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Ferguson Big Board

The Big Board (1980) was a Z80 based single-board computer designed by Jim Ferguson and sold primarily through Digital Research Computers of Texas (not to be confused with Digital Research, Incorporated aka Intergalactic Digital Research, creators of the CP/M operating system.) The design was a success in both the amateur and professional computer communities. It was used by many amateurs to obtain an inexpensive and simple to assemble computer system, and was used by professionals as an embedded controller as well as the basis for value-added systems sold to commercial clients. It was also licensed to Xerox, which used it as a basis for their Xerox 820 desktop computer. The Kaypro II portable computer's design was so similar that some considered it an unlicensed copy.

The success of the Big Board spawned Micro Cornucopia magazine and later designs such as the Big Board II, which added a hard disk drive interface, enhancements to system speed (increased to 4MHz from the 2.5MHz of the stock Big Board I), and enhancements to the built-in terminal interface. The Big Board designs also inspired the Ampro Little Board designs, which provided similar functionality in a board sized to match the form factor of a mini floppy disk drive. The Ampro boards sacrificed the on-board terminal functions of the Ferguson Big Boards to achieve this size, however.

The Big Board design was simple enough to build a system around that many people with no prior electronics experience were able to build and bring up a capable computer system of their own at a cost far less than that of a fully assembled system of the time. In this way, the Big Boards anticipated the DIY PC clones that became popular later. In its most popular form, the fully assembled and tested Big Board need only be connected to a power supply, one or two eight inch floppy disk drives, a composite monitor, and an ASCII encoded keyboard in order to provide a fully-functioning system. A serial terminal could be used in place of the monitor and keyboard, further simplifying assembly. The only tool required for basic assembly was a screwdriver for the terminal block power connections.

The design was also simple to modify for the sake of system expansion and enhancement. Many different modifications to increase the system clock speed were possible, including some that required nothing more than jumpers (e.g. the 3.5MHz speed upgrade obtained by jumpering the clock divider, with no software modifications or changes to the ICs on the board.) There was also a minor industry in user-installable system upgrades such as real time clocks, 4MHz upgrades, double density floppy upgrades, character enhancements for the display (reverse video, blinking, etc.), and the addition of hard disk interfaces such as SASI and SCSI. Most of these upgrades were accomplished through the use of daughter boards that plugged into existing IC sockets on the board, with the original IC either replaced by a more capable IC or placed into a socket on the daughter board.

With a little more effort it was possible to upgrade the memory to 256 KB, which was extremely large for the time. While not directly supported by CP/M, the extra memory could be used to implement a ram disk, cacheing of the operating system image (to greatly improve warm boot time), and even a print spooler.

The Big Board II (1983) incorporated many of the most popular upgrades into its design. It also featured a small breadboard area that allowed for many simple upgrades to be performed without the addition of daughter boards.


The Big Board was designed primarily to run the CP/M operating system, version 2.2. It came with a monitor program in ROM called PFM-80 which was the "software front panel" of the system. The source code listing of PFM was a feature of the first issue of Micro Cornucopia. PFM featured many well-documented routines that could be employed in user code.

The board featured 3 spare 2K ROM sockets that allowed for the addition of additional firmware. Popular additions were Tiny BASIC, FORTH, and enhanced versions of PFM.


The Big Board came with a full set of schematics, a document titled "Theory of Operation", the PFM-80 User's Manual, instructions for assembly and testing of the Big Board, a parts list, and addenda to these. The Theory of Operation described the details of the operation of the system, including the CRT controller, floppy disk controller, serial communications, memory bank switching, and connector pinouts.


Big Board I

  • Z80 CPU at 2.5MHz.
  • 64 KB dynamic RAM in 32 4116's or equivalent.
  • An ASCII keyboard interface.
  • A TTL / composite video CRT interface allowing a display of 24 lines by 80 characters.
  • A SS/SD floppy disk interface allowing addition of up to four 8" Shugart Associates SA800 compatible 8" floppy disk drives.
  • Two (optional) RS-232 serial ports.
  • A parallel Centronics compatible printer port.
  • An optional real time clock.
  • Board dimensions 8.5" by 13.75". (Compare to SA-800 disk drive: 9.5" x 14.5".)

Big Board II

  • Z80 CPU at 4MHz.
  • 64 KB dynamic RAM in 8 4164's or equivalent.
  • SASI/SCSI hard disk interface, also usable as a general purpose I/O or for control of other SCSI devices.
  • An ASCII keyboard interface.
  • A TTL / composite video CRT interface allowing a display of 24 lines by 80 characters.
  • A DS/DD floppy disk interface allowing addition of up to four 8" or 5.25" floppy disk drives.
  • Two RS-232 serial ports via a Z80 SIO.
  • A parallel Centronics compatible printer port via a Z80 PIO.
  • An optional real time clock.
  • Board dimensions 8.875" x 14.5"
  • 6 ROM/EPROM sockets, including ability to program EPROMs in place.

Third Party Products

Shugart Associates was a common source of floppy disk drives, as was Tandon.

Taylor Electric Company provided the "Better Board", including floppy disk drive interface enhancements, enhancements to PFM, and corrections to the original assembly and testing instructions bundled with the Big Board.

SWP Microcomputer Products of Arlington, TX (formerly Software Publishers) provided the Bigboard Dual Density upgrade, which provided both hardware and software to allow the Big Board I to use dual density formats on its drives.

Micro Cornucopia provided many products to enhance the Big Board computers, including speed upgrades, utiity software and development tools both on ROM and on disk, and I/O enhancements.

AB Computer Products sold enclosures, monitors, and pre-punched I/O panels targeted at Big Board users.

D&W Associates of Rome, NY sold monitors, ASCII-encoded keyboards, and power supplies targeted to the Big Board market.

Paradise Valley Electronics of Moscow, ID sold a version of FORTH, as well as graphics upgrades and utility software for the Big Board.

Several manufacturers, including JBW and Andy Bakkers sold SASI interface kits.

Kuzara Enterprises of San Diego, CA (formerly Design Technology) sold printer interfaces that allowed the Big Board to use the full feature sets of the Xerox Diablo printers.

Several manufacturers provided real time clock upgrades as a CPU daughter board.

Andy Bakkers sold a 1 MB RAM Disk daughter board for Big Board II.

External links

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