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Which comms protocols can be isolated?

cupoftea

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Hi,
Which of the comms protocols dont need the RX PCB and TX PCB to have the same ground?
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Also, which is best for noise rejection, balanced signalling or differential signalling?

Presumably the only difference between differential signalling and balanced signalling si that balanced signalling is bipolar, and each signal is equally distant from ground?

CAN is differential, unipolar?
RS485 is balanced?
USB is differential, unipolar?

I am looking these up on google, but making the exact comparisons is a little troublesome.

Also, you cant send a balanced signal down coax cable.....because you need 3 wires.....or would you classify twisted pair cable with a ground shield as being similar in noise immunity to coax cable?
--- Updated ---

Also, why doesnt RS485 also send the ground?....surely a ground return for both the opposing signals woudl make EMC/Noise immunity better?

Which is the most noise immune way to send a signal along a cable thats going off a PCB? (the signal was a TTL 0-5V coming from a micro on the PCB) The wire is 2 metres long and the data rate is 10000 bits/sec...What woudl you convert it to, to get max noise immunity.?
 
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FvM

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All interfaces that don't use optocoupler or transformer isolation need a common ground, no matter if you use single ended or differntial signalling because differential receivers have limited common mode voltage range. For this reason, a ground wire is included in the interface cable between non-isolated nodes (the cable shield can act as ground connection), nevertheless a common mode voltage between both grounds, specifically high frequent noise must be expected. Differential signalling e.g. RS-485 or RS-422 has the advantage of tolerating higher common mode voltage differences.

Referring to question title, all interfaces can use opto couplers, transformer isolation is only suitable for DC balanced bit-streams, e.g. ethernet.
 

KlausST

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Hi,

best isolation: POF = plastic optical fiber.
I use HFBRxxxx types. They work for your distance and for your datarate.
Either buy cable with assembled connectors
.. or assemble them on your own. (sharp knife, and something to crimp the ring should work for first tests)

Klaus
 

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