80C85 and MMU for RC2014 / BP80

An 80C85 CPU card for the RC2014 bus that includes a simple MMU for use with linear memory cards.

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The board is really two boards in one. The MMU and 80C85 whilst sharing some gates can be built as a pure CPU card, an MMU card for use with another CPU card or a combo. The CPU can run on the basic RC2014 bus, the MMU needs the full 80pin extended bus

The 80C85 side is fairly basic, demux the address/data lines and turn a few signals from 808x into Z80 format. It's based on the Ancient Computing 8085 card design except that interrupts are instead routed to one of the RST x.5 lines so they actually work.

The MMU is nothing more than a latch at 0xFF that generates two sets of A16-A19 bits and a 4bit 2 way mux driven from a 3 input and gate (0000-DFFF/E000-FFFF) to select the memory used. At boot time the reset line sets the bits to 0 so the system powers up with the memory card ROM mapped for the full 64K and the boot ROM then moves to RAM.

Tested to the full 8MHz with a Tundra 80C85. It works fine that way but the CF adapter only works reliably at lower speeds. The PPIDE adapter which produces proper IDE timings unlike the CF hack works reliably at 8MHz.

ROM firmware, Fuzix and CP/M 3 are on github.


Zip Archive - 95.21 kB - 04/02/2020 at 17:59



Kicad PCB layout

x-kicad-pcb - 391.39 kB - 04/02/2020 at 17:57


lib - 17.95 kB - 04/02/2020 at 17:57



Schematic (Kicad)

x-kicad-schematic - 24.37 kB - 04/02/2020 at 17:57


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View all 8 files

  • 1 × 80C85
  • 1 × 74HCT573 Electronic Components / Misc. Electronic Components
  • 6 × 0.1µF capacitors
  • 1 × Crystal (2 x CPU frequency)
  • 1 × 20pF capacitor (only for lower crystal frequencies)

View all 13 components

  • PPIDE and real hardware

    EtchedPixels10/05/2019 at 18:32 0 comments

    Some changes to the boot firmware to get the CF card boot up stable. With those I'm running CP/M and Fuzix happily on the real platform. Fuzix also needed some other minor changes.

    All the relevant pieces are now pushed to github

View project log

  • 1

    There should be no surprises other than the orientation of some components.

    If you are building just an MMU then omit RN1, C1, Y1, U1 and U2. If you are building just a CPU omit U3, U5, U6, U7.

    Only populate C1 if you are running at a lowish crystal speed (try without first). Remember that the 80C85 crystal input needs to be twice the clock frequency. You may want to pick double the classic RC2014 frequency for it if you have cards that need the clock 'right'.

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