Floppy-Disk-Controller

A floppy disk controller ( abbreviated FDC) is a special integrated circuit ( IC) for controlling said floppy disk drives ( FDD) and serves for reading and writing data to floppy disks. The increasing diffusion of the personal computer in the 1980s and 1990s, and the time for data exchange standard floppy disk drives, floppy disk controller found on first own cards and later housed directly on the motherboard, wide distribution.

The floppy disk controller is primarily used in the PC area corresponded to the integrated circuit manufactured by Western Digital FD1771 ( WD1771 ) in a 40 -pin dual- in-line chip package which was introduced in 1976 and produced in subsequent years by various manufacturers and in several variants been. The FD1771 was sold on 19 July 1976, a single unit price of U.S. $ 60 in quantities of 100 pieces. Compatible and functional Advanced Controller published in the following years, including the μPD765 of the NEC Corporation and the derivatives i8072A and i8272A from Intel. At the time of the Cold War Soviet replicas existed under the name КР1818ВГ93.

Functions

Floppy disk controller type WD1771 and their replicas can control up to four floppy drives. To connect to the main processor, it has a system in which computer systems can be connected directly to the then usual ISA bus. The actual data transfer, depending on the disk format and write density are different bit rates up to 500 kbit / s, via a DMA channel in combination with an interrupt. In addition to the floppy disk controllers were on the first ISA expansion cards, additional control logic and line driver, which was partially integrated into the later replicas in the FDC, and as a result in the chipset of the motherboard.

The tasks of the FDC are:

• Implementation of the data to be written for the magnetic recording. In this case, coding methods such as digital frequency modulation (FM), modified frequency modulation (MFM) or Group coded recording ( GCR ) is applied.
• The low-level formatting new disks. In this case be written to the magnetic memory synchronization data for the subsequent reading.
• When reading data in addition to the decoding results in a synchronization of the various sectors in the FDC by means of phase -locked loops (PLL ).
• Error correction process and error detection process by means of cyclic redundancy check (CRC).
• Control of the drive mechanics such as moving the read-write head of a stepper motor in the drive, so as to approach a specific track. Furthermore, the turning on and off the drive motor.

Advanced features which are present in some floppy disk controllers, including the write Precompensation are located around the inner tracks with a lesser extent at a higher magnetic flux density to be described.