Floppy disk drive interface

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Each generation of floppy disk drive (FDD) began with a variety of incompatible interfaces but soon evolved into one de facto standard interface for the generations of 8-inch FDDs, 5.25-inch FDDs and 3.5-inch FDDs.[1] For example, before adopting 3.5-inch FDD standards for interface, media and form factor there were drives and media proposed by Hitachi, Tabor, Sony, Tandon, Shugart and Canon.[2]


8 inch[edit]

The de facto standard 8 inch FDD interface is based upon the Shugart Associates models SA800/801[3] FDDs and models SA850/851[4] FDDs.[5]: 171 The signal interface uses a dual in-line 50-pin PCB edge connector which mates to a flat ribbon cable connector; separate connectors are provided for both AC and DC power.[3][4]

5.25 inch[edit]

Connectors for 5.25 inch or 3.5 inch FDD (Drive "A") at end of universal two drive FDD cable. Note twist in flat cable.

The de facto standard 5.25 inch FDD interface is based upon the Shugart Associates SA400[6] FDD.[5]: 169 The signal interface uses a dual in-line 34-pin PCB edge connector which mates to a flat ribbon cable connector; a separate connector is for DC power.[6] The 34-pin connector is similar in pinout to the standard 50-pin connector for 8 inch FDDs.

3.5 inch[edit]

The de facto standard for 3.5 inch drives uses a dual in-line pin style connector mating to a socket connector, collectively slightly smaller than the PCB edge pin connector and mating socket used for the 5¼ inch standard but with the same 34 pin definitions as the 5¼ inch standard.[7] A 'universal' cable would have four drive connectors, two for each size of FDD, although cables which have only two drive connectors are common. The cable is normally formed into a ribbon, and a twist located between the pairs of connectors for the drives (see image) is usually applied to the conductors for pins 10 to 16 inclusive. This allows two drives connected to the same cable to be addressed by the host controller. Only two drives may be connected to such a cable. If there are four drive connectors at least two must remain unused. A separate connector is provided for DC power.[1]

Signal and control interface[edit]

3.5-inch and 5.25-inch drives connect to the floppy controller using a 34-conductor flat ribbon cable for signal and control; a separate cable provides d.c. power. Most controllers support two floppy drives, so a cable could have 5.25-inch style connectors, 3.5-inch style connectors, or a combination. After IBM introduced the "twist" to floppy cables, and when both 5.25-inch and 3.5-inch drives were in common use, many cables had four connectors: one of each type before the twist, and one of each type after the twist. These cables still only supported two drives, one before and one after the twist, but they allowed using one cable for any combination of drives with differing connectors. This type of cable is called a universal cable.[8]

When multiple floppy disks are connected, many pins are shared, including the read and write data pins. As a result, early floppy drives required jumpers to be set on the drive to tell it which controller commands it should receive. When introducing the PC, IBM sliced the cable between the first and second drive, and twisted seven of the conductors, effectively flipping the four conductors which specifically addressed the first or second drive. (The remaining three were ground only, so were not affected by the twist.) As a result, all drives could have their jumpers set to be drive "B", but if they were connected after the twist, they would appear to the controller as drive "A". This eliminated the need to change selection jumpers in the drive, and eventually many floppy drives were manufactured without jumpers at all, instead being hardwired as drive "B". As the IBM PC created a market for clones and compatibles, many manufacturers adopted the same cable twist system, although jumpers may still be required on systems that are older, or not based on the IBM PC.[9]

The drive that is at furthest end of the cable additionally would have a terminating resistor installed to maintain signal quality.[10]

Floppy drive connector pinout (host controller as a reference)
Pin number Abbreviation Description Notes Type
2 DENSEL Density Select 1=Low/0=High The default use is 0 Output
4 RSVD Reserved No connection or connect to the ground
6 RSVD Reserved No connection or connect to the ground
8 INDEX# Index 0=Index Input
10 MOTEA# Motor A Enable 0=Motor Enable Drive 0 Output
12 DRVSB Drive Select B Output
14 DRVSA Drive Select A Output
16 MOTEB# Motor B Enable 0=Motor Enable Drive 1 Output
18 DIR# Direction Select Low Current/Direction in uPD765 controller Output
20 STEP# Head Step Fault Reset/Step in uPD765 controller Output
22 WDATA Write Data Output
24 WGATE# Floppy Write Enable 0=Write Gate Output
26 TRK0# Track 0 Fault/Track 0 in uPD765 controller Input
28 WPT# Write Protect 0=Write Protect Input
30 RDATA Read Data Input
32 HDSEL#/SIDE Head Select / Side select Two uses, see application or use for details.(Side select:1=Side 0/0=Side 1) Output
34 DSKCHG# Disk Change 1=Disk Change/0=Ready Input
3 RSVD Reserved No connection or connect to the ground
5 N/C No connection Pins usually do not exist here to prevent the male plug from being inserted in the opposite direction
Odd pins 1 thru 33 GND Ground Except for the 3rd and 5th pins Power

