Right now my team is planning on using 9 cylinders on our robot. The problem is that there is only 8 ports on the PCM. I’m wondering if it is possible to use 2 cylinders on one solenoid port, and would it break any rules. I’ve read the rules and it doesn’t say that I can’t do that, but I just want to double check with this wonderful community.
It is very possible to do this. My team did this last year.
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This is definitely legal. Just remember that all air will be traveling through that 1 port so it will be a little slower.
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Also remember to keep the tubing length the same for all cylinders on one port.
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You can also use a second PCM.
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Two generations of control systems back (the IFI system) we used Spike relays - see the Getting Started with the Control System doc - to control solenoids. Spikes are still legal, and any team that’s been around a while will have a box of Spikes but not know what they are 
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Thank you! I never knew we could use 2 PCM’s. I just reread the rules, and we can do that. 
Please keep in mind that any pistons requiring a double solenoid (ie, any piston which you’re trying to use air to both push and pull) will require two ports each.
I was uninformed. See below.
You can use a single action solenoid to push and pull a “cylinder.”
Hmm, I may be a little uninformed on this topic. We’ve always used doubles; how would you accomplish this with a single?
A single action solenoid will be in one position until activated and switch to the second position when activated. Turn off and it will return to it’s starting state.
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Yep, that makes sense. For some reason I had always assumed that single actions were for things like spring-loaded cylinders. We don’t really have any single action solenoids on hand, so the idea of them still being able to have two outputs hadn’t really occurred to me (though it did seem silly that such a thing didn’t exist, so… I guess I should’ve done more research).
In the context of FRC, then, would the main difference between the two be that single actions will always revert to their starting state when disabled at the end of a match, whereas doubles will hold their state?
Yup. They have used in the past for end-of-match actions like in 2007, some robots would have ramps that would elevate after the buzzer.
But really in FRC we don’t use them enough, most actions can use single action solenoids unless you need your cylinder to stay in whatever state it’s in when disabled or you need to choose what state you want them in before the start of a match such as choosing what gear to start in on a shifting gearbox or being able to manually switch to hold onto something.
You can use one solonoid, as long as you want the two cylinders to be synchronized
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Yep I’m going to put the two gearbox shifters together.
Jumping back to OP - yes, you can use one solenoid valve to actuate two cylinders or other actuators. Note that the solenoid valves have a limited amount of airflow because of the port size limits, so if you do this, it has to either be cylinders where you don’t mind them filling at half speed, or cylinders which are small enough that half speed is still pretty fast. (Shifters are a perfect example of them being small enough.)
What is NOT allowed in this sort of realm is combining the outputs of two solenoid valves to drive one side of a cylinder (or for any other purpose which combines those airflows) per R94.
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