S100 Floppy Controller Question

M H Stein dm561 at torfree.net
Mon Nov 12 01:39:06 CST 2007

-------------Original Message:
Date: Sun, 11 Nov 2007 19:32:38 -0500
From: "Richard A. Cini" <rcini at optonline.net>
Subject: Re: S100 Floppy Controller Question

Yes, I understand the 5-for-8 substitution. One of the challenges I'm having
is finding a suitable disk image to try. If I get an 8" image I plan on
jamming it onto a 5.25" HD disk and giving that a whirl.

I made the "8"-only" distinction because one of the boards I have (the
Versafloppy) has both connectors. It just saves a little time in not having
to make a cable.

This is quite possibly not relevant to _your_ controller, but FWIW my
Cromemcos have both the DD and the HD drives on the same cable,
and the 50 pin connector is only used for the 'real' 8" drive.

Although some of the signals have separate drivers/receivers, the
5 1/4 and 8" drives are effectively on the same bus; the only difference
is that the 8" drives supply (and the FDC expects) a /READY signal on 
pin 22 (?) whereas the TM-100s did not supply that signal and the FDC 
does not look for it on the 5 1/4" cable; I suspect that's how the system 
distinguishes between the drives in order to select the correct speed.

Modern HD drives _do_ supply a /READY signal on pin 34, although you'll
probably have to change a jumper since PCs use it for Disk Change; the
jumpers are in fact often labelled DC and RDY.

Therefore all that is required is a jumper on the FDC from pin 34 of the 5"
connector to pin 22 of the 8" (and of course any DD 5 1/4 drives must 
have their /READY signal disabled if it's active).

The only other FDC I have any experience with is a Micropolis, and it
_does_ use the /READY signal from its DD drives (although it looks for
it on pin 6, which is normally DS3 or 4), so this wouldn't work.


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