PWM

[jmiller18]

How many jags or other actuaters that use pwm's can share a +5 and ground for pwm, ie run 1 +5 and 1 ground and however many signals needed for the actuators???

[flargen507]

Really you should only have one device per power input/output. The rules read specifically on Jaguars that you cannot connect any other devices to a Jaguar PWM port. This really applies to all large motor controllers like Victors and Talons at inspection. My initial response is that you try to use only one power slot. I have yet to see an official ruling on motor control PWM splitters but I try to stay on the safe side.

I can see two reasons why you would need splitters. Either a) the programmers can't get 4 PWM ports to open, or b) you are running out of PWM ports or wire space on your robot. If the former, tell the programmers to use an open 4 motors block in the WPI robotics library. If the latter, there are several things you can try. First, make sure all motor outputs are on the open side that says PWM OUT and that all digital sensors are on DIGITAL I/O ports. You should have 14 of each, which I have personally found quite enough. If it isn't though, you always have the option of a second digital sidecar. If you don't have the crio slots or the board space, however, I would then try the splitters. Note: by splitting PWM signals you may be increasing the risk of magnetic interference from your other electronics.

[Mark McLeod]

Can't agree with the previous post. You'll need to quote a rule on PWM limitations.
FIRST supplies us with PWM "Y" splitters in the KOP just for this purpose.

The recommended max for the signal line is probably 2 of the same controller type, i.e., (2) Jaguars or (2) Victors or (2) Talons. Here's a post on the topic by the Digital Sidecar designer. The Y cables they give us share the signal and ground between two speed controllers, so (2) obviously works.

Jags/Talons/Victors don't use the power from the +5v pin on a PWM output from the Digital Sidecar, so your question might be a bit simpler. Power for those devices comes from the 12v feed they get directly from the Power Distribution Panel.

[Alan Anderson]

None of the FRC-legal motor speed controllers require +5 volts. The only pins they use are SIG and (-). I'd personally avoid trying to share the (-) return wires, but I don't think it would be a problem electrically to do it.

Why are you asking? PWM cables aren't that heavy, are they?

[apalrd]

There's no reason to not Y two motors mounted to the same gearbox. You would then reduce the number of PWM ports you are using, and reduce the number of connections around the DSC.

Our 2012 robot had 17 PWM devices and a single Digital Sidecar (10 PWM outputs). Thus, we had 7 Y-cables. We also ran 3x Victors on a single PWM port and never complained about it.

[efoote868]

Originally Posted by apalrd

There's no reason to not Y two motors mounted to the same gearbox. You would then reduce the number of PWM ports you are using, and reduce the number of connections around the DSC.

Make sure they're running the same direction. Otherwise you'll have a pretty nice oven.

[Al Skierkiewicz]

j,
I would not recommend you group PWMs in such a fashion. The common wiring will make each input susceptible to noise currents present on other devices. You end up with causing unrelated failures in other devices. Individual PWM cables bring all this noise and interference back to a common, low impedance point on the DSC. However, you could position the controllers in the robot and not on the arm eliminating the need to run PWM signals to the arm. It doesn't matter where in the path the controller is located.

[Ether]

Al, what post are you responding to? Your post is linked here:

http://www.chiefdelphi.com/forums/sh...3&postcount=11

... but I don't think that's the one, is it?

[jmiller18]

Ok then thank you.

Originally Posted by Alan Anderson

Why are you asking? PWM cables aren't that heavy, are they?

We are thinking about doing an arm and i know we have cable carriers but those things are impossible to work with and limit the number of wires we can use so if we can have 1 +5 1 ground and a few signals, then that would make my life oh so much simpler

[Joe Ross]

Originally Posted by Alan Anderson

None of the FRC-legal motor speed controllers require +5 volts. The only pins they use are SIG and (-). I'd personally avoid trying to share the (-) return wires, but I don't think it would be a problem electrically to do it.

The Vex Motor Controller is legal, and uses the power pin.

[Alan Anderson]

Originally Posted by Joe Ross

The Vex Motor Controller is legal, and uses the power pin.

Yeah, you're right. I guess I tend to think of the Vex motor in the servo category.

(It still doesn't use +5v, though. It uses +6. )

[cgmv123]

Bottom line: don't split power leads or drive 2 high power motors with the same controller. Split the PWM into 2 separate controllers on separate circuits.

[jmiller18]

Also, what is the gauge of a standard pwm cable??