R54 and R69 motor control and PWM servos regards to the MXP port

Technical Electrical
1

The team this year maybe going over the 10 PWM ports on the main breakout of the Roborio. They are planing what channels the motors and servos will be assigned to to be sure they have enough. Quite possibly they will be using a few small RC-servos to preform some small tasks along with the usual Talons ect.

R54 states servos MUST be connected to the PWM ports on the roboRio and MUST NOT connect to the MXP unless using a WCP Spartan Sensor Board.

R69 states if a motor is controlled via the MXP its power regulation device must be connected by one of the following: a: directly to any PWM pin, B…, C…

I understand a Motor is different than a Servo.

  1. Is a motors power regulating device the Talon or speed Controller?

  2. should the team just plan to put servos on the PWM breakout pins first and any over flow speed controllers directly to the MXP?

3.a. Does the MXP PWM pins simply not have the driver circuit to run a servo since they are multi purpose?

3.b. Does a WCP Spartan Sensor Board have special singnal conditioning ie logic shifting to protect what ever is on the other side of the MXP

2

1 - yes.

Regarding the rest: Servos take more current, and there is not enough current from the RoboRio via the MXP port. According to the description, the WCP Spartan Sensor Board has a dedicated power supply, so that must be able to provide the additional current required to drive servos.

3

When you hook up a motor controller like the REV SPARK or any other (talon, victor, etc) it does not use the power pin from the MXP, just the signal and ground. Which is why it is totally safe to use the pwm in the MXP port using a breakout board like the MoreBoard.

If you are running servos, I would just use the standard PWM ports instead of buying a $100 + board just for this purpose (unless you need and want it’s other functionality.

4

The standard PWM outputs on the RoboRio have a power connection to supply the ~7 volts needed by servos. It is limited to 2.2 amps as I remember. That will limit your use of servos. You need to stay under that specification. As Greg has point out above, motor controllers do not use the power connection that is required for servos and therefore may be better wired to the MXP port PWM outputs.

5

So then using the PWM pin in the MAP or Standard PWM ports to drive a motor controller we only need two wires. ( signal and ground)

6

Yes, however, it is far easier (and more reliable) to use the three wire standard PWM cabling and connectors.

7

Can you still leverage the VEX voucher to get 2 Talon SRXs?
http://www.vexrobotics.com/pdv-2016.html
That would be two more CAN motor controllers, that don’t need to use PWM channels. And they are FREE.