Re: Did Commodore cheat with the quad density floppies?

----- Original Message ----- 
From: "smf" <smf@null.net>
To: <cbm-hackers@musoftware.de>
Sent: Saturday, January 05, 2019 6:00 PM
Subject: Re: Did Commodore cheat with the quad density floppies?


> On 05/01/2019 06:50, Mike Stein wrote:
>> A normal daisy chain cable directly connects all lines, including  the motor on/off line, of the two drives together in parallel; how could a jumper anywhere turn on one motor while leaving the other turned off?
> 
> There are two motor control lines and two drive select lines on the PC 
> end, the twist swaps pins 10-16 so the floppy drive gets them on the 
> same pins.
> 
> http://old.pinouts.ru/HD/InternalDisk_pinout.shtml
> 
> If you use jumpers on the floppy drive that tell it which of the two 
> sets to use, then you don't need a twisted cable.
> 
> I prefer the twist, but lots of systems used jumpers on the drive instead.
> 

I have never seen a standard drive that has a jumper to use any pin other than 16 to turn on the motor. Systems that used drives like a TM100 on an untwisted daisy chain cable would have to run both motors at the same time while using the Shugart standard for DS0 to DS3.

On a PC Drive B is connected normally, with DS1 on pin 12 and Motor On on pin 16. Drive A also uses DS1 on pin 12 and Motor On on pin 16, but the twist actually connects pin 12 to pin 14 on the controller (DS2 in a non-PC controller) and the Motor On pin 16 to pin 10 (DS0 in the non-PC world). This only works because the 'standard' happens to have the three DS lines adjacent to the Motor On line.

If you used an untwisted cable on a PC you could set the jumpers to use DS1 and DS2 but the motor would only run when using DS1 unless you modified the drive or the cable.

The external connector uses the same twist but the controller uses Drives C and D instead of A and B (and a 37 pin D-sub connector).