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485 Grounding In Harness With Power and Ground Question

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jmeyer84

jmeyer84

Electrical
Aug 28, 2009
17
I am working on developing a system including a RS485 network. All microcontroller-based devices will be powered by a 24VDC lead acid battery system. Each system will contain a RS485 driver powered by an a 5V DC-DC buck converter. The device in question will be added to the end of a network, so the termination resistor will be built in.

I would like to power(battery supply and ground) and communicate(RS485 A and B) using the same harness and connector(4 conductor).

My question are:

1) Is a two-pair RS-485 cable harness sufficient for this application? Two conductors would be 485 twisted pair and the other two would be supply and ground. The conductors would be 22AWG. Max current would be 1.5A. Nominal current is about 50 mA.

2) Would the RS-485 twisted pair have to be separately shielded from the conductors supplying the power?

Thank you all for taking the time to read this post.
 
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Can't say with just the supplied info...

1.5A on 22AWG over an extended distance could certainly be a problem.

In principal what you want to do will work.

What are your data rates and how far are you trying to go?

Keith Cress
kcress -
 
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For any typical system, you could run power next to the data line without much issue, but you need to make sure the data line is twisted appropriately and the power line is not.

Dan - Owner
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The distance will be about 4 feet 99% of the time. There will be situations where we'll have to run about 20-25 feet.

Data rates will be 115kbps or lower, half-duplex.

Why can't the power and ground cables be twisted?
 
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The power/ground can be twisted together (though to what purpose, other than convenience of using a pre-twisted cable, I have no idea), but should not be twisted with the comm lines... that would allow any power line noise to be induced directly into the comm lines.

Dan - Owner
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Thanks for the info. The reason I ask is for the selection of a cable. A two-pair 22AWG is fairly common from what I am finding. If I could use one pair for power/ground and the other for half-duplex 485, that would be ideal.
 
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RS485 running a couple of feet could be run over four coat hangers.. You can use whatever you want. I would use whatever two twisted pairs you have laying around.

Keith Cress
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