Blown talon srx modules

[GregGarner]

I have a new theory:

Last night it occurred to me that our team is probably doing one thing quite differently than any other team out there. We elected to make our own encoder/limit switch cables, rather than using the pre-made cables that come from Vex. The reason we did this is because our team is cash limited, and we could save a good chunk of change making our own cables.

Therefore we bought the .025 ribbon cable in bulk, and purchased the correct .050 spacing dual row IDC connectors for that cable. We then proceeded to make our own cable to go all the way from the Talon SRX out to the Encoder, which in some cases ended up being 8 feet long. We then terminated the ribbon cable with the correct 5 pin connector into the encoder.

Since the ribbon cable has all 10 conductors in it, this strategy implies that every signal on the Talon is being routed all the way out to the end of the 8 foot cable, even though there are some signals we didn't need. In particular, the 3.3V signal is connected to the ribbon cable. The 3.3V signal is not used by our encoder, and it is not terminated at the end of the cable. This effectively means that we have a 8 foot antenna on the 3.3V wire, which could pick up any signal in the area and connect it into the Talon 3.3V rail.

In addition, we put the motor wires into the same wiring duct with the encoder cables. This means that the current pulses in the motor wires could be inductively coupling into the encoder ribbon cable, and in particular the 3.3V line going straight to the Talon SRX CPU.

We have noticed that out of the 5 blown Talon SRX modules, at least 4 of them were on the longer encoder cables. We have not seen any Talon SRX module have a problem when it had a shorter encoder cable (i.e. The Talon SRX modules are physically closer to the drive motors so the cables are shorter. These closer Talon SRX modules have not failed).

We have seen the Talon SRX modules fail when functioning, and also when not functioning but we have been manually rotating the motors by manipulating the robot. Again the failures are only seen when there are long cables to the encoder.

My goal at this point is to shotgun the problem and try lots of different things at once to try to stop the problem. We have one week to bag and tag, and so it means we can't debug the problem in the standard way of changing one thing at a time.

I am going to do at least one more test before tearing apart the wiring. I will put a digital scope on the 3.3V line at the Talon and look and see if I can see any significant induced spikes on the 3.3V line when moving the motor under program control and under manual control, on the talon SRX with the longest attached ribbon cable. I will report back on what I see.

In order to try to stop the problem from happening, we intend to use shorter ribbon cable out of the Talon SRX, and then splice in a more robust wire to go on out to the Encoder. This means we will not be routing out all the unused signals from the Talon SRX to the rest of the robot, and in particular we will not route out the 3.3V line. Also, we will run the encoder cables in a separate wiring ducts, so there is physical separation between the encoder signals and the large current power wires for the motors.

Finally we do intend to follow industry standard ESD protocol as part of the shotgun approach.

I want to emphasize again that the guys at CrossTheRoadsElectronics have been very helpful, and are replacing the blown Talon SRX modules. In addition they are assisting us with trying to understand why this is happening. They have also sent us some cables and breakout boards to try, since that is what other teams are probably using.

GregGarner
Team 3612