These are the drive control signals for each type of disk drive:

Signal	Function	     Unidisk	//c	//gs	Disk ][
GND	ground               1-4	1-4	1-3	1,3,5,7
+5V	+5 V     	     6,16	6	6	11,12
-12V	-12 V    	     5		5	5	9
+12V	+12 V   	     7,8	7,8	7,8	13,15,17,19
PH 0-3	stepper motor phases 11-14	11-14	11-14	2,4,6,8
/DRVEN	drive enable	     17 (9)	17	17 (9)	14
/WREQ	write request	     15		15	15	10
/WRPROT	write protected      10		10	10	20
WRDATA	write data	     19		19	19	18
RDDATA	read data	     18		18	18	16

The UniDisk uses pin 9 to select the second drive.  Inside each UniDisk,
the signal from pin 9 at the computer is connected to pin 17 of the
daisy- chain drive connector.  When the computer selects drive 2 by
activating pin 9, the first drive passes this through and the second
drive sees its enable signal on pin 17.  Thus all drives are identical.

The Disk ][ controller has two drive connectors, and the same pin (14)
is used on each connector to select the appropriate drive.  This signal
is the only difference between the connectors - all other signals are
connected in parallel.

Controllers with IWM chips can use the step motor phase outputs PH0..3
for special purposes, this is done when controlling Twiggies or 3.5"
drives. The Disk II controller is a subset of the IWM functions, that
means every IWM chip can emulate a Disk II controller card.


Drive connector numbering: 

D19:
1    2    ...   9    10
  11   12   ...   19

20 pin header (Disk II)
2    4   ...  20
1    3   ...  19


Patrick