Date   : Thu, 20 Oct 2005 17:22:39 +0100
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Dual Floppies On A Master

Richard Gellman wrote:
> Jules Richardson wrote:
> 
>> That's a strange one - I'm not quite sure why you'd get one drive in 
>> one situation and both drives in another.
>>
>> Definitely untwist the cable though first and then see what's what.
>>
> I concur with untwisting. The PC modification to the standard floppy 
> cable does more than just swap drive IDs (though I can't remember enough 
> of the shugart interface spec. to say what).

Well it'll tweak the motor enable lines too - but I recall that the 
twist does really strange things to the signal paths if you're using a 
system with more than two drives (rather, I think it makes it impossible 
to do so)

 > Use an untwisted cable and
> try to set the drive IDs to 0 and 1 (being PC floppies, they'll be A: 
> and B: which won't map directly to :0 and :1 or something equally 
> irritating done to make it harder for the unqualified to break something).

In PC-land they'll both be set as drive 1 (not drive 0) as it's the 
cable twist that makes a drive 1 look like a drive 0 to the system.

It's a nasty hack, with no sensible reason behind it (as back when the 
twist was introduced, drives of the time could be jumpered to do just 
about anything).

Indeed, it would have been introduced in the days when you weren't even 
allowed to add a second drive yourself, and an IBM field engineer would 
presumably come out and do the job (these are the days before clone PCs 
existed). Plus the drives were all the standard full-height ones, so 
it's not like there were jumper combinations on hundreds of different 
drives for them to learn beforehand.

Seems a strange thing to do, particularly as the end drive on the bus 
(drive A:) should be terminated and so technically should be set up 
differently to the first drive on the bus (drive B:)

(I can't remember if the IBM PC would work without drive termination or 
not - some systems are more picky than others, plus the cable length 
will matter)

cheers

Jules