The Polysix-M CPU board

Here are the overall schematics for the CPU board. (These schematics were drawn by Martin Wise, who kindly let me use them on this page. Please note that if you choose to build the retrofit using his PCB design, there are updated schematics with the artowrk files). Basically, this is a standard run-of-the-mill microprocessor system. The following main components can be identified:

  • A standard Z80A CPU, clocked at 4MHz.
  • A standard 27128 EEPROM (although in practice the code will fit in a 2764, which can also be used)
  • A standard 6264LP CMOS low power RAM, battery backed up. The Polysix battery is utilized for the battery power.
  • A 6850 UART (or ACIA as Motorola prefers to call it) for the MIDI communication. It is clocked at 2MHz, and can interrupt the CPU using its INT* pin.
  • Two 8255 24-bit parallel I/O ports. In principle, the first 8255 (8255-I) is connected to the 8048 socket in the Polysix using a 40-pole ribbon cable. Similarily, the second 8255 (8255-II) is connected to the 8049 socket in the Polysix using another 40-pole ribbon cable.
  • The 5V power for the new CPU board is derived from the 8048 socket.
  • A TIL111 optocoupler, for incoming MIDI.
  • A 74LS04 hex inverter, which is utilized as a clock oscillator in conjunction with an 8 MHz crystal, as an inverter for the reset signal, and as a MIDI buffer.
  • A 74LS00 quad NAND gate, which is used to generate the E signal for the 6850 from the CPU RW* and RD* signals, as well as for MIDI buffering.
  • A 74LS139 dual two-to-four line decoder, used for memory and I/O decoding
  • Two 74LS393 dual four-bit counters, which are used to generate the clock signals from the crystal oscillator. The output signals are 4 MHz for the CPU, 2 MHz as a time base for the MIDI communication, and a (roughly) 2 kHz time base which is fed to the CPU Non-Maskable Interrupt (NMI*) pin.

    The board is attached to the Polysix with the following connections:

  • One 40-pole ribbon cable plugged into the Polysix 8048 socket (header1).
  • One 40-pole ribbon cable plugged into the Polysix 8049 socket (header2).
  • One wire running from the battery positive pole.
  • A 10-pole ribbon cable going to the MIDI DIN jacks on the back of the synthesiser. It terminates in a 14-pin DIL socket on the CPU board (header3).

    A couple of comments on the schematics. First of all, some of the discrete logic chips have been written as ALS chips; the retrofits I've built use LS chips throughout. Also, the MIDI connector header3 is actually a 14-pin DIL socket. You can of course use any connector you want as long as you get the signals right.

    All in all, it's not a terribly complex circuit, but it is easy to get lost in all the wiring, so if you're building it by hand on veroboard, I've made a series of netlists for the different signals and power lines on the CPU board. There's also a list of pin numbers and signals for the wiring between the two 8255 I/O chips and the Polysix 8048 and 8049 sockets. Also, there is a detailed drawing of the MIDI connections, which you'll probably need even if you're building the PCB design. (Please note that the pin numbering for the CPU board's MIDI connector in this diagram does not match the numbering on the schematics above. Note also that the middle pin of the OUT and THRU connectors must be connected to ground, whereas the middle pin of the IN connector must not be connected to ground.)

    Veroboard-based design

    This is what the CPU board looks like when built on a 160x100 mm prototype board. Note that the 220uF filter cap had not been mounted when this picture was taken, and also that sockets were used for all large chips. 99% of the signal wiring is on the top, and all power supply wiring is on the bottom of the board. I used copperless veroboard (no copper strips or 'islands') for the prototype, a method I favour for prototype building. Note that to save space, the two '393 counter chips have been piggybacked.

    CPU board picture

    The chips are, from top to bottom: to the left, the Z80, 2764/27128 and 6264. In the middle, the 6850 and the two 8255's. To the right, the '00 (with the TIL111 at its side), the '193, the '04 (with the crystal visible to its left), and finally the two piggybacked '393's. At the far right is the 14-pin MIDI connector.

    This is what the board looks like with the ribbon cables attached. Note carefully the orientation of the 40-pin connectors, and the lengths of the cables: in the picture, the left 40-pin header has its pins upwards, and the right header has its pins downwards. Also, the unused wires in the 40-pin cables have not been cut yet, lest something has been incorrectly wired. They should be cut before final installation.

    CPU board with 40-pole wires

    This is what it looks like when installed in the Polysix. In order to suspend the board above the existing circuit boards in the bottom of the synthesiser, I used 1mm piano wire which I bent into shape and attached to four of the existing mounting screws. Note how one of the ribbon cables wraps around to plug into the 40-pin 8048 socket underneath the board.

    CPU board mounted in Polysix

    Note: in the pictures there are two diodes and a pullup resistor connected to the CPU reset pin. These are only needed when used with my Z80 development system, and are not included in the schematics or netlists.

    PCB-based design

    In order to build the CPU board using Martin Wise's PCB layout, you'll have to download a .zip file of the artworks. Within the .zip file, you'll find images of the copper traces and silk screen for the board, updated schematics and some notes on how to assemble it all, including a list of components. Once completed, the CPU board will look like this. It is mounted in the same position as the veroboard-based design.

    Martin's CPU board

    This page is (c) Copyright 1999 by Ricard Wolf

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