Using a PWM cable splitter for 2 Jaguar motors

[Randy Forgaard]

We are using one of the AndyMark gearboxes that allows the use of 2 CIM motors at one time for extra driving power on a single side of the robot.

We are starting to run out of PWM channels on the Digital Sidecar, and we would prefer not to use a second Digital Sidecar, to save on some space.

One possibility is that we could use an off-the-shelf PWM servo splitter cable, so that one channel of the Digital Sidecar could control both CIM motors that are used in a single gearbox, since both CIM motors need to receive the same signals anyway. Then, we would simply program our robot as though it has two motors (one on each side) rather than four motors.

Can anyone think of a reason why we shouldn't use a PWM splitter cable for this purpose? In this thread of the Q&A forum, the Game Design Committee says that there is no rule saying you can't do this, but they suggest there might be better technical alternatives.

Any thoughts, yea or nay? Many thanks!

[NOV8R]

The better technical alternative is CAN.

[apalrd]

It will work fine, we are doing it and have done it in the past. The point of having a Y-Cable is to drive two Jaguars or Victors or Servos that must be run continuously, such as when using said AM transmissions. With one level of Y cables (two Jaguars or Victors) you will be fine, but with more the signal gets too low to drive all of the inputs on the Jaguars or Victors. So if you have two motors, its fine.

You could also use CAN, but thats more work to setup if you already have everything on PWM and impossible using Victors.

[Alan Anderson]

The Q&A response was about a 3-way splitter. I'd have given a similar answer -- it seems strange to talk about having three separately driven motors just because you have three wheels.

Using a two-way splitter is not unusual for your application. You don't even have to buy one. Two of them were included in the Kit of Parts (see page 12 of the KoP checklist).

[Randy Forgaard]

Chuck, Palardy, and Alan: Thanks so much for all your responses, they were very helpful.

I'm glad to hear it's not unusual to use a 2-way PWM splitter to drive 2 Jaguars from a single PWM output on the Digital Sidecar. That will help us avoid using up all the PWM outputs on the Digital Sidecar and thus avoid having to use two Digital Sidecars, which would mean using more room on our robot control board (already a bit squished for space this year).

Alan, I had forgotten that we received those two 2-way PWM splitter cables in the Kit of Parts. Thanks for pointing that out. Emphasizes that it's very common to do this. We found those two PWM splitter cables in our servo cable inventory, and I thought we must have purchased them at some point and forgotten about it.

Indeed, as you folks point out, moving to use CAN for the Jaguars would be another excellent way to avoid using PWM slots on the Digital Sidecar. We worked on doing that for a couple of days earlier this season, but even though we carefully read all the info at www.luminarymicro.com/jaguar about using CAN with Jaguars, read the threads here at Chief Delphi on it, and carefully created the required adapter and cables using the approved DigiKey part numbers, we for some reason were not able to get the "bdc-comm" application to "see" the black Jaguar so we could update its firmware. We finally had to set the project aside so we could focus on building and programming the robot. However, CAN would be very cool, give us more Jaguar control, give us plenty of extra PWM slots on the Digital Sidecar, and provide an excellent educational opportunity for our team on an important and often-used industrial network technology, so we will work on CAN again in the off-season, and hopefully use it for next year's robot (if the rules still allow it).

Thanks again!

[Teched3]

Randy,
Just some additional info regarding PWM plugs and circuits. Many teams have found they can disengage during competition (not a good thing), and use silicone adhesive/caulking to help ensure they remain retained in their sockets. Doesn't need much, and they can still be removed for repair or troubleshooting.

In addition, the PWM outputs, if I remember correctly, only put out about .75 amp of current. Since we have doubled up servos and have run Y cables so the servos operate simultaneously (transmission shifting), there is a limit to how much current the PWM output can deliver. This will not be a problem when used to signal two Jags, but can be if you are using them on more than two servos which will require more current.