View source for Floppy disk drive interface

{{Short description|Computer storage bus}}
{{Redirect|Shugart bus|the SASI see|Shugart Associates System Interface}}
{{Use dmy dates|date=December 2021|cs1-dates=y}}
{{Use list-defined references|date=January 2022}}
Each generation of [[floppy disk]] drive (FDD) began with a variety of incompatible [[Electrical connector|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.<ref name="Scott_2006"/>  For example, before adopting 3.5-inch FDD standards for interface, media and form factor there were drives and media proposed by [[Hitachi]], [[Tabor Corporation|Tabor]], [[Sony]], [[Tandon Corporation|Tandon]], [[Shugart Associates|Shugart]] and [[Canon Inc.|Canon]].<ref name="Abraham_1983"/>

== Sizes ==

=== 8 inch ===
The de facto standard 8 inch FDD interface is based upon the Shugart Associates models SA800/801<ref name="OEM_SA800"/> FDDs and models SA850/851<ref name="OEM_SA850"/> FDDs.<ref name="Porter_1982"/>{{rp|page=171|quote=SA800, SA801, SA850, SA851, Industry standard for size and interface}} The signal interface uses a 50-pin PCB [[edge connector]] which mates to a flat ribbon cable connector. Separate connectors are provided for both [[Alternate current|AC]] and [[Direct current|DC]] power, as many 8 inch drives used AC spindle motors.<ref name="OEM_SA800"/><ref name="OEM_SA850"/>

=== 5.25 inch ===
[[File:Floppy_buskabel.jpg|thumb|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<ref name="OEM_400"/> FDD.<ref name="Porter_1982"/>{{rp|page=169|quote=SA400, Industry standard for size and interface}} The signal interface uses a 34-pin PCB edge connector which mates to a flat ribbon cable connector. DC power is provided on a separate connector.<ref name="OEM_400"/>  The 34-pin connector is similar in pinout to the standard 50-pin connector for 8 inch FDDs.{{clear}}

=== 3.5 inch ===
[[File:3.5-inch_floppy_disk_drive_(3).JPG|thumb|34-pin data connector (right side) on the back of a 3.5" floppy drive. It looks similar to the [[Parallel ATA]] connector.]]
The de facto standard for 3.5 inch drives uses a 34-pin [[Pin header|header]] mating to an [[IDC (electrical connector)|IDC]] [[Berg 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.<ref name="Davis_2015"/> A separate connector is provided for DC power.<ref name="Scott_2006" /> 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 a [[ribbon cable]]. For IBM-compatible floppy controllers, a twist in the cable reverses the order of conductors 10 through 16 for the second connector.  This allows two drives connected to the same cable to be addressed by the host controller without having to select drive assignments with [[Jumper (computing)|jumpers]] on the drives themselves.  Only two drives may be connected to such a cable.  If there are four drive connectors at least two must remain unused.{{clear}}

== Signal and control interface ==
3.5-inch and 5.25-inch drives connect to the [[Floppy-disk controller|floppy controller]] using a 34-conductor flat ribbon cable for signal and control. Most controllers support two floppy drives, although the Shugart standard supports up to four drives attached to a single controller. 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.<ref>{{cite web |last1=Davis |first1=Larry |date=June 13, 2015 |title=Floppy Drive Pinout, Signal names, Pin out Description and Cable twist wiring |url=http://www.interfacebus.com/PC_Floppy_Drive_PinOut.html |access-date=January 29, 2019 |website=www.interfacebus.com}}</ref>

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.<ref name="Farquhar_2021"/>

The drive that is at furthest end of the cable additionally would have a [[terminating resistor]] installed to maintain signal quality.<ref>Scott Mueller, ''Upgrading and Repairing PCs, Second Edition'', Que, 1992, {{ISBN|0-88022-856-3}},page 487</ref>

The following explanation of pinout is for reference only.

{| class="wikitable" style="border:1; display:inline-table"
|+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.{{huh|date=January 2025}} (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.

{| class="wikitable" style="border:1; display:inline-table"
|+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==
* [[Floppy disk controller]]
*[[Berg connector|Floppy drive power connector]]

