Writing 8" floppies with SuperCard Pro

Jay Jaeger cube1 at charter.net
Wed Aug 12 08:26:49 CDT 2015

You and I were thinking of two different things:  I was thinking of the
reduced write current on the inside tracks, and you were thinking of
changing the bit timing.  On the latter point, I was incorrect.

For reduced write current, all the controller does is to tell the drive
whether or not it is on an inside track, the drive handles the rest.  I
note that the pin used for this varies from drive to drive.  On a
Shugart 850, this is on pin 2.  On a Siemens FDD 100-8, this is on pin
16, so this is something that the original correspondent should check
for each of his/her drives.

The Shugart 801 does not seem to have this feature.  Also, there is a
model of the FDD 100-8C, which counts the tracks ON ITS OWN, and DOES
handle this automatically, and that is where I got confused.  There are
also some 5" drives which do this, e.g. the Tandon 848, either using a
signal from the controller (on pin 2) or on their own.

Then, when this reduced write current signal is active-low, the drive
reduces the write current.  This is the "TRK 43" signal that the
correspondent actually referred to.  Most of my 8" floppy experience was
with the single-density FD1771, which does NOT change the bit timing,
and ONLY has this "TG43" signal.

But, you are also right in that the later controllers, e.g., the FD
1791, for example, also provide two additional signals, "EARLY" and
"LATE" for write precompensation.  As you point out, these signals do
not go to the drive, but instead are typically used inside the
controller to change the timing of the write pulses.

The correspondent should probably check with the SuperCard Pro folks to
make sure BOTH have been implemented.  It is quite possible neither have.

On 8/12/2015 4:05 AM, Christian Corti wrote:
> On Tue, 11 Aug 2015, Jay Jaeger wrote:
>> Aside from the write pre-comp thing which you seem to have in hand  (and
>> which is generally handled by the *drive*, not the controller, i.e., the
>> bit timing from the controller does not change, as far as I know), one
> Au contraire.
> Write precompensation is entirely handled by the controller. The drive
> has absolutely no idea of what it records or reads.
> (We're talking about floppy disk drives, not intelligent hard disks with
> integrated encoder/decoder and PLL like SMD or ESDI)
> Christian

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