Floppy-disk controller

A floppy disk stores binary data not as a series of values, but a series of changes in value. Each of these changes, recorded in the polarity of the magnetic recording media, causes a voltage to be induced in the drive head as 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, the clock. 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 use clock recovery during reads to recreate the original signal. Some controllers require this encoding to be performed externally, but most designs provide standard encodings like FM and MFM.

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, determining the speed of rotation and attempting to keep it relatively steady, racking 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 the Apple II and IBM PC, this is controlled by software running on the computer's host microprocessor and the drive interface is connected directly to the processor using an expansion card. On other systems, like the Commodore 64 and Atari 8-bit family, there is no direct path from the controller to the host CPU and a second processor like the MOS 6507 or Zilog Z80 is 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 with Shugart Associates for a simplified drive that lacked most of its normal circuitry. 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.

The IBM PC took a more conventional approach, their adaptor card could support up to four drives; on the PC direct memory access (DMA) to the drives was performed using IRQ 6. The diagram below shows a conventional floppy disk controller which communicates with the CPU via an Industry 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 a super I/O chip which communicates via a Low Pin Count (LPC) bus.

Block diagram showing FDC communication with the CPU and the FDD.

Most of the floppy disk controller (FDC) functions are performed by the integrated circuit but some are performed by external hardware circuits. The list of functions performed by each is given below.

The FDC has three I/O ports. 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.

The controller connects to the drive using a flat ribbon cable with 34 connectors split between the host, the 3.5" drive, and the 5.25" drive. This type of cable is called a universal connector.[1] 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 interchange the drive select line at the socket. The drive that is at furthest end of the cable additionally would have a terminating resistor installed to maintain signal quality.[2]

More detailed descriptions of the interface signals including alternative meanings are contained in manufacturer's specifications for drives.

Many mutually incompatible floppy disk formats are possible; aside from the physical format on the disk, incompatible file systems are also possible.

[13]

Sides:

• SS (or 1S) – Single sided
• DS (or 2S) – Double sided

Density:

• SD (or 1D) – Single density (FM)
• DD (or 2D) – Double density (most often MFM)
• QD (or 4D) – Quad density
• HD – High density
• ED – Extra-high density
• TD – Triple density

A setup disk of Japanese Microsoft Office 4.3, provided with 3.5" 1.2 MB and 1440 KB formats.

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 mode 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.[14] While the more common 1440 KB format spun at 300 rpm, the 1.2 MB format instead spun at 360 rpm, thereby closely resembling the 1.2 MB format with 15 sectors per track previously found on 5.25" high-density floppy drives. Later Japanese floppy drives incorporated support for both high-density formats (as well as the double-density format), hence the name 3mode. Some BIOSes have a configuration setting to enable this mode for floppy drives supporting it.[15]

• Western Digital FD1771
• Integrated Woz Machine (IWM)
• Paula (Amiga controller)
• Floppy disk drive interface
• List of floppy disk formats

• 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 from the original (PDF) on 2017-03-20. Retrieved 2017-03-20.
• Shah, Katen A. (1996) [September 1992, April 1992]. Intel 82077SL for Super-Dense Floppies (PDF) (Application Note) (2 ed.). Intel Corporation, IMD Marketing. AP-358, 292093-002. Archived from the original (PDF) on 2017-06-19. Retrieved 2017-06-19.

• viralpatel.net A Tutorial on Programming Floppy Disk Controller
• isdaman.com Programming Floppy Disk Controllers