PC floppy cable twists...

[Allison]

>
>Subject: PC floppy cable twists...
>   From: Jules Richardson <julesrichardsonuk at yahoo.co.uk>
>   Date: Thu, 20 Oct 2005 17:48:04 +0100
>     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>
>
>.... discussion about this on another list got me curious - what *was* 
>the point of that cable twist in a (IBM clone) PC floppy cable, when 
>every other system on the planet was using straight-through cables?
>
>1) Great, it means both drives in a system can be jumpered for the same 
>ID - but someone's still got to go in and jumper/modify the last drive 
>in the chain so that it's terminated, so it's not like the twist 
>eliminates messing around with jumpers.

Jumpers were not messed with.  Drive arrived, plugged in and go.  Cost 
of delivering service was by then high enough that giving the drive away
to avoid the call was actually becomming loss avoidence! 

>2) when the twist was introduced, there were presumably no clone 
>machines around (it was there from day 1 IIRC) - and wouldn't the 
>addition of a second floppy drive to an IBM machine have been a field 
>service call anyway? So it's not like it was the general public changing 
>jumpers, but a trained engineer...

There was no jumper change. All had pullups and they are sized to allow 
two in parallel without problems.

>3) IBM seemed to use a very small range of drives in the PC / XT / 286 
>days, so it's not like there'd be a million jumper combinations to 
>figure out. If a customer tried to add their own drive rather than 
>buying through IBM, surely IBM couldn't care less if they struggled to 
>figure the drive jumpers on their 'non-standard' unit out?

2 is greater than 1.  Cost to stock two is greater than one.

>It's got me curious as it seems like a hack that doesn't completely 
>solve any kind of problem whilst introducing a difference between IBM 
>and the rest of the industry.
>
>cheers
>
>Jules

You missed the most basic reason.  One part, one bin, one stocking number
and less standing stock.  Logistics of warehousing costs and space not
electrical design. 

Everyone seems to forget or even miss that not too long after the PC was
introduced and clones appeared the costs of producing, stocking and servicing
them were under great pressure.  Anything that cost, even pennies, could put
a vendor at risk.  Why did some vendors disappear?


Allison