOS unit 1.

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