[Coco] Educate me about Y-Cables

[RETRO Innovations]

On 2/20/2015 4:20 PM, Gene Heskett wrote:
> On Friday, February 20, 2015 02:48:04 PM Joe Grubbs wrote:
>
>
> ===============Caveat=================
>
> Because tieing all 4 of these lines together also ties all 4 of the pullup
> resistors together, the output gate in the individual pack is heavily
> loaded, so much so that you can see the interrupts as bars in the gime's
> video output, it shakes the supply lines that badly.
>
> Most MPI's had 4.7k pullup resistors, and when all 4 are trying to keep
> the line high, the pack has to sink around 50 milliamps.  So the common
> practice when putting in the jumpers, is to locate those 4 resistors along
> the edge of the MPI's board, and clip one end of 3 of them, leaving only
> one, which is much easier on the pack doing the pulldown to exert the IRQ.
> Not all MPI's had 4.7k there, some had 2.7k and I ran into one way back
> that had 27k resistors there.  All are adequate to get the job done, and
> in terms of power supply loading, the higher the better. A single 220k
> might even be enough, the recovery time at the end of the IRQ would be the
> controlling factor.  It is buffered in the MPI's logic anyway so its not
> like you are driving 2 feet or wire to actually arrive at the 6x09 socket.
It would seem that making a new byte in the memory map able to select 
this behavior would be of interest on a newer MPI replacement option.  
Seems like a CPLD could do:

assign CART = ((!CART1 + !CART2 + !CART3 + !CART4) ? 0 : 1'bz);  // 
Verilog to set CART to 0 if any CART lines are low, Hi-Z otherwise)