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View Full Version : Jury rigging a floppy drive terminator



klipty
May 10th, 2017, 04:50 PM
I have been working on restoring an old IBM PC, but have run into an issue. It has two floppy drives; the A: drive is a half height unit, and the B: drive is one of the original Tandons. The A: drive had issues with it's lever, and wouldn't work. I unplugged that drive and swapped the connector, making what was the B: drive the A: drive of a one-drive system. However, the terminating resistor on the half-height unit is jumper based, while the full-height uses an IC shaped one. This means I can't simply swap the terminators.

This brings me to my question: since the terminator is just a specific set of resistors, is it possible just to use a set of axial lead resistors in its place? It seems to me that they would function about the same, but I don't know the ohmage, how they should connect through the circuit, etc. Or would this not work, and I'm oversimplifying too much? I would love any answers you have. Thanks!

kgober
May 11th, 2017, 11:04 AM
I think the floppy cable is short enough, and the data rate is slow enough, that it won't matter that the terminator is not actually at the very end of the cable. Floppy termination isn't as critical as, say, SCSI bus termination.

If you're really concerned, I'd try leaving the drives at their original positions (terminated drive at the end of the cable) and use the Drive Select jumpers instead to change which drive responds to what. This will leave you with drives that are improperly jumpered for any other PC, but they should work for you and they'll be properly terminated.

-ken

Chuck(G)
May 11th, 2017, 12:27 PM
"terminator" is a bit of misnomer for the floppy drive variety. Floppy interface lines are driven by open-collector drivers; that is, there's no connection within the driver to Vcc (+5). So you need a pullup on each driven line. For the old 8" and 5.25" drives, the driver was usually a number 7438s and the pullup was 150 ohms. However, that was the spec suitable for a 3 meter cable. You don't have anything like that in a PC, so a much higher resistance pullup can be used to save on power. Since PC floppy cables are so short < 1 m usually, even a 1K pullup works--and this is what 3.5" drives use--on each drive.

So, pick a value, from 150 ohms to 1 or 2 Kohms. If what you need is a DIP terminator, you can get DIP headers and solder the individual resistors in. If you have a SIP terminator, you can still find SIP resistor packs to fill the need. Ideally, the terminator pack should be on the most distant drive from the controller, to minimize reflections. In practice, however, it probably doesn't matter where the thing is on a PC.