Afloppy-disk controller(FDC) is a hardware component that directs and controls reading from and writing to a computer'sfloppy disk drive(FDD). It has evolved from a discrete set of components on one or more circuit boards to a special-purposeintegrated circuit(IC or "chip" ) or a component thereof. An FDC is responsible for reading data presented from the host computer and converting it to the drive's on-disk format using one of a number of encoding schemes, likeFM encoding(single density) orMFM encoding(double density), and reading those formats and returning it to its original binary values.
Depending on the platform, data transfers between the controller and host computer would be controlled by the computer's ownmicroprocessor,or an inexpensive dedicated microprocessor like theMOS 6507orZilog Z80.Early controllers required additional circuitry to perform specific tasks like providing clock signals and setting various options. Later designs included more of this functionality on the controller and reduced the complexity of the external circuitry; single-chip solutions were common by the later 1980s.
By the 1990s, the floppy disk was increasingly giving way tohard drives,which required similar controllers. In these systems, the controller also often combined amicrocontrollerto handle data transfer over standardized connectors likeSCSIandIDEthat could be used with any computer. In more modern systems, the FDC, if present at all, is typically part of the many functions provided by a singlesuper I/Ochip.
History
editThe first floppy disk drive controller (FDC) like the firstfloppy diskdrive (the IBM 23FD) shipped in 1971 as a component in the IBM 2385 Storage Control Unit for theIBM 2305fixed head disk drive,[1]and of the System 370 Models155and165.The IBM 3830 Storage Control Unit, a contemporaneous and quite similar controller, uses its internal processor to control a 23FD.[2]The resultant FDC is a simple implementation in IBMs'MSThybrid circuits on a few printed circuit cards.[2]The drive, FDC and media were proprietary to IBM and although other manufacturers provided early FDDs prior to 1973 there were no standards for FDCs, drives or media.
IBM's 1973 introduction of the3740 Data Entry Systemcreated the basic media standard for the 8-inch single sided floppy disk, IBM's"Type 1"diskette, which coupled with rapidly increasing requirements for inexpensive, removable direct access storage for many small applications caused a dramatic growth in drive and controller shipments.[3]
Prior to the introduction of special purpose integrated circuit versions, most FDCs consisted of at least one printed circuit implemented with 40 or more ICs.[4]Examples of such FDCs include:
- 1973: The FDC inIBM's 3741is a type of microcontroller that accepts commands from the system's microprocessor ( "MPU" in IBM's terminology) and executes them on the attached 33FD as independently as possible. It accepts and executes the following commands, select/stop, write check, seek lower, seek higher, read data, read I D, write data, write control, write I D, set ready, reset access counter, and nothing (no-op). It was implemented using IBM'sMST hybrid circuitson the motherboard plus a separate data separator (VFO) pcb.[5]This IBM FDC did establish the IBM Type 1 diskette as the first industry standard floppy disk medium but neither its interface to the host microprocessor nor its interface to the 33FD were adopted as industry standards.
- 1974: iCOM's FD360 contained an early FDC, the CF 360, that generated industry standard media, connected to industry standard host busses, and supported industry standard FDDs.[6][7]Its FDC was implemented on a pcb approximately 12x9 inches as a state machine using 30 ICs.[8]
- 1976: Scientific Micro Systems' FD0300 FDC[9]built on an 8-inch by 12-inch circuit board contains a microprocessor and approximately 50 integrated circuits and is designed to provide easy attachment to a number of host buses.[10]
- 1976:Shugart Associatesintroduced the first 5¼-inch floppy disk drive along with an associated and first FDC for this form factor, the SA4400.[11]The SA4400 performs control functions to transfer data between a host system and up to 3 disk drives using an 8-bit general purpose host interface which format disks according to a modified IBM 3740 type media format specifications. The FDC is microprocesser controlled and implemented on a 5.75 by 9.50 inch pcb with 45 ICs.[12]The drive interface and media form factors became industry standards with the media then evolving over time to support a number ofdifferent formats.
