Quote:
Originally Posted by GregGarner
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.
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It can be very difficult to get useful information doing this. Your scope and it's probe can inject noise into the system since it acts like an antenna too. How you ground your scope probe to the system is critical to how clean signal you get. A lot of what you may see may be induced in the ground lead of the scope probe. You are probably better off putting a high-frequency current probe around the cable coming out of your Talon since the current probe will not be Gavanically connected to your system.
Secondly, the 3.3V line is probably a power supply line. The impedance looking into the Talon on that line will probably be quite low making it hard for any outside interference source to have a significant influence on the internal circuitry of the Talon. Basically the internal impedance of the 3.3V line forms a divider with the impedance coupling to your external noise source.
Quote:
Originally Posted by magnets
Interference is no longer an ESD problem.
Also, a wire that isn't terminated at one end isn't likely to result in much inductive coupling, and it will be much, much less than a wire that's terminated at both ends. Having it terminated at only one end will make inductive coupling almost disappear. There's no effective loop area if you don't have a loop!
You may have parasitic capacitive coupling though.
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Yes.
Quote:
Originally Posted by s1900ahon
Greg,
Could your custom cables be shorting adjacent wires and therefore be shorting things within the Talon? If misaligned, this could be a problem since they're insulation displacement connectors.
Moreover, if these cables are faulty, you'd continue to propagate failures.
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Quote:
Originally Posted by GregGarner
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.
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Making a new cable assembly may make the problem go away if there is a short in your existing cable; i.e. the new one does not have the same fault. You can use the same ribbon cable material and the same IDC connector to make a cable with just the wires you need. The "more robust wire" will not be any more resistant to external interference. If the pins in the connector that you need are all adjacent, then rip the ribbon cable to have only the number of wires you need then crimp it into the IDC connector. If the wires you need are not adjacent, then crimp a width of ribbon cable that encompasses the wires you need, use a sharp knife to split out the wires you do need from the ones you don't need and cut off the ones you don't need near the connector.
You may want to twist the new cable assembly (2-4 twists per inch) to make it more resistant to external inductive noise.
Running the cable so that runs that are parallel to power wires are 2-3 inches away from the high power wires can also help reduce noise coupling.
Because FIRST wants everyone to have a sense of accomplishment even if you didn't accomplish anything during the match. - Koko Ed [more] 
10-02-2015, 12:48
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