==References==
{{reflist|refs=
<ref name="Scott_2006">{{cite book |title=Upgrading and Repairing PCs |chapter=Floppy Disk Drives, Past and Present |author-last=Mueller |author-first=Scott |via=flylib.com |edition=17 |date=2006-03-24 |publisher=[[Que Publishing]] |isbn=0-7897-3404-4 |id={{ISBN|978-0-7897-3404-4}} {{EAN|2147483647}} |url=https://flylib.com/books/en/4.57.1.105/1/ |access-date=2022-01-01 |url-status=live |archive-url=https://web.archive.org/web/20220108230635/https://flylib.com/books/en/4.57.1.105/1/ |archive-date=2022-01-08 |quote-page= |quote=[…] all PC floppy disk drives are still based on (and mostly compatible with) the original [[Shugart Associates|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.}}</ref>
<ref name="Abraham_1983">{{cite magazine |title=Microfloppy Drives Achieve High Densities and Faster Data Access |author-last=Abraham |author-first=Robert |date=January 1983 |magazine=Computer Technology Review |volume= |number= |publisher= |issn= |page=239}}</ref>
<ref name="OEM_400">{{cite book |title=SA400L Minifloppy Diskette Storage Drive |series=OEM Manual |date=November 1982 |orig-date=1981 |publisher=[[Shugart Associates|Shugart]] |publication-place=Sunnyvale, California, USA |id=P/N 39019-1 |url=http://www.bitsavers.org/pdf/shugart/SA4xx/39019-1_SA400L_OEM_Manual_Nov82.pdf |access-date=2022-01-02 |url-status=live |archive-url=https://web.archive.org/web/20200727174108/http://bitsavers.org/pdf/shugart/SA4xx/39019-1_SA400L_OEM_Manual_Nov82.pdf |archive-date=2020-07-27}} (2+iv+29+1 pages)</ref>
<ref name="OEM_SA800">{{cite book |title=SA800/801 Diskette Storage Drive |series=OEM Manual |date=May 1980 |publisher=[[Shugart Associates|Shugart]] |publication-place=Sunnyvale, California, USA |id=P/N 50574-4 |url=http://www.bitsavers.org/pdf/shugart/SA8xx/50574-4_SA800_OEM_May80.pdf |access-date=2022-01-02 |url-status=live |archive-url=https://web.archive.org/web/20210309183338/http://bitsavers.org/pdf/shugart/SA8xx/50574-4_SA800_OEM_May80.pdf |archive-date=2021-03-09}} (1+iv+40+1 pages)</ref>
<ref name="OEM_SA850">{{cite web |title=SA850/851 Bi-Compliant Double Sided Diskette Storage Drive |series=OEM Manual |date=November 1980 |publisher=[[Shugart Associates|Shugart]] |publication-place=Sunnyvale, California, USA |id=P/N 39017-0 |url=http://www.bitsavers.org/pdf/shugart/SA8xx/39017-0_SA850_851_OEM_Manual_Nov80.pdf |access-date=2022-01-02 |url-status=live |archive-url=https://web.archive.org/web/20201129225035/http://bitsavers.org/pdf/shugart/SA8xx/39017-0_SA850_851_OEM_Manual_Nov80.pdf |archive-date=2020-11-29}} (1+iv+50+1 pages)</ref>
<ref name="Porter_1982">{{cite magazine |author-last=Porter |author-first=James |date=February 1982 |title=Floppy-disk drives: a truly flexible industry standard
|magazine=[[Mini-Micro Systems (magazine)|Mini-Micro Systems]] |volume= |number= |publisher=[[Cahners Publishing Company]] |issn= |pages=169, 171 |quote-pages=169, 171 |quote=[…] SA400, Industry standard for size and interface […] SA800, SA801, SA850, SA851, Industry standard for size and interface […]}}</ref>
<ref name="Farquhar_2021">{{cite web |title=Floppy drive pinout |author-last=Farquhar |author-first=David "Dave" L. |date=2021-11-24 |work=The Silicon Underground |url=https://dfarq.homeip.net/floppy-drive-pinout/ |access-date=2022-01-04 |url-status=live |archive-url=https://web.archive.org/web/20220108230157/https://dfarq.homeip.net/floppy-drive-pinout/ |archive-date=2022-01-08 |quote=The pinouts for all these drives are all based on the original [[Shugart Associates|Shugart]] floppy drive [… of the SA800 Series], but many manufacturers changed them slightly to suit their own purposes.}}</ref>
<ref name="Davis_2015">{{cite web |title=Floppy Disk Drive Pinout |author-last=Davis |author-first=Larry |date=2015-06-13 |website=www.interbus.com  |url=http://www.interfacebus.com/PC_Floppy_Drive_PinOut.html |access-date=2022-01-06 |url-status=live |archive-url=https://web.archive.org/web/20220107035902/http://www.interfacebus.com/PC_Floppy_Drive_PinOut.html |archive-date=2022-01-07}}</ref>
}}