- 1977: The Apple Disc II FDC, the"Woz Machine",is built with only 8 ICs.[4][13]It, like the much earlier IBM 3830 FDC, achieved the reduction in components thru use of the host processor and firmware. Its interface to the Apple host as well as its interface to the Apple 5¼-inch floppy disk drive is unique and it was not adopted as an industry standard.
The first FDC implemented as a special purpose integrated circuit is theWestern Digital FD1771[14]announced on 19 July 1976.[15]The initial design supported a single format and required additional circuitry but over time, as a family, the design becamemulti-sourcedandevolvedto support many formats and minimize external circuitry.
The NEC μPD765 was announced in 1978[16]and in 1979 NEC introduced the μPD72068, which was software compatible with the μPD765, incorporating a DigitalPLL.[17]The μPD765 became a quasi-industry standard when it was adopted in the originalIBM PC(1981); the FDC was physically located on its own adapter card along with support circuitry. Other vendors such as Intel produced compatible parts. This design evolved over time into a family offering an almost complete FDC on a chip.[18]
As of March 1986, Sharp had commercialized the FDC LH0110.[19]
In early 1987, Intel introduced the 82072 CHMOS High Integrated Floppy Disk Controller for use in industry standard PC computers.[20][21]
Ultimately in most computer systems the FDC became a part of aSuper I/Ochip or aSouthbridgechip.[18][22][23]However, in later motherboards, as floppy disks were phased out by personal computer users, this interface was eliminated. Some manufacturers developedUSB-based floppy disk controllers.[24]
Overview
editAfloppy diskstores binary data not as a series of values, but a series ofchangesin value. Each of these changes, recorded in the polarity of the magnetic recording media, causes a voltage to be induced in thedrive headas the disk surface rotates past it. It is the timing of these polarization changes and the resulting spikes of voltage that encode the ones and zeros of the original data. One of the functions of the controller is to turn the original data into the proper pattern of polarizations during writing, and then recreate it during reads.
As the storage is based on timing, and that timing is easily affected by mechanical and electrical disturbances, accurately reading the data requires some sort of reference signal, theclock.As the on-disk timing is constantly changing, the clock signal has to be provided by the disk itself. To do this, the original data is modified with extra transitions to allow the clock signal to be encoded in the data and then useclock recoveryduring reads to recreate the original signal. Some controllers require this encoding to be performed externally, but most designs provide standard encodings likeFMandMFM.
The controller also provides a number of other services to control the drive mechanism itself. These typically include the movement of the drive head to center over the separate tracks on the disk, tracking the location of the head and returning it to zero, and sometimes functionally to format a disk based on simple inputs like the number of tracks, sectors per track and number of bytes per sector.
To produce a complete system, the controller has to be combined with additional circuitry or software that acts as a bridge between the controller and the host system. In some systems, like theApple IIandIBM PC,this is controlled by software running on the computer's hostmicroprocessorand the drive interface is connected directly to the processor using anexpansion card.On other systems, like theCommodore 64andAtari 8-bit computers,there is no direct path from the controller to the host CPU and a second processor like theMOS 6507orZilog Z80is used inside the drive for this purpose.
The original Apple II controller was in the form of a plug-in card on the host computer. It could support two drives, and the drives eliminated most of the normal onboard circuitry. This allowed Apple to arrange a deal withShugart Associatesfor a simplified drive that lacked most of its normal circuitry.[4]This meant that the combined cost of a single drive and controller card was roughly the same as on other systems, but a second drive could be connected for a smaller additional cost.[citation needed]
The IBM PC took a more conventional approach, their adaptor card could support up to four drives; on the PCdirect memory access(DMA) to the drives was performed using DMA channel 2 andIRQ6. The diagram below shows a conventional floppy disk controller which communicates with the CPU via anIndustry Standard Architecture(ISA) bus or similar bus and communicates with the floppy disk drive with a 34 pin ribbon cable. An alternative arrangement that is more usual in recent designs has the FDC included in asuper I/Ochip which communicates via aLow Pin Count(LPC) bus.
