What Does A Twist In A 34-pin Data Cable Mean?

Question

Summarizing everything

tl;dr

The bulldoze before the twist volition be drive B while the one on the cease volition be A. This way, there is no need to "configure" the drives which drive (A or B) are they going to be and what they should heed to. They can be configured identically and the twist volition swap the controlling input for them.

Or quoting sawdust, from this comment:

The cable twist allows both floppy drives to exist configured identically (for drive selection) when installed (for manufacturing convenience), yet operationally, tin can be uniquely selected as either the first bulldoze or the 2d drive based on cablevision position.

Pins and cabling

The twisted pins are from pin 10 to pin sixteen.

Pin setup

Cabling setup

Explanation

Without the twist, we take to configure the drives and set them to be drive A for one and B for the other, because when the motherboard selects for example drive A, both drives would receive the select signal if they are both configured every bit a drive A. To avoid this, we should setup them by jumpers or by hard-wiring their role and then there would be a drive prepare to be drive A that would listen to signals on the select A wire, while the other drive would exist drive B that would listen to signals in select B.

This is totally achievable, simply we don't want to mess with the setting of the drives, just want to throw them into the PC case and plug in the cables.

Let'southward say both drives are hard-wired to be drive B. Now we don't have to setup them, but they both listened to the select B signal, while the motherboard would still want to send a indicate to select A to select drive A. Hither comes the twist! After the get-go drive, we twist the select cables so the bulldoze A (that is still a hard-wired drive B) volition listen to the select A controls, because we connected the select A pin to its select B pin (the only pin it listens to).

Now the drive earlier the twist will work every bit a drive B listening to select B signals, while the drive subsequently the twist will piece of work as drive A listening to select A signals. They're both difficult-wired bulldoze Bs that mind to their select B pivot, but for 1 drive we connected the select A pin to its select B so the motherboard can command it through the select A motorbus.

With hobbs's words hither:

The pins are in fact "Bulldoze Select A", "Drive Select B", "Motor Enable A", and "Motor Enable B". The twist swaps Drive Select A<->B (pins fourteen and 12, respectively) and Motor Enable A<->B (pins 10 and sixteen, respectively). All are outputs on the floppy controller and inputs on the drives.

and hither:

The residual of the pins (read and write information, stepper motor control, caput select, etc.) are bussed in the normal mode, which is why the bulldoze select pins are so disquisitional. A drive has to ignore all input and produce no output when it's not selected

While difficult-wired drives are normally a drive B, there is the take chances to exist a bulldoze A as said past Tonny here:

I in one case had a whole afternoon of amusement trying to figure out why a drive which came from a working arrangement wouldn't work in another figurer... It turned out to be hardwired for A and the original computer used a normal cable, but had the signals twisted on the motherboard itself!

Also, please note what Michael Hampton wrote here:

Certain non-PC-compatible systems (like the Radio Shack Color Computer) did actually utilize floppies without the cablevision twist, just required manually setting the jumpers, and could indeed use four drives at once. While this hack allows for the end user to not have to mess with jumpers, it also restricts the system to two floppy drives.