Date   : Sun, 12 Nov 2006 11:35:19 -0000
From   : jason.watton@... (Jason Watton)
Subject: 5 1/4" disc on PC

There's a bit of overlap here from another thread(!)...

> ... For
> the 1770 the values 0, 1, 2 and 3 mean respectively 6, 12, 20 and 30 msec,
> whereas on the 1770 the same numbers mean 6, 12, 2 and 3 msec. The common
> value for BBC drives is 6 msec, which requires the same value to be 
> written
> in either case, though some 40-track drives might require 12 msec, but 
> again
> this is common.)

Standard PC timings for 5.25" 1.2Mb 80-track HD drive:

SRT = 0xD

This means a step rate of 3.0ms or 5.01ms is being used (depending on the 
clock (data) rate the FDC is set to).

The corresponding slowest possible step rates on the PC would be 16ms and 
26.7ms...

> On the PC the same is set in the BIOS by the top 4 bits of the first byte 
> of
> the disk-base table for the drive. With the drive in use this will be
> pointed to by vector 1Eh (at location 0000:0078). If you want to try a
> different value, then set up your own disk-base table (11 bytes), point 
> this
> vector to it, and call INT 13h with AH=0 and the drive number in DL.


**Note to other thread** This is where the theory of people using the BIOS 
came from...

>> Put OmniFlop to use....
>
> I would certainly agree with this.

Ask for a version with "slowest possible step rates for 1.2MB 5.25" drive" 
if you want to try... can't hurt...

(rewind)
>> As part of my recent efforts to get software accross to my master and
>> beebs I tried connecting a 5 1/4" disc to my PC - this did not work:


Have you any evidence that the drive (stepper motor) isn't simply 
knackered?(!)

J.