==Further reading==
* {{cite web |title=Tech information on floppy disks drives and media |author-first=Herbert "Herb" R. |author-last=Johnson |publication-place=New Jersey, USA |website=retrotechnology.com |date=2021-07-21 |orig-date=2006 |url=https://www.retrotechnology.com/herbs_stuff/drive.html |access-date=2022-01-04 |url-status=live |archive-url=https://web.archive.org/web/20220101165112/https://www.retrotechnology.com/herbs_stuff/drive.html |archive-date=2022-01-01}}
<!-- removing redundant links not likely to be used in article
* {{cite web |title=Floppy Disk Drive / Bus Interface |author-first=Thomas |author-last=Brase |publication-place=Sarstedt, Germany |date=2021-02-10 |website=retrocmp.de |url=https://retrocmp.de/fdd/general/floppy-bus.htm |access-date=2022-01-04 |url-status=live |archive-url=https://web.archive.org/web/20220101185218/https://retrocmp.de/fdd/general/floppy-bus.htm |archive-date=2022-01-01}}
* {{cite web |title=Modify NMS8245/00 from 14-pins Philips floppy drive to 34 pin Shugart bus |author-first=Hans |author-last=Oranje |date=2003 |publication-place=Waddinxveen, Netherlands |url=http://cdn.preterhuman.net/texts/computing/msx/fddnms8245e.pdf |access-date=2022-01-04 |url-status=live |archive-url=https://web.archive.org/web/20161108141349/http://cdn.preterhuman.net/texts/computing/msx/fddnms8245e.pdf |archive-date=2016-11-08}} (5 pages)
* {{cite magazine |title=Anschluß gesucht, Floppy-Laufwerke zum Laufen gebracht – vom Shugart-Bus und ähnlichen Standards |language=de |trans-title= |author-first1=Willi |author-last1=Wagemuth |author-first2=Detlef |author-last2=Grell |magazine=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |issue=7 |volume=1987 |date=July 1987 |pages=169–}}
* {{cite magazine |title=Das Floppy-Laufwerk in Theorie und Praxis, Teil 1 |language=de |author-first=Gerd E. |author-last=Neumann |magazine=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |issue=12 |volume=1983 |date=December 1983 |id=ark:/13960/t37133p2f |pages=68-70 |url=https://archive.org/details/ct8312/mode/2up?q=Shugart}} "Shugart-Bus", "Shugart-kompatibler Bus"
* {{cite magazine |title=Das Floppy-Laufwerk in Theorie und Praxis, Teil 2 |language=de |author-first=Gerd E. |author-last=Neumann |magazine=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |issue=1 |volume=1984 |date=January 1984 |pages=77-79 |url=https://www.yumpu.com/de/document/read/7199787/ct-1984-01pdf-54686kb-may-16-2012-0317-free-hack-library |access-date=2022-01-01 |url-status=live |archive-url=https://web.archive.org/web/20220101161858/https://www.yumpu.com/de/document/read/7199787/ct-1984-01pdf-54686kb-may-16-2012-0317-free-hack-library |archive-date=2022-01-01}}
* {{cite magazine |title=Das Floppy-Laufwerk in Theorie und Praxis, Teil 3 |language=de |author-first=Gerd E. |author-last=Neumann |magazine=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |issue=2 |volume=1984 |date=February 1984 |pages= |url=}}
* {{cite magazine |title=Was die Floppys speichern |language=de |department=Praxis & Tipps / Tipps & Tricks |date=2010-02-13 |author-first=Detlef |author-last=Grell |magazine=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |issue= |volume=2010 |url=http://heise.de/-923961 |access-date=2022-01-01}}
* {{cite magazine |title=Floppy-Symphonie |language=de |author-first=Philip |author-last=Steffan |department=Bastelprojekt |magazine=c't Hacks |publisher=Heise Zeitschriften Verlag |issue=1 |volume=2014 |date=2014 |pages=149 |url=https://maker-faire.de/wp-content/uploads/2020/03/Musik-mit-Floppies.pdf}}
* https://www.ndr-nkc.de/download/docs/FLO3_BAU.PDF
* {{cite web |title=Floppy-Disk-Interface für den PC/M |language=de |author-first1=A. |author-last1=Mugler |author-first2=H. |author-last2=Mathes |date=February 1990 |issue=2 |volume=1990 |issn=0016-2833 |magazine=Funkamateur - Radiosportzeitschrift |url=http://www.ac1-info.de/literatur/fa_90_02_1.htm |access-date=2022-01-01 |url-status=live |archive-url=https://web.archive.org/web/20220101162812/http://www.ac1-info.de/literatur/fa_90_02_1.htm |archive-date=2022-01-01}}
* https://torlus.com/floppy/forum/viewtopic.php?t=3276 "floppy bus"
* https://retrocomputing.stackexchange.com/questions/5302/why-is-the-rdy-signal-on-floppy-disk-drives-disabled "original Shugart floppy bus specification"
* https://de.set18.net/Gepard_(Computer).html
* https://forum.classic-computing.de/forum/index.php?thread/14477-gould-logic-analyzer-k105-d-k115-d-disassembler-software-und-doku-gesucht/ "Standard-Shugart-Bus"
* https://www.c64-wiki.de/wiki/Alan_Shugart "Shugart-Bus" (Shugart bus), "Shugart-Stecker" (Shugart connector), "Floppy-Bus" (floppy bus)
* https://www.stcarchiv.de/stc1992/09_gehtdoch.php "Shugart-Bus"
* https://www.mikrocontroller.net/topic/458932 "Shugart-Bus", "Floppy-Bus"
-->

[[Category:Computer buses]]