Replacing drives on classic systems was Wild-hair floppy drive

[Tony Duell]

> 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.

-tony