"#" indicates that the low electric level is effective (aka "active low").

Motor A,B is also known as Motor 0,1.

Since floppy disks are rarely used nowadays, "MOTEB#" and "DRVSB" pins are not connected in motherboards designed with floppy disk data interfaces, and only one floppy disk drive can be connected.

Floppy drive A/B twist pinout
Wire Controller Drive A Drive B Description
1-9 1-9 1-9 1-9 No Change
10 10 16 10 Motor Enable Drive 0/1
11 11 15 11 Ground, No Change
12 12 14 12 Drive Select 0/1
13 13 13 13 Ground, No Change
14 14 12 14 Drive Select 0/1
15 15 11 15 Ground, No Change
16 16 10 16 Motor Enable Drive 0/1
17-34 17-34 17-34 17-34 No Change

See also[edit]


  1. ^ a b Mueller, Scott (2006-03-24). "Floppy Disk Drives, Past and Present". Upgrading and Repairing PCs (17 ed.). Que Publishing. ISBN 0-7897-3404-4. ISBN 978-0-7897-3404-4 EAN 2147483647. Archived from the original on 2022-01-08. Retrieved 2022-01-01. […] all PC floppy disk drives are still based on (and mostly compatible with) the original Shugart designs, including the electrical and command interfaces. […] The standard interface that all PC floppy disk drives use is called the Shugart Associates SA400 interface. It was invented in the 1970s and is based on the NEC 765 controller chip. {{cite book}}: |website= ignored (help)
  2. ^ Abraham, Robert (January 1983). "Microfloppy Drives Achieve High Densities and Faster Data Access". Computer Technology Review. p. 239.
  3. ^ a b SA800/801 Diskette Storage Drive (PDF). OEM Manual. Sunnyvale, California, USA: Shugart. May 1980. P/N 50574-4. Archived (PDF) from the original on 2021-03-09. Retrieved 2022-01-02. (1+iv+40+1 pages)
  4. ^ a b "SA850/851 Bi-Compliant Double Sided Diskette Storage Drive" (PDF). OEM Manual. Sunnyvale, California, USA: Shugart. November 1980. P/N 39017-0. Archived (PDF) from the original on 2020-11-29. Retrieved 2022-01-02. (1+iv+50+1 pages)
  5. ^ a b Porter, James (February 1982). "Floppy-disk drives: a truly flexible industry standard". Mini-Micro Systems. Cahners Publishing Company. pp. 169, 171. pp. 169, 171: […] SA400, Industry standard for size and interface […] SA800, SA801, SA850, SA851, Industry standard for size and interface […]
  6. ^ a b SA400L Minifloppy Diskette Storage Drive (PDF). OEM Manual. Sunnyvale, California, USA: Shugart. November 1982 [1981]. P/N 39019-1. Archived (PDF) from the original on 2020-07-27. Retrieved 2022-01-02. (2+iv+29+1 pages)
  7. ^ Davis, Larry (2015-06-13). "Floppy Disk Drive Pinout". www.interbus.com. Archived from the original on 2022-01-07. Retrieved 2022-01-06.
  8. ^ Davis, Larry (2015-06-13). "Floppy Drive Pinout, Signal names, Pin out Description and Cable twist wiring". www.interfacebus.com. Retrieved 2019-01-29.
  9. ^ Farquhar, David "Dave" L. (2021-11-24). "Floppy drive pinout". The Silicon Underground. Archived from the original on 2022-01-08. Retrieved 2022-01-04. The pinouts for all these drives are all based on the original Shugart floppy drive [… of the SA800 Series], but many manufacturers changed them slightly to suit their own purposes.
  10. ^ Scott Mueller, Upgrading and Repairing PCs, Second Edition, Que, 1992, ISBN 0-88022-856-3,page 487

Further reading[edit]