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Unread 18-02-2011, 22:56
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Would This work? wiring pwm question

I wanted to ask CD before I went ahead and started wiring my pwm cables and potentially messing something up. Would it be ok to splice two relay pwm grounds together? The reason I want to do this is because i have a bit of 8 conductor 24 awg cable. If I spiced the grounds for 2 pwm cables running to spikes I would be able to run pwm cable for 2 spikes and one victor though this one cable. Heres a quick doodle in paint if people don't understand.
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Unread 19-02-2011, 02:51
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Re: Would This work? wiring pwm question

Yes, but....

Yes, this would work. All the grounds are tied together inside the cRio at some point anyway, and there is no significant current travelling along the line, so I can see no technical or engineering issue that would prevent this from working.

However I do note

<R57> Every relay module, servo, and PWM speed controller shall be connected via PWM cable to the Digital Sidecar and be controlled by signals provided from the cRIO-FRC via the Digital Sidecar. They shall not be controlled by signals from any other source.

This is more intended to explain that the cRio has to control the relays and speed controllers, however it also says "shall be connected via PWM cable". The catch is that it doesn't define PWM cable. Is PWM cable the standard Red/White/Black three conductor cable, or is PWM cable any cable that carries a PWM signal?

I can see an inspector claiming that if it isn't standard, unmodified "PWM cable" similar to that supplied in the KoP that is is not PWM cable. While I would interpret things differently, I also couldn't say that the other inspector's interpretation would be "wrong".

So I think you are correct in assuming that this wiring plan would work, but you have good reason to be concerned about taking it to tech inspection. My suggestion is to a) Ask the Q&A system for official guidance b) Bring "PWM Cables" of the correct length, in the event a ruling on this does not go your way or c) just use the standard cable if you've got it kicking around.

Jason
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Unread 19-02-2011, 04:41
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Re: Would This work? wiring pwm question

While it may or may not be legal, I would not do it either way. Whenever you put a bunch of wires into a big cable, you have problems identifying which one is which. If two PWM cables somehow come out (common in spikes), you don't want someone plugging them in backwards and then deploy when you try to open your claw.

It's best to have each wire be routed along its own path. That makes you absolutely sure that it's the right wire without looking at colors or reading tags, and looks really nice. However, if your relays and things are a ways away from your Digital Side Car, you should probably just go for it, since it's so near the end of the build season.

See if you can't obtain some "real" PWM cables though.
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Unread 19-02-2011, 08:28
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Re: Would This work? wiring pwm question

I will go with Jason's interpretation above as the correct read on the rules. Although electrically there should be no reason you can't do this, mixing common wiring in small diameter cable tends to have noise introduced by the common signals traveling through the same wire. It is unlikely that any real misfire would occur, it is just not common practice.
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Unread 20-02-2011, 00:59
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Re: Would This work? wiring pwm question

Yea I ended up not doing this. Thanks for the suggestions.
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