PWM Connection Legality?

[Toa Circuit]

Would it be legal to connect just the PWM signal and ground from the Digital Sidecar to motor controllers? I know it works, but would it be legal?
Also, just as a thought, would it be possible just to connect the PWM signal (since everything is commonly grounded, right?)? (Would this be legal?)

[MaxMax161]

It seems like the 2013 manual is no longer up and I don't have a copy handy. I'm not sure what it says specifically on the matter but I would expect it to be strongly discouraged if not illegal. I think it's also something an inspector could take issue with at an event. If I find a copy of the game manual I'll update you.

[EricH]

If I find a copy of the game manual I'll update you.
Look in the archived documentation, same place on the FIRST site as the Manual.

[DarrinMunter]

The 2013 manual is still on the FIRST web page it's just under the archived section: http://www.usfirst.org/roboticsprograms/frc/archived-game-documentation-and-event-results

I know that you must control all motors using a FIRST approved motor controller or spike relay. The only execption is controlling a servo. Then that is connected directly to the side car, with the jumper added to put power to the PWM cable.

[MaxMax161]

Look in the archived documentation, same place on the FIRST site as the Manual.

Thanks didn't know about that!

Anyway quick search through the manual leads me to this

4.1.9.13 R67
Every relay module, servo, and PWM motor controller shall be connected via PWM cable to the Digital Sidecar and be
controlled by signals provided from the cRIO via the Digital Sidecar. They shall not be controlled by signals from any
other source.

I think the issue would be an incomplete PWM cable is not a PWM cable and therefore illegal to connect speed controllers and the digital sidecar with.


Out of curiosity what brings up your question?

[Ether]

FWIW, inside the motor controller (unlike a servo) there is no connection to the PWM power pin.

[AustinSchuh]

FWIW, inside the motor controller (unlike a servo) there is no connection to the PWM power pin.




In addition, the PWM ground and signal inputs are connected to an opto-isolator. This is at least how it is implemented on the Jaguar for sure, I have seen evidence on the Talon that this is also true, and I would expect the same for the Victor. This means that ground on the PWM cable isn't the same ground as the speed controller ground. Which is a good thing, since it breaks what would otherwise be a nasty ground loop.

[Al Skierkiewicz]

You would severely limit the current available for the motor controller by using the DSC as a power common. Not only that, the DSC would then be forced to generate noise as the DSC common varies with motor usage. Remember there is no "ground" on the robot. There is only power common which is something altogether different. As all motor current returns to the battery through normal (two wire branch circuits) wiring to the PD, noise is not a problem for the DSC, Crio or the radio in most cases.
Also for all the newbies lurking here, the robot frame may not be used as a power pathway. You must provide two wires to all loads connected to the battery and they must be insulated from the frame.

[nuttle]

In addition, unless you place a jumper to get 6V on the power wire (when using a servo, for example), the power wire is not connected at the DSC. Spikes and (I suspect) all motor controllers use an opto-isolator, so by design there is no common ground. You have to use ground and signal but there is no need for the power wire. Note that for a Spike, there is actually one ground and two signal wires and the signals are on/off only (there is no Pulse-Width Modulation happening and this is why there are separate places for Spikes and motor controllers/servos to plug into the DSC), even though you see this called a PWM cable. When wiring a Spike for the compressor, my personal opinion is it is good practice to only connect the signal for the forward direction, so there is no way to run the compressor in reverse (which is very bad for most compressors). When we've made and used custom cables, we always only included the required number of wires and never had this come up in an inspection, FWIW.