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Date   : Sat, 18 Nov 2006 15:05:31 +0000
From   : pete@... (Pete Turnbull)
Subject: Unusual piece of equipment available.

Jonathan Graham Harston wrote:
> Richard Gellman <splodge@...> wrote:

>> What we have on offer here is a floppy disc drive cable (of the BBC
>> Micro variety) with one edge connector on the end, but fitted with a
>> switch to convert an 80-track drive into a 40/80 switchable.
>  
> How? There are no connections on the disk drive connector to
> control the number of tracks or the stepping rate. Some disk
> drives have links or pads on them that a switch can be wired to,
> but that is on the PCB, not on the drive connector.

I believe I have the (possibly dubious) distinction of having designed 
and sold the first 48/80 add-on switch for the BBC Micro in early 1983, 
so I can tell you how it works.  Of course it would have worked for 
other systems too, but the Beeb was the one mostly likely to benefit 
from a 40/80 drive setup.

It has a little logic circuit to generate two step pulses from each one 
sent by the controller.  A 74LS123 monostable is triggered by the 
incoming step pulse, and at the end of the monostable period (about 
3ms), its falling edge triggers a second monostable which generates the 
extra step pulse.  The original step signal and the extra one are gated 
together in a 7438 open-collector buffer and sent to the drive.  A 
simple switch enables/disables the second monostable.

It actually needs to be a little more complicated than that, in reality.

Because the circuit might be switched from 80-track (single-stepping, 
with the extra step pulses disabled) to 40-track (double-stepping) while 
the drive was on an odd-numbered track, the DIR(ection) signal and TRK0 
signals are also gated onto the circuit to ensure that an extra pulse is 
never generated while the drive is stepping back to track zero.  This 
prevents damage to drives that don't disable their actuator -- like most 
5.25" drives.  Without this, some drives are apt to go beyond track 
zero, bash the actuator against the stop, and repeat ad nauseam because 
the FDC doesn't get a track zero indication (the drive now being at 
track minus-one) and retries.  Not good for head alignment.  At least 
one commercial manufacturer (Canon) didn't get this right on their 
switchable drives, and they suffer as a result.

Secondly, the Drive 0 signal is gated with the pulse generator, so 
normally only Drive 0 is ever double-stepped.  It would be easy to 
modify so either drive select could enable the circuit, with a switch 
for each, but my board allowed Drive 1 to be switched between the second 
drive in a dual-drive box, and a separate genuine 40-track drive 
connected to an extra connector provided on the circuit board.  This was 
to avoid the problems that can arise if a disk originally 
formatted/written on a real 40-track is (re)written on a double-stepped 
80-track drive, which typically doesn't completely erase the old data.

The last complication is that in those days, some drives were too slow 
to work with intervals between the step pulses of as little as 3ms. 
Fortunately almost all 80-track drives from reputable manufacturers were 
specced for 3ms pulse intervals, but it was possible to change the 
timing on my board (by soldering a different resistor) even for 6ms 
drives -- but then you'd have to ensure the controller never sent step 
pulses at less than 12ms intervals.  The default on a Beeb is 20ms 
intervals but that's rather slow and lots of people changed it (two 
links on the keyboard do that).

I can't remember how many of these I sold but it was certainly less than 
200 because I still have some of the second batch of unpopulated PCBs 
somewhere.  Mitsubishi (IIRC) brought out a smaller version in 1985 or 
so, and in about the same time frame some drives were available that 
were inherently switchable.  Then, of course, the problem went away, at 
least for new purchasers, when the Master Series came out.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York
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