[Coco] Educate me about Y-Cables

[RETRO Innovations]

I laid out all of the equations in Verilog tonight, and I believe I have 
replicated the 4 slot MPI functionality with 5 ICs (1 '245, 3 541s, and 
1 xc9572xl CPLD). A couple thoughts:

  * Found some nice triple output brick-like PSUs, so that does not seem
    to be a major challenge.  To make for more options, I put a simple
    2.5mm coax connector on for 5VDC as well and can easily add a
    Mini-USB for 5VDC power.  I think Mini USB wall warts only do 1Amp,
    though, so not sure if that would be enough to run 4 carts
  * I am not convinced more than 4 switched ports are needed, as it
    seems like 5 ports of an 8 port unit could be "shared", on the last
    port, assuming the various peripherals will coexist on the bus (I
    understand the ROMs won't, but an RS232 cart, modem cart, FDC (with
    ROM), and some other simple cart look to play nice with each other
    without switching.
  * That said, I can extend the design to do 8 switched ports (it'll
    drive the IC count up a bit).
  * I need to find a cart that has the grounding tabs on it (the Coco I
    bought only  has a few game carts, which don't have them, so I can't
    measure.)  Any suggestions?
  * Looking at the original MPI schematic, it looks like an external
    peripheral cannot reset the computer, which seems odd, as they would
    be able to if they were plugged directly into the Coco instead of
    into the MPI.

Jim