3.5" and 5.25" Drive cable pinouts

Much of this information comes from page 90 of Open-Apple Volume 1, number 11 
(1985).

The following table lists all of the drive control signals for each type of 
controller card/disk port.

signal       function             Unidisk   IIc       IIgs      Disk ][

GND      ground reference         1-4       1-4       1-3       1,3,5,7
-12V     -12 volts DC             5         5         5         9
+5V      +5 volts DC              6,16      6         6         11,12
+12V     +12 volts DC             7,8       7,8       7,8       13,15,17,19
WRPROT   write protect            10        10        10        20
PH 0-3   stepper motor phases     11-14     11-14     11-14     2,4,6,8
WREQ     write request            15        15        15        10
DRVEN    drive enable             17 (9)    17        17 (9)    14
RDDATA   read data                18        18        18        16
WRDATA   write data               19        19        19        18
EXTINT   external interrupt       -         9         -         -
3.5DISK  Apple 3.5 drive enable   -         -         4         -
HDSEL    3.5" drive head select   -         -         16        -
not connected                     -         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.

Despite the IIgs having special functions for pins 4 and 16, they may be 
ignored when dealing with 5.25" drives, and treated as a UniDisk controller 
(i.e. connect pin 4 to ground, and pin 16 to +5V).  The Apple 3.5 drive 
disconnects these signals internally, so that they will not interfere with 
its operation.

The UniDisk, IIc external drive and equivalents use a DB-19 connector, in 
which the pins are numbered along the connector, i.e.

 -------------------------------------------------
\ 1    2    3    4    5    6    7    8    9    10 /
 \  11   12   13   14   15   16   17   18   19   /
   ---------------------------------------------

The Disk ][ uses an IDC-20 (20 pin insulation displacement connector) in 
which the pins are numbered in columns, i.e.
--------------------------------------------------------------------------------
+-------------------------------------------------+
| 2    4    6    8    10   12   14   16   18   20 |
| 1    3    5    7    9    11   13   15   17   19 |
+-------------------------------------------------+

The above numbering is from the back of the plug (where the ribbon cable 
connects to the plug).  If you are looking at the front of the plug (the 
socket side), reverse the rows in the above diagram (top row is 1, 3, ...)

The pin numbering of the DB-19 connector does NOT correspond to the wire 
numbers in the cable.  Pin 1 of the cable goes to pin 1 of the connector, 
but pin 2 of the cable goes to pin 11 of the connector.