Floppy Diskdrive pinout

34 pin IDC male connector layout
34 pin IDC male connector at the motherboard & diskdrives


34 pin IDC female connector layout
34 pin IDC female connector

Controller pinout:

Pin Name Dir Description
2 /REDWC --> Density Select
4 n/c   Reserved
6 n/c   Reserved
8 /INDEX <-- Index
10 /MOTEA --> Motor Enable A
12 /DRVSB --> Drive Sel B
14 /DRVSA --> Drive Sel A
16 /MOTEB --> Motor Enable B
18 /DIR --> Direction
20 /STEP --> Step
22 /WDATE --> Write Data
24 /WGATE --> Floppy Write Enable
26 /TRK00 <-- Track 0
28 /WPT <-- Write Protect
30 /RDATA <-- Read Data
32 /SIDE1 --> Head Select
34 /DSKCHG <-- Disk Change/Ready

Floppy Diskdrive pinout (Shugart interface):

Pin Name Dir Description
2 /DCD --> Disk Change Detect
3 Key   no pin in this position
4 /DS3   Device Select 3. Not sure but Amiga 500s schematics
reveal that this signal might be used for motor control of internal DF1:
on the Amiga 2000
6 /INUSE   A common open-collector LED driver signal? I have never seen this signal used anywhere.
8 /INDEX <-- Index
10 /DS0 --> Device Select 0
12 /DS1 --> Drive Sel B
14 /DS2 --> Device Select 2
16 /MTRON --> Motor On
18 /DIR --> Direction
20 /STEP --> Step
22 /WDATE --> Write Data
24 /WGATE --> Floppy Write Enable
26 /TRK00 <-- Track 0
28 /WPT <-- Write Protect
30 /RDATA <-- Read Data
32 /SIDE1 --> Head Select
34 /RDY --> Drive Ready/Disk Changed

On a standard floppy drive there is absolutely no way to remap the
motor on signal to another pin with jumpers. Therefore to have
independent motor control for two drives the cable must provide this
remapping with the traditional seven-conductor twist. If you jumper a
floppy drive to work as drive A (unit 0) and connect it to an IBM PC
controller with a direct cable, the drive will be selected when the
controller tries to turn on the motor of drive A but nothing will happen
and the drives motor will rotate when drive Bs motor is turned on.

The original Shugart interface (from which the IBM PC floppy
interface is derived a long time ago) doesnt have separate motor on
signals for the floppy drives but it does have a total of four device
select lines. I have also seen floppy drives that wont turn on their
motors unless the according device select signal is driven low. My guess
is that this kind of drives strictly follow the original Shugart
standard.

Also many synthesizers that have floppy drives use the standard
Shugart interface pinout. This is why after replacing a faulty drive
many people ask around the Internet why their new floppy drive doesnt
work when connected to the synth but works fine on their PC.

The same thing affects the classic Amiga. It uses a very slightly
modified Shugart interface pinout at the motherboard (the other /MTRON
on pin 4) and a PC drive just doesnt work correctly unless the /DCD is
remapped to its original pin. The correctly mapped /DCD is enough for
AmigaOS but many trackloaders (X-Copy Pro for example) require the /RDY
signal which the drive should set low when the motor rotation has
stabilised. This signal does exist on most drives but at worst it
requires relocating a soldered SMD jumper on the circuit board.

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 how the connectors are used:

  Controller Connector: The
single connector on one end of the cable is meant to connect to the
floppy disk controller, either on a controller card or the motherboard.
  Drive A Connectors: The
pair of connectors (or single connector in the case of a
three-connector cable) at the opposite end of the cable is intended for
the A: floppy drive. This is explained in more detail below.
  Drive B Connectors: The
pair of connectors (or single connector in the case of a
three-connector cable) in the middle of the cable is intended for the B:
floppy drive.

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 more common pin header (IDC) connector is shown below.

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:.

Heres 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:

  Line 10 Line 12 Line 14 Line 16
Controller Signals Motor Enable A Drive Select B Drive Select A Motor Enable B
Drive Before the Twist Sees Motor Enable A Drive Select B Drive Select A Motor Enable B
Drive After the Twist Sees Motor Enable B Drive Select A Drive Select B Motor Enable A

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.

One might ask why the twist was needed. In short, 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, for two floppy
drives to be used, one had to be jumpered as A: and the other as B:.
With the twist, it was possible to leave them both jumpered as B:, and
whichever was after the twist will appear to the system as A: because
the control lines are inverted. Changing which drive is A: and which is
B: is as easy as switching the cable. In systems with only one floppy
drive, only the connector after the cable should be used. 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 pre-jumpered as B: drives so that they will
work with this setup.

If this whole idea sounds similar to the seldom-used cable select
protocol for IDE/ATA hard disks, thats because it is essentially the
same thing. 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.

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.

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.

On some mainboards pin 3 is used as the key (missing pin) and on some
pin 5 is used as the key pin, while a lot of mainboards dont have the
key pin removed at all. This can all cause problems when using cables
which have the key pin hole closed. As all odd pins are ground there are
no technical implications in modifying such cables by removing the key
pin closure by force.

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