MikeS
Veteran Member
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I'll try: Since you can have several drives in parallel, you can't have one drive pulling a signal low while another pulls the same line high, so the drivers for these lines are what's called "open collector". That means that any drive (the selected one) can only pull a signal line low like a single pole switch; all the unselected drives are like open switches, i.e. disconnected, and a resistor is connected to +5V to pull the line high when the selected drive "opens the switch" to send a high level (or when all drives are inactive). To avoid drawing too much current and also to avoid reflections when the signal gets to the end of the cable, only one drive would have these resistors on each line, and it should be on the drive at the end of the cable (usually around 100-330 Ohms).
If more than one drive has them installed, it requires more current to "ground" the line and it may not go low enough; if none are installed, there is nothing to pull the line high except the high impedance of the line receiver and it may not go high enough.
On the older drives these resistors were removable packs that look like ICs (although some, and many hard disks, used SIP packs which had all the pins in one row), and you removed them from all drives except the last one. Later models had permanently installed resistors and a jumper enabled or disabled them. On most modern drives they are permanently installed and enabled, but a higher resistance with different driver and receiver chips to match.
So, to make a long story short, if it's a fairly old drive you should probably have an IC socket with something in it that looks similar to but not exactly like an IC, and with numbers like 330 or 102 instead of a normal IC number, especially if there's only one drive connected.
Although it doesn't apply to most 360K drives, pin 34 was another thing that IBM did differently from everyone else at that time (along with the twisted cable); when it was used at all, it was usually a signal that told the controller that the drive was ready for use (disk in and spinning), but IBM decided to use it to indicate that the drive door had been opened (and tell the controller that the disk might have been changed).
Most drives that had this feature had a jumper labelled RY/DC or similar (Ready/DiskChg).
3.5" drives used it somewhat differently again...
Probably doesn't have anything to do with your problem, but worth checking. I'd try that drive in another computer if you have one that'll accept it.
Whew!
Good luck!
m
