Western Digital FD1771

Western Digital datasheet for the FD1771 floppy disk controller

The FD1771 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.^[1][2]

Derivatives

The FD1771 was succeeded by many derivatives that were mostly software-compatible:

• The FD1781 was designed for double density, but required external modulation and demodulation circuitry, so it could support MFM, M2FM, or other double-density encodings.^[3][4]
• The FD1791-FD1797 series added internal support for double density (MFM) modulation, compatible with the IBM System/34 disk format. They required an external data separator.^[5]
• The WD1761-WD1767 series were versions of the FD179x 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-in (200 mm) floppy drives or the later "high-density" 5.25-inch (133 mm) or 90 mm 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. They took a 1 MHz or 2 MHz clock and were intended for 8-inch (203 mm) and 5.25-inch (133 mm) drives.^[6]
• The WD1770, WD1772, and WD1773 added an internal digital data separator and write precompensator, eliminating the need for external passive components but raising the clock rate requirement to 8 MHz. They supported double density, despite the apparent regression of the part number, and were packaged in 28-pin DIP packages.^[7]
  • The WD1772PH02-02 was a version of the chip that Atari fitted to the Atari STE which supported high density (500 kbit/s) operation. After production at WD could not be sustained, Atari decided to license the design and modify it to get high density and extra density (1MBit/s) operation. The chip was codenamed "Ajax", had the number C302096 and was produced by Toshiba.

Compatible chips

Many compatible chips were available from other vendors:

• FD179x series from SMC Microelectronics
• MB887x series from Fujitsu
• VL177x series from VLSI Technology.
• Soviet КР1818ВГ93 was a WD1793 clone

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, the Commodore 157x/1581 for the Commodore 64 and the Atari XF551 for the Atari XL/XE.

References

1. Michalopoulos, Demetrios A (October 1976). "New Products: Single-chip floppy disk formatter/controller". Computer (IEEE) 9 (10): 64. doi:10.1109/C-M.1976.218414."The FD1771 is a single-chip floppy disk formatter/controller that interfaces with most available disk drives and virtually all types of computers."
2. "Recent IC Announcements". Computer (IEEE) 9 (10): 66. October 1976. doi:10.1109/C-M.1976.218417. The FD1771 was announced on July 19, 1976, and sold for $60 each in lots of 100.
3. "Recent IC Announcements". Computer (IEEE) 10 (12): 140. December 1977. doi:10.1109/C-M.1977.217618. The FD1781 was announced on September 21, 1977. Supported Dual density; accommodates MFM, M2FM.
4. "Dual Density Floppy Controller Chip". Byte 3 (3): 216. March 1978.
5. FD 179X-02 datasheet
6. WD279X-02 datasheet
7. WD177X-00 datasheet images