An MC68020/68881 Coprocessor Card for the Apple ][

In late 1985 I designed a MC68020/68881 coprocessor card for the Apple ][, based on an earlier 68008 card designed by Richard Ottosen. Richard and I built the card.

The MC68020 and MC68881 are run from a common clock, which is jumper selectable from either an on-board oscillator or the Apple ][ 7M clock.

The card has a 2764 boot EPROM, and 32 Kbyte of SRAM organized as 8K*32.

Communication with the Apple ][ is by means of four eight-bit parallel I/O ports, two in each direction, and by one interrupt "doorbell" in each direction. Typically one port in each direction is used for data, and the other for status and/or handshaking.

On system reset, the MC68020 CPU is held in reset until the Apple ][ releases it by writing to an I/O location.

We used two PAL16L8 chips (U2 and U12) for the address and byte strobe decode, bus error, and interrupt auto-vector.

Schematic:

Here's a scan of the hand-drawn schematic (B size, 354K PDF file).

Although I no longer have it in machine readable form, I have scanned a printed copy of the schematic redrawn using MacDraw (A size, 400 DPI scan, 80K PDF file).

Construction:

We didn't have wire-wrap sockets for the PGAs, so Richard constructed custom ones by removing the pins from machined-pin DIP wire-wrap sockets, and inserting them into suitably sized (13x13, 10x10) pieces of perf board. Most of the remaining circuitry was wired using 3M Scotchflex insulation-displacement prototyping.

PAL Programming:

This was the first time either Richard or I had used PALs. Even though the concept seemed fairly simple, we weren't 100% confident that we'd get it right the first time. We purchased five parts, in order to have three spares. We borrowed a Structured Design SD10 programmer. This was a self-contained box that had PALASM built-in, and a wafer-tape drive for storing your source code and/or JEDEC files. All you need with the SD10 is a dumb terminal. In practice, though, we used the Apple ][ as the terminal, and kept our source files on disk.

Debugging the hardware:

I checked and rechecked the wiring. When we plugged it into the Apple, there was a problem with lots of noise on the +5V supply, and the card didn't run the boot code. In short order Richard and I traced this to a fault on the R/W* line. Once this was fixed, the hardware worked. And the PALs did in fact work the first time.

Software:

A simple monitor program was put in the EPROM, which provided a means for the Apple ][ to load code into the card.

Loren Blaney modified the XPL0 compiler to generate native 68020 and 68881 code.

Epilogue:

Unfortunately the card was disassembled for parts in 1987. I should have at least taken a photo of it. Sigh.

Some thought was given to a "mark two" card, which would have had DRAM and direct access to the Apple ][ memory (handy for graphics). However, the design was not completed.

Back to my retrocomputing page

Back to my home page

Last updated July 21, 2002

eric@brouhaha.com

check now