The 9114B drive has a 34 pin dual row receptacle connector, doesn't it? Tony Duell and I were discussing the use of some of the odd numbered pins on the 34 pin data connector to carry power to the drive. Here's what I wrote about the way it was done on some Sony drives that Tandy used:
"You can see on the PCB where there are places for optional zero-ohm resistors which either connect all the odd pins (except pin 1) of the data connector to ground or connect one group of odd pins (3,5,7,9,11) to +5V and another group (29,31,33) to +12V (all the remaining odd pins except pin 1 [providing a logic signal] are directly connected to ground). When the drive has the option for power on the data cable, the 4 pin power connector is deleted. Tandy used a data cable that ran to the first FDD first (remember that the standard IBM "twisted" cable has the connector for the first drive at the far end - better for transmission line termination with removable 150 ohm resistors installed on the drive at the end of the cable, but later abandoned, with permanent 1K resistors on all drives, and a shorter cable used), then after that first data connector (or possibly after a second receptacle connector to allow a second 3.5" drive of the same type), there are punches in the ribbon cable to isolate those odd pins on additional devices from the +5V and +12V coming from the motherboard."
The even numbered pins should be the standard Shugart bus:
2 -
4 -
6 -
8 - Index Pulse from drive
10 - Drive Select 0
12 - Drive Select 1
14 - Drive Select 2
16 - Motor On signal to drive
18 - Head Carriage Direction signal to drive - I think high=in
20 - Head Carriage Step Pulse to drive
22 - Write Data to drive
24 - Wrote Enable to drive
26 - Track Zero signal from drive
28 - Write Protect signal from drive
30 - Read Data from drive
32 - Head Select signal to drive - high=head 1 (top surface)
34 -
IBM's floppy disk drive controller generates two Motor On signals and two Drive Select signals assigned like this at the controller end of the cable:
10 - Motor On A
12 - Drive Select B
14 - Drive Select A
16 - Motor On B
These four wires on the cable are split apart from the other wires and twisted 1/2 turn between the two drive connectors so the connector nearest to the controller has the untwisted signals and goes to drive B, and the connector at the other end of the cable from the controller has the twisted signals and goes to drive A. Both drives have their jumpers or switches set for Drive Select 1 (signal on pin 12 selects the drive, signal on pin 16 turns on the spindle motor).
The signals I have described above are the ones used in the PC which had drives that only worked at low density and didn't have a disk change signal. On the AT there are two new signals - a Density Select signal from the controller to the drive that tells the 1.2 MB 5.25" drive whether to treat its diskette as 360KB or 1.2MB media; the i.44 MB 3.5" drive only listens to the sensor that detects the density hole in the diskette shell. I think the density Select is on one of the low numbered even numbered lines. The AT drives also provide a Diskette Change signal from the drive that I think is on pin 34.
Also, there is a fourth Driive Select signal.