WD1771

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The WD1771 is the first in a line of floppy disk controllers produced by Western Digital. It uses single density (FM) modulation and supports the IBM 3740 disk format. It is packaged in a 40-pin DIP.

The WD1771 was succeeded by many derivatives that were mostly software-compatible:

• The WD1781 was designed for double density, but required external modulation and demodulation circuitry, so it could support MFM, M2FM, or other double-density encodings.
• The WD1791-WD1797 series added internal support for double density (MFM) modulation, compatible with the IBM System/34 disk format. They required an external data separator.
• The WD1761-WD1767 series were versions of the WD179x series rated for a maximum clock frequency of 1 MHz, resulting in a data rate limit of 125 kbit/s for single density and 250 kbit/s for double density, thus preventing them from being used for 8-inch (203 mm) floppy drives or the later "high-density" 5.25-inch (133 mm) or 90-millimetre (3.5 in) floppy drives.
• The WD2791-WD2797 series added an internal data separator using an analog phase-locked loop, with some external passive components required for the VCO.
• The WD1770, WD1772, and WD1773 used an internal digital data separator, eliminating the need for external passive components. They supported double density, despite the apparent regression of the part number, but had the same 1 MHz clock rating as the WD176x. They were packaged in 28-pin DIP packages.
  • The WD1772PH02-02, or 'Ajax', was a derivative chip fitted to the Atari STE which supported high density (500 kbit/s) operation.

Many compatible chips were available from other vendors:

• FD179x series from SMC Microelectronics
• MB887x series from Fujitsu
• VL177x series from VLSI Technology.

These families were used in many microcomputers and home computers including the Radio Shack TRS-80, Acorn Electron, BBC Master, Atari ST, Acorn Archimedes and the SAM Coupé, as well as the +D and DISCiPLE disk interfaces for the Sinclair ZX Spectrum and the Commodore 1581 for the Commodore 64.