Is anyone aware of a "motherboard" for lack of a better term, that can have multiple Arduino Mega 2650 shields (and the Mega itself) plugged into it, to allow you to "break out" the boards to facilitate troubleshooting? In other words, I want to "unstack" a group of modules and have them laid out so I can access all of them at once with a logic probe, multimeter, etc.
-Bill
Fiddling with Pc for 30 years the name "motherboard" is well known.
I'm afraid that You will need to create that "motherboard" Yourself. What would be the connection pins between the Megas and the motherboard?
While possible, you'll need a massive custom PCB to facilitate all of the modules. You can create PCBs with the software Eagle. Due to how big the PCB will need to be, the PCB will probably be very, very expensive to fabricate.
Why do you need to "unstack" and troubleshoot?
Might be easier to make a smaller pc board to shift a single shield to the side, then chain those boards together for multiple shields. You may get into trouble when the distance gets too great, or you get too much crosstalk between adjacent pins.
If you insist on using the word "shield", it is clearly simpler to go vertical, as intended by the makers, than horizontal, and I don't think the need, real or imagined, of access is anywhere near as bad is you think it might be. The worst that can happen is that you may be obliged to put dummy long-pin headers under short-pin boards.
The idea of spreading them horizontally is just absurd, and may even cause self-inflicted grief leading you to actually need the multimeters and logic probes that may otherwise have been unnecessary.
If this is actually just to demonstrate an intellectual exercise, the components on shields are all available individually or on breakout boards. These could all be soldered onto a perf board, along with a Pro Mega, which has standard pin-spacing. Otherwise, read reply #2 again - last line first.
I could see a Mega shield, three Megan’s wide, that lets you put two shields off to either side of the Mega, and then let you stack more boards like that above it. Then you could have each pin jumperable to be connected or not to resolve any pin conflicts with shields.
"Is anyone aware of a "motherboard" for lack of a better term, that can have multiple Arduino Mega 2650 shields (and the Mega itself) plugged into it, to allow you to "break out" the boards to facilitate troubleshooting?"
Sure, just get a small piece of thin plywood (or thick cardboard) that the mega and the shields will fit on side by side, and tack them to the plywood (cardboard) with a little bit of hot glue. The mega and most shields have top side pin headers where simple pieces of hookup wire can be used to connect any of the header pins. I had a small piece of plywood setup that way with a mega and two arduinos just to try different things. Include a small USB hub for them to connect to, and that connects to the PC.
culturedropout:
Is anyone aware of a "motherboard" for lack of a better term, that can have multiple Arduino Mega 2650 shields (and the Mega itself) plugged into it, to allow you to "break out" the boards to facilitate troubleshooting? In other words, I want to "unstack" a group of modules and have them laid out so I can access all of them at once with a logic probe, multimeter, etc.-Bill
Hi, are you going to use the "mother board" prototyping, POC exercise?
If so then you could possibly use a group of protoboards to hold your shields and external circuitry.
Tom... 
Railroader:
Fiddling with Pc for 30 years the name "motherboard" is well known.
I was thinking of more like an S-100 motherboard, which is what my first system used. 
Power_Broker:
Why do you need to "unstack" and troubleshoot?
This particular setup (which would probably have better been done on something other than shields, in hindsight) consists of an Arduino Mega 2560 and four shields. All four shields are pretty well populated wall-to-wall, as well as some ribbon cable headers along the front edges to connect to the test fixture they're going to drive. As I designed and built all the circuits myself, and have found a few bugs down the road, I have to unstack them, troubleshoot, fix, and restack. I'm not sure how many more repetitions either the boards or my nerves have left...
Nick_Pyner:
The idea of spreading them horizontally is just absurd, and may even cause self-inflicted grief leading you to actually need the multimeters and logic probes that may otherwise have been unnecessary.
Thanks for sharing your opinion. :-\ It doesn't really help answer my question, however.
