Unexpected floppy behavior...

[Eric Smith]

Tony wrote about floppy drives:
> The correct way to drive the signal lines (both on the drives and on the
> controller) is with open-collector drivers, pulled up/terminated by 150
> ohm resistors to +5V at the far end of the cable (outputs from the
> controller), or at the controller (outputs from the drive).

Note that one is supposed to use 48 mA drives.  Suggested drivers are
the 7438 quad 2-input NAND buffer, or the 7406 or 7407 hex inverting
or non-inverting buffer.  The 74LSxx, 74HCxx, 74HCTxx, etc.
"equivalents" are NOT acceptable, as they have significantly lower
rated drive current.  Fortunately the 7406, 7407, and 7438 are still
being manufactured, even though many other "plain" TTL parts have
been discontinued.

It is recommended to use 7414 schmitt-trigger inverters or equivalent
as the receivers.  For this, a 74LS14 is acceptable.  Other 74xx14
parts will probably work, though most of them have less hysteresis
than the 7414 and 74LS14 so they may be less tolerant of noise.

> The use of open-cpllector drives means it should be safe to power down
> individual drives without causing signals to be asserted.

Provided that the drive uses interface circuits that present a high
impedance when powered down.  Not all drives do that, so it is not
generally recommended to power down some drives on the bus while using
others.

Eric