20 Oct
2005
20 Oct
'05
9:48 a.m.
... 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.
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...
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?
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
20 Oct
20 Oct
10:18 a.m.
On 10/20/2005 at 5:48 PM Jules Richardson wrote:
... 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?
It's actually pretty simple--there's only a single "motor on" line for
the
standard floppy pinout. IBM wanted to control drive motors individually.
The only ways to do that are with individual drive cables or using the
"twist" (or some modification of it) so that the individual drive motor
enables are given different conductors in the cable.
Other systems using "straight through" cables switched all drive motors on
and off--and many consequently used drives with head-load solenoids. IBM's
solution let them economize a bit on power-supply design and use a cheaper
(and quieter) drive without the head-load mechanism. FWIW, IBM makes no
mention of terminator removal or addition and I don't even recall if the
standard 5.25" drive allowed one to play with termination.
On one of the ISA floppy/hard controllers--I think it was DTC--you could
add a third drive to the string using a special cable and a drive jumpered
for DS0. (I'd have to check my doc files to get the rest of the details on
the cable construction).
Cheers,
Chuck
11:18 a.m.
Chuck Guzis wrote:
On 10/20/2005 at 5:48 PM Jules Richardson wrote:
Brilliant - most informative, thanks :) (although I've seen mention of
the hack-like nature of the cable twist on this list before, I don't
remember anyone ever saying why it was done!)
That does beg the question of whether there's any other jumpering to
worry about when putting a PC-setup floppy onto a classic system (other
than getting the drive select jumpering right).
cheers
Jules
... 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?
It's actually pretty simple--there's only a single "motor on" line for
the
standard floppy pinout. IBM wanted to control drive motors individually.
The only ways to do that are with individual drive cables or using the
"twist" (or some modification of it) so that the individual drive motor
enables are given different conductors in the cable. 
1:53 p.m.
... 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
That was part of it. The jumper hlock on the original Tandon drives was
of those infernal things where you break the metal bars with a sharp
screwdriver.
in the chain so that it's terminated, so it's
not like the twist
eliminates messing around with jumpers.
Didn't it? OK, there was still the termination pack that had to be fitted
on the last drive on the cable (which was A:), but there were no jumpers
to fiddle with.
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...
Remember IBM did make the service manual (boardswapping) and TechRef
available to anyone. I think it was expected that some users would do
such upgrades themselves (but then again such users would have known how
to set jumpers).
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?
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.
You've missed the real reason...
The twist doesn't just swap over 2 of the the select lines, it also swaps
the motor-on line with one of the other, unused, select lines. This means
the motor-on lines for the 2 drives end up on different pins of the
controller connector, so the motors can be controlled seperately (and
independantly of drive select).
Now quite why you'd want to do this I don't know, but that, IMHO, is the
reason for the twist. It does something you can't do with link settings
alone.
-tony
3:12 p.m.
On Thu, 20 Oct 2005, Jules Richardson wrote:
... 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?
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...
IBM sold through Computerland, Businessland, and a few others.
They had to simplify it enough that Computerland "technicians"
could add a drive without damaging anything.
SOME Computerland technicians were competent.
Some others failed entry exams at Fry's.
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?
When somebody adds a B: drive, they could blame IBM when the A: drive
would no longer work.
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.
. . . and you were looking for a REASON?
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