Replacing drives on classic systems was Wild-hair floppy drive
ard at p850ug1.demon.co.uk
Tue Jun 14 19:00:28 CDT 2005
> This issue only affects 3.5" drives which no longer use the same termination
> (but is compatible with).
I am pretty sure I've seen some recent-ish 5.25" drives that have the
same pull-ups (not termination, strictly) as the PC-type 3.5" drives.
> I'll have to look up the technical details (hopefully someone will speak up
> first) but basically the floppy drivers on PC's have gone from open
> collector TTL to CMOS changing the characteristics of the line drivers.
Termination is related to the cable type, and not the drivers!. You
should terminate the cable with its characteristic impedance at the far
end (from the driver), this was typically a 150 Ohm resistor. It also
acted as a pull up for the open-collector TTL drivers.
Problem was, users couldn't be relied upon to put a termination resistor
pack in the last drive on the cable. So later drives (including most 3.5"
ones) just have 1k pull up resistors on all the lines. These are not
terminators, they do not remove reflections on the cable. It seems to
work OK, but it's a kludge...
It probably is OK if you have the drives in the same box as the
controller, and thus a fairly short cable. I wouldn't want to use it on a
couple of metres of cable, though. When I put the external 3.5" drives on
my XT, I desoldered the (surface mount) resistors from the drives and
made up a terminator to plug into the end of the cable. It may have
worked without doing that, but I don't like risking my data.
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