Most of the floppy disk controller (FDC) functions are performed by theintegrated circuitbut some are performed by external hardware circuits. The list of functions performed by each is given below.
Floppy disk controller functions (FDC)
edit- Translate data bits intoFM,MFM,M²FM,orGCRformat to be able to record them
- Interpret and execute commands such as seek, read, write, format, etc.
- Error detection withchecksumsgeneration and verification, likeCRC
- Synchronize data withphase-locked loop(PLL)
External hardware functions
edit- Selection of whichfloppy diskdrive (FDD) to address
- Switching-on the floppy drive motor
- Reset signal for the floppy controller IC
- Enable/disable interrupt and DMA signals in the floppy disk controller (FDC)
- Data separation logic
- Write pre-compensationlogic
- Line driversfor signals to the controller
- Line receivers for signals from the controller
Input/output ports for common x86-PC controller
editThe FDC has threeI/Oports. These are:
- Data port
- Main status register (MSR)
- Digital control port
The first two reside inside the FDC IC while the Control port is in the external hardware. The addresses of these three ports are as follows.
| Port Address [hex] |
Port Name | Location | Port type |
|---|---|---|---|
| 3F5 | Data port | Bidirectional I/O | |
| 3F4 | Main status register | FDC IC | Input |
| 3F2 | Digital control port | External hardware | Output |
Data port
editThis port is used by the software for three different purposes:
- While issuing a command to the FDC IC, command and command parameter bytes are issued to the FDC IC through this port. The FDC IC stores the different parameters and the command in its internal registers.
- After a command is executed, the FDC IC stores a set of status parameters in the internal registers. These are read by the CPU through this port. The different status bytes are presented by the FDC IC in a specific sequence.
- In the programmed and interrupt mode of data transfer, the data port is used for transferring data between the FDC IC and the CPU IN or OUT instruction.
Main status register (MSR)
editThis port is used by the software to read the overall status information regarding the FDC IC and the FDD's. Before initiating a floppy disk operation the software reads this port to confirm the readiness condition of the FDC and the disk drives to verify the status of the previously initiated command. The different bits of this register represent:
| Bit | Representation |
|---|---|
| 0 | FDD 0: Busy in seek mode |
| 1 | FDD 1: Busy in seek mode |
| 2 | FDD 2: Busy in seek mode |
| 3 | FDD 3: Busy in seek mode |
| 4 | FDC Busy; Read/Write command in progress |
| 5 | Non-DMA mode |
| 6 | DIO; Indicates the direction of data transfer between the FDC IC and the CPU |
| 7 | MQR; Indicates data register is ready for data transfer |
| Explanations | |
|---|---|
| MQR | 1 = data register ready, 0 = data register not ready |
| DIO | 1 = controller has data for CPU, 0 = controller expecting data from CPU |
| Non-DMA | 1 = Controller Not in DMA Mode, 0 = Controller in DMA Mode |
| FDC Busy | 1 = Busy, 0 = Not Busy |
| FDD 0,1,2,3 | 1 = Running, 0 = Not Running |
Digital control port
editThis port is used by the software to control certain FDD and FDC IC functions. The bit assignments of this port are:
| Bit | Representation |
|---|---|
| 0 and 1 | Device number to be selected |
| 2 | RESET FDC IC (Low) |
| 3 | Enable FDC interrupt and DMA request signals |
| 4 to 7 | Turn ON the motor in disk drive 0, 1, 2 or 3 respectively |
Interface to the floppy disk drive
editA controller connects to one or more drives using a flat ribbon cable, 50 wires for 8 "drives and 34 wires for 3.5" & 5.25 "drives. A" universal cable "has four drive connectors, two each for 3.5" & 5.25 "drives.[25]In the IBM PC family and compatibles, a twist in the cable is used to distinguish disk drives by the socket to which they are connected. All drives are installed with the same drive select address set, and the twist in the cable interchanges the drive select lines at the socket. The drive that is at the far end of the cable would also have aterminating resistorinstalled to maintain signal quality.[26]
- More detailed descriptions of the interface signals including alternative meanings are contained in manufacturer's specifications for drives or host controllers.
When the controller and disk drive are assembled as one device, as it is the case with some external floppy disk drives, e.g.,Commodore 1540and USB floppy disk drives,[27]the internal floppy disk drive and its interface are unchanged, while the assembled device presents a different interface such asIEEE-488,parallel portorUSB.
Format data
editMany mutually incompatible floppy disk formats are possible; aside from the physical format on the disk, incompatible file systems are also possible.
| Drive | Format | Capacity | Transfer speed [kbit/s] |
RPM | Tracks | TPI | Comment |
|---|---|---|---|---|---|---|---|
| 8-inch SD | 8-inch SD | 80 KB | 33.333 | 360 | 32 | 48 | Only on old controllers.[28] |
| 5.25-inch SD | 5.25-inch SD | 160 KB | 125 | 40 | Only on old controllers. | ||
| 5.25-inch SSDD | 5.25-inch SSDD | 171 KB | 250–308 | 300 | 35 | 48[29] | Only onC1541compatibles. |
| 5.25-inch SD | 5.25-inch SD | 180 KB | 150 | 40 | Only on old controllers. | ||
| 5.25-inch DD | 5.25-inch DD | 320/360/400 KB | 250 | 300 | 40 | 48 | [30]8/9/10 512 byte sectors respectively. |
| 5.25-inch DD(96 tpi) | 5.25-inch QD (2DD) | 800 KB | 250 | 300 | 80 | 96 | [31] |
| 5.25-inch HD | 5.25-inch DD | 360 KB | 300 | 360 | 40 | 48 | [32][33] |
| 5.25 "HD | 5.25 "HD | 1200 KB | 500 | 360 | 80 | 96 | Up to 83 tracks. Different biasing current.[32][33] |
| 5.25 "HD | 5.25 "HD | 720 KB | 300 | 360 | 80 | Up to 83 tracks.[30] | |
| 3.5 "DD | 3.5 "DD | 720 KB | 250 | 300 | 80 | 135 | Up to 83 tracks.[30][34] |
| 3.5 "DD | 3.5 "DD | 800 KB | 394–590 | 80 | Used by AppleMacintosh.[35] | ||
| 3.5 "DD | 3.5 "DD | 800 KB | 250 | 300 | 80 | Used byCommodore 1581. | |
| 3.5 "DD | 3.5 "DD | 880 KB | 250 | 300 | 80 | Up to 83 tracks. Used byAmigacomputers. | |
| 3.5 "DD | 3.5 "DD | 360 KB | 250 | 300 | 40 | [30] | |
| 3.5 "HD | 3.5 "DD | 720 KB | 250 | 300 | 80 | Up to 83 tracks.[30] | |
| 3.5 "HD | 3.5 "HD | 1280 KB | 500 | 360 | 80 | 135 | Up to 83 tracks."3mode" |
| 3.5 "HD | 3.5 "HD | 1440 KB | 500 | 300 | 80 | 135 | Up to 83 tracks.[30][36] |
| 3.5 "HD | 3.5 "HD | 1760 KB | 250 | 150 | 80 | Used byAmigacomputers. | |
| 3.5 "ED | 3.5 "ED | 2880 KB | 1000 | 300 | 80 | 135 | Up to 83 tracks.[34][37] |
Sides:
- SS (or 1S) –Single sided
- DS (or 2S) –Double sided
Density:
- SD (or 1D) –Single density(FM)
- DD (or 2D) –Double density(most oftenMFM)
- QD (or 4D) –Quad density
- HD –High density
- ED –Extra-high density
- TD –Triple density
3-mode floppy drive
edit
Primarily in Japan there are 3.5 "high-density floppy drives that support three modes of disk formats instead of the normal two – 1440 KB (2 MB unformatted), 1.2 MB (1.6 MB unformatted) and 720 KB (1 MB unformatted). Originally, the high-density modes for 3.5" floppy drives in Japan only supported a capacity of 1.2 MB instead of the 1440 KB capacity that was used elsewhere.[39]While the more common 1440 KB format spun at 300 rpm, the 1.2 MB formats instead spun at 360 rpm, thereby closely resembling the geometries of either the 1.2 MB format with 80 tracks, 15 sectors per track, and 512 bytes per sector previously found on 5.25 "high-density floppy disks or the 1.2 MB format with 77 tracks, 8 sectors per track, and 1,024 bytes per sector previously found on 8" double-density floppy disks. Later Japanese floppy drives incorporated support for both high-density formats (as well as the double-density format), hence the name 3-mode. SomeBIOSeshave a configuration setting to enable this mode for floppy drives supporting it.[40]
See also
edit- Western Digital FD1771
- Integrated Woz Machine(IWM)
- Paula(Amiga controller)
- Floppy disk drive interface
- List of floppy disk formats
References
edit- ^"IBM 2835 Storage Control and IBM 2305 Fixed Head Storage Module Reference Manual"(PDF).October 1983.Retrieved22 July2022.
The control unit contains a miniature direct access device which provides read-only storage for control logic backup and storage of nonresident microdiagnostics. The recording medium is an inexpensive Mylar-coated disk cartridge.
- ^ab"IBM Maintenance Library – Storage Control, Model 2, Volume 2"(PDF).4 June 1973. pp. MPL 25A, MPL 200, MPL 245, MPL 260.Retrieved29 July2022.
Hardware has already added 64 words (Track 0, Sector 0) and this microprogram will load the remainder of control storage.
- ^Porter, James N. (August 1977). 1977 DISK/TREND REPORT – FLEXIBLE DISK DRIVES (Report). p. 26.
- ^abcGregg, Williams; Moore, Rob, eds. (1984)."THE APPLE STORY, PART 2, An interview with Steve Wozniak".Byte.Retrieved6 August2022.
At the time, all the existing floppy-disk controllers were 40 or 50 chips…
- ^3741 Data Station, Theory-Maintenance.IBM. 15 May 1974. pp. 14–2.14–15 (488/599).Retrieved9 August2022.
- ^"First floppy disc peripheral made for microcomputers".Electronic Design.27 September 1974. p. 138.Retrieved11 August2022.
- ^"CF 360 FLOPPY DISK CONTROLLER"(PDF).iCOM Microperipherals.Retrieved19 August2022.
the controller is fully IBM 3740 and 3540 compatible with all formatting and deformatting accomplished automatically within the controller.
- ^SCHEMATIC AND LOGIC DIAGRAMS MODEL FD360(PDF)(Report). iCOM Microperipherals. March 1976. pp. 8, 34.Retrieved11 August2022.
- ^"Discs".INTERFACE AGE.November 1976. pp. 65–66.Retrieved24 July2022.
- ^"FD0300 FLOPPY DISK CONTROLLER"(PDF).1976.Retrieved24 July2022.
A General Purpose Host Interface is also provided for easy interface to host systems such as minicomputer, microprocessor I/O busses, CRT terminals, instruments, TTL/MSI microprocessors, industrial controllers and other byte oriented systems.
- ^"State of the Art Disk Technology"(PDF).Byte.December 1976.Retrieved1 September2022.
- ^"SA4400 ministreaker FloppyDisk Drive Controller"(PDF).1977.Retrieved1 September2022.
- ^Craig, David T (April 1978)."Apple II Computer Family Information, Schematic: Disk 2 Interface Card"(PDF).Retrieved6 August2022.
- ^Joe, Jaworski (1985). "Floppy Disk Controllers".1985 Controller Concepts – Volume 1(PDF)(Technical report). p. SEMI-1 (67/160).Retrieved9 September2022.
Pioneering this field was Western Digital Corporation who, in 1976, began sampling the first LSI floppy disk controller, the FD1771.
- ^"Recent IC Announcements".Computer.IEEE. 1976.Retrieved6 August2022.
- ^"μPD765 SINGLE/DOUBLE DENSITY FLOPPY DISK CONTROLLER"(PDF).NEC. December 1978.Retrieved9 September2022.
- ^"NEC Electronics Inc. μPD72068 Floppy-Disk Controller"(PDF).Retrieved24 January2024.
- ^abNecasek, Michal (26 May 2011)."The floppy controller evolution".Retrieved2 September2022.
- ^"Sharp 1986 Semiconductor Data Book"(PDF).pp. 279–295.Retrieved14 January2024.
- ^Intel Corporation, "New Product Focus Components: Single-Chip Disk Controller Squeeze More into Less", Solutions, January/February 1987, page 14
- ^Kearns, Patrick, Begur, Sridhar, and Fischer, Steve, "High Integration/High Performance Floppy Disk Controller Subsystem with the 82072", Intel Corporation, Microcomputer Solutions, November/December 1987, page 20
- ^Mueller, Scott (2005)."Motherboard Components".Scott Muellers Upgrading and Repairing Laptops, Second Edition.Retrieved5 September2022.
- ^"FDC37C78 Floppy Disk Controller"(PDF).SMSC. 2007. Archived fromthe original(PDF)on 13 December 2007.Retrieved9 September2022.
Licensed CMOS 765B Floppy Disk Controller
- ^"USB Floppy Disk Controller USB97CFDC2-01"(PDF).keil.13 December 2001.
- ^Davis, Larry (13 June 2015)."Floppy Drive Pinout, Signal names, Pin out Description and Cable twist wiring".interfacebus.Retrieved29 January2019.
- ^Scott Mueller,Upgrading and Repairing PCs, Second Edition,Que, 1992,ISBN0-88022-856-3,page 487
- ^Fisher, Tim (18 January 2022)."What Is a Floppy Disk Drive?".Retrieved20 September2022.
- ^Ableman, Genna (2005). Elert, Glenn (ed.)."Angular speed of a floppy disk".The Physics Factbook.Retrieved25 January2022.
- ^"C 64 Workshop / C= 8 Bit & Peripherals".19 May 1998.Retrieved18 April2016.
- ^abcdef"unifr.ch – sys/src/kernel/floppy.c".Archived fromthe originalon 19 July 2011.Retrieved5 May2011.
- ^"Product specification TM100-4 flexible disk drive 96, tpi"(PDF).Retrieved8 January2022.
- ^abiesleonardo.info – This diskette tutorial provides technical information concerning diskettes
- ^aboldskool.org – Let HD 5,25 "FDDs operate at 300 rpm instead of 360 rpm
- ^abintel – Intel 82077SL for Super Dense FloppiesArchived8 October 2012 at theWayback Machine
- ^Johnson, Herbert R. (22 December 2016)."Floppy Drive Tech Info".Retrieved14 January2017.
- ^yi.org – High Density Floppy Disks Mf2hd Disk 3 5 1 Pk[permanent dead link]
- ^mcamafia.de – IBM Personal system/2, 3,5 "-inch Diskette Drives, Technical Reference
- ^"Linux-2.6.17/drivers/block/floppy.c".Archived fromthe originalon 23 August 2008.090504 gelato.unsw.edu.au
- ^books.google – Fix Your Own PC by Corey Sandler
- ^rojakpot – 3mode floppy support
Further reading
edit- de Boyne Pollard, Jonathan (2003)."There is no such thing as a 3.5-inch floppy disc".Frequently Given Answers.
- ISO/IEC 8860-1:1987 Double-Density (DD)
- ISO/IEC 9529-1:1989 High-Density (HD)
- ISO 10994-1:1992 Extra-high-density (ED)
- ECMA-147
- NEC μPD72070 – Floppy Disk Controller Specification Version 2.0(PDF).2.0 preliminary.NEC Corporation.October 1991. Archived fromthe original(PDF)on 20 March 2017.Retrieved20 March2017.
- Shah, Katen A. (1996) [September 1992, April 1992].Intel 82077SL for Super-Dense Floppies(PDF)(Application Note) (2 ed.).IntelCorporation, IMD Marketing. AP-358, 292093-002. Archived fromthe original(PDF)on 19 June 2017.Retrieved19 June2017.
- Johnson, Herb (2009)."The first floppy controller for CP/M and S-100?".Retrieved9 September2022.