If I have two Spikes to control an RGB LED strip, is it legal for them to share a ground connection on the output of the Spikes? I don’t see any problems with this electrically, I just want to make sure it doesn’t violate any rules. I couldn’t find anything about this, so if you could share your source, that would be much appreciated 
While you could probably argue that sharing a ground would not constitute controlling an electrical load, it would be much better to stay on the safe side and wire them separately if possible. Most likely it would depend on the inspector as to whether it is deemed legal.
Look again.
The Spike is an H-bridge relay circuit. It does not have a unique ground output.
To clarify, the H-bridge circuit has 4 permutations, since the White and Red wires stand for Fwd and Rev respectively. You can see this in the User Manual.
http://i.imgur.com/uHjsO1Q.png
If M- is pulled to +12V (Reverse, or Forward+Reverse), you will short out your connections if you try to ‘share’ a ground.
This is a GREAT example of why the only “dumb” electrical question is the one not asked…
-George
IMO the only dumb question is, “Is this a dumb question?”
“ground” is a concept that is not really applicable to FRC robots, outside of designing circuit boards.
I agree.
The terms I prefer are “battery return”, or “battery minus”, for the black wire connections.
“Chassis”, “frame”, or “dead metal” are the nearest terms we have to “ground” on an FRC robot, and the electrical system is required to be isolated from that.
Alex,
While there is no robot rule that expressly addresses this, standard electrical practice dictates that it not be done. By now you have seen that the output of any controller, (Spike or motor controller) has no fixed connection to the negative lead of the battery. Jaci provided a chart of the possible outputs of the Spike so you can see that it is possible for both outputs to be either flavor, positive or negative with respect to the battery.
Jim is expressing the principle that there is no “ground” on the robot because there is no conductor that is attached to the earth. The correct term is “return” in this application.
One issue with sharing the return between multiple spikes is that return wire should be sized for the maximum load. IE the sum of the positive feeds from (PD board x CB size) So spikes controlling motors should have individual returns to the PD board.
LGB lights are custom circuits. If you are feeding them through multiple spikes from single branch circuit then you will have to common both the supply and return since you are only allowed one wire in the wago PDB connector. The spike in this case would be a custom circuit rather than a motor controlling spike.
To clarify the spike connections a little. Although the spike outputs can be either + or return: there is only one + battery connection to the spike and only one battery return connection. When using the spike to control two separate outputs (IE 2 LED light strings) the returns do not go through the Spike.
Thank you for your help everyone! I saw in the Relay class each output could be set to 12v or ground, but I wasn’t sure if it applied to the spike since the outputs are labeled M+ and M-. Thinking about it now, the other motor controllers are labeled like that too, but the motors can be run either forwards or reverse, so I guess it makes sense. ¯_(ツ)_/¯
Next time I just need to read the datasheet 
Just to clarify one more thing, does this mean I can hook up the outputs of the spike to the red and blue channels, and the ground/return to the ground/return on the side of the spike connected to the PDP? Or would I wire it to another channel on the PDP with the positive side empty (this sounds very wrong to me).
I’m not entirely sure what you are trying to achieve, but from what I gather you have an RGB strip that you want to change color from R to B. We did a similar thing a few years ago with a spike and the R was wired + to + and - to -. The B was wired in reverse + to- and - to +. Then you set the Spike forwards to do red, and backward to do blue, neutral to do off. Not sure if this helps, but if should work.
While the black wagos on the PDB are all internally connected, all the current from the red channel should return to its corresponding black channel. If for no other reason that it satisfies my sense of completeness. So no. You should not have a wire going to the black wago with its corresponding red Wago empty.
Ok, thank you!
Now that the SPIKE is discontinued, will anything take its place?
In our team’s case, no. We have a box of them left, so we can spare one 
In general, the Spark is a full motor controller, not just a relay, and it’s only $10 more than the Spikes were. http://www.revrobotics.com/spark/
Alex,
Spike application notes show a way to connect both outputs of the Spike to different controlled devices and use a common return. This is one method to use the full features of the device. However, when powering high current loads like the compressor, I would not do this. Event the little Thomas compressors can draw near to 20 amps during start. Addign another electrical load may put this over the spec of 20 amps continuous load for the Spike.
And in the case of the compressor, its against R35.
Unless otherwise noted, each power regulating device shall control one and only one electrical load.
The compressor row does NOT have such a note.