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Floppy Interface Cable
The floppy disk interface uses what is considered, by most people, a truly strange cable. It is similar to the standard IDE cable in that it is usually a flat, gray ribbon cable. It is unusual in terms of the number of connectors it has and how it is used to configure the setup of the floppy disks in the system.
The floppy cable has 34 wires. There are normally five connectors on the floppy interface cable, although sometimes there are only three. These are grouped into three "sets"; a single connector plus two pairs of two each (for a standard, five-connector cable) or three single connectors. This is what they are used for:
The reason that the standard cable uses pairs of connectors for the drives is for compatibility with different types of drives. 3.5" drives generally use a pin header connector, while 5.25" drives use a card edge connector. Therefore, each position, A and B, has two connectors so that the correct one is available for whatever type of floppy drive being used. Only one of the two connectors in the pair should be used (they're too close together to use both in most cases anyway). The three-connector cables are found either in very old systems or in ones where the manufacturer was trying to save a few pennies. They reduce the flexibility of the setup; fortunately these cables can be replaced directly by the five-connector type if necessary.
You will also notice that there is an odd "twist" in the floppy cable, located between the two pairs of connectors intended for the floppy drives. Despite the fact that this appears to be a "hack" (well, it really is a hack), this is in fact the correct construction of a standard floppy interface cable. There are some cables that do not have the twist, and it is these that are actually non-standard! What the twist does it to change the connection of the drive on the far end of the twist so that it is different than the drive before the twist. This is done to cause the drive at the end of the cable to appear as A: to the system and the one in the middle to be as B:.
Here's how it works in detail. Traditionally, floppy drives used a drive select (DS) jumper to configure the drive as either A: or B: in the system. Then, special signals were used on the floppy interface to tell the two drives in the system which one the controller was trying to talk to at any given time. The wires that are cross-connected via the twist are signals 10 to 16 (seven wires). Of these, 11, 13, and 15 are grounds and carry no signal, so there are really four signals that are inverted by the twist. The four signals that are inverted are exactly the ones that control drive selection on the interface. Here is what happens when the twisted cable is used:
Since the signals are inverted, the drive after the twist responds to commands backwards from the way it should; if it has its drive select jumpers set so that it is an A: device, it responds to B: commands, and vice-versa.
Why on earth would anyone have wanted to do this? Basically, because it was a big time-saver during setup back in the days when it was quite common to find two floppy drives in a machine. Without the twist, if you wanted to use two floppy drives one had to be jumpered as A: and the other as B:. With the twist, you just leave them both jumpered as B:, and whichever one you put after the twist will appear to the system as A: because the control lines are inverted. If you want to change the setup so that the other drive is A: instead, you just switch the cable. If you only want one drive, you only use the connector after the twist. Large manufacturers, therefore, could arrange to have all of their floppy disks configured the same way without having to pull jumpers as the PC was assembled.
In order for this system to work, both drives must be jumpered as B: drives. Since the floppy cable with the twist is standard, this jumpering scheme has become the standard as well. Virtually all floppy disks that you purchase come prejumpered as B: drives so that they will work with this setup.
If this whole setup sounds like it is the same thing as the cable select protocol for IDE/ATA hard disks, that's because it is basically the same idea. IDE/ATA hard disks require you to change the master/slave jumpers in a similar manner, and cable select was invented to do away with this. The difference is, as usual, just one of inertia and history; the floppy drive system is the standard while cable select never caught on for hard disks.
Tip: Some newer BIOSes have
taken things a step further. They include a BIOS
parameter that will invert the A: and B: signals within the controller itself. When
enabled, this lets you reverse whichever drive is A: with the one that is B:, without
requiring you to even open the case. Note however that this is not compatible with all
operating systems: in particular, both Windows NT and Linux can malfunction with this swap
feature set, which can cause serious problems when trying to install the operating system.
The reason this happens is that the swap setting only affects the way the BIOS handles the
floppy drive, and confuses operating systems that go directly to the hardware.
Note: Apparently, there is yet
another floppy cable variant out there, that is used by some manufacturers. In this setup,
there are actually two twists in the floppy cable. The drive placed after the
first twist, in the middle of the cable, is A:, much as it is with the standard one-twist
cable. The drive placed after the second twist is B:. The second twist
"reverses" the effect of the first one and makes the connector at the end of the
cable operate the same way a drive that appears before the twist in a regular cable does.
I have no idea what the benefits of using a cable like this are over a standard floppy