RS485 Wiring Guidelines - nathanmarlor/foxess_modbus GitHub Wiki
The theory...
Many industrial communication standards, including RS485, recommend or require using twisted pair cables for optimal performance. Adhering to standards ensures compatibility and reliability.
Following recommended practices helps meet performance expectations for several important reasons:
Noise Immunity: Twisted pair cables, with two insulated copper conductors twisted together, cancel out electromagnetic interference (EMI) and radio frequency interference (RFI) from external sources.
Common-Mode Rejection: The twisting of the pair enhances common-mode rejection in RS485 communication, preserving the desired signal while rejecting interference affecting both conductors equally. Long cable runs make them susceptible to common-mode