[Coco] MPI Progress

[RETRO Innovations]

Found and fixed a few issues in the board over the weekend:

  * XC9572XL parts are 5V tolerant only if powered.  If someone should
    power the Coco with the MPI connected but unpowered, any line
    directly connected to the Coco bus will be at 5V and could damage
    the CPLD.
      o All but 2 pins are behind buffers:  SLENB and CART.  Buffers are
        powered by the MPI PSU, so only the two signals are of concern.
          + CART is a CPLD output, so I used one of the remaining
            buffers to isolate the line.  Since the buffers are not open
            collector, I put a diode on the output facing towards the
            buffer so the buffer can only drive the line low.  Since the
            CART line can be disconnected from the MPI slots, I put a
            pullup resistor on the buffer input to ensure a normally
            high state
          + SLENB is a CPLD input, so it was only necessary to put a
            diode facing away from the CPLD on the line.  a low level on
            SLENB will drive the SLENB input of the CPLD to .2V (when
            using a schottkey diode).
  * Beefed up ground returns for better signal clarity

I sent the board design off for final quote and manufacturing of 5 
prototypes.

Jim


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