Awesome stuff guys!

Yes, when I got in and tried the thought that I had of just clocking in the set up register stuff first:

Code:
nCSA = 0                'Enable MCP23S17-A
    SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA]   ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATA]    
    SHIFTOUT SDO, SCLK, 0, [VoltageFst]     ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageFst]    
    nCSA = 1                'Disable the MCP23S17
It worked just like Tabsoft and Tumbleweed both surmised (or tested!) and yes, the byte order was wrong as well, so I just swapped the two alias bytes:

Code:
    nCSA = 0                'Enable MCP23S17-A
    SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA]
    SHIFTOUT SDO, SCLK, 0, [VoltageSec]
    nCSA = 1                'Disable the MCP23S17
    pause 1
    nCSA = 0                'Enable MCP23S17-A
    SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATB]
    SHIFTOUT SDO, SCLK, 0, [VoltageFst]        
    nCSA = 1                'Disable the MCP23S17
And that gave me the full picture!

So yay for my tired little brain at least showing some spark of life there with my hunch! Sometimes it's just good to put stuff up here to get the right thought patterns going.

And thank all of you for helping to fill in the blanks! You guys rock!

Now to get to tackling all the other stuff this thing needs to do.

Like I posted above, the PIC is handling a setup byte and then a 16 bit value that ends up setting a 16 bit DAC at the other end of a fiber optic link. The card my circuit is talking to is an OEM ADC/DAC card that uses two pulse widths to represent a one or zero. The widths are tight and the entire pulse train is pretty narrow, so the PIC couldn't begin to help with that. So its a mess of shift registers and one-shot chips that do the critical timing, the PIC presents the full 24 bits to a string of parallel shift registers via the MCP23S17's and sends an enable pulse to transmit when everything is set.

I'm working on another card that does the RX side by turning the pulse widths back into parallel data, and then I'll read them with a MCP23S17 again. Just 16 bits on the return.

I'll likely end up redoing the board for the send side since it's got some "blue wires" going on, and the fact it requires some trickery in code to map everything just right. But at least I can do some more debugging with it now.

My major mistake was the fact that I breadboarded the digital logic circuitry to figure that all out and never bothered to do the PIC stuff on there as well!!!

I've learned my lesson!

Thanks again guys! You are a wealth of knowledge and way more help than anyone could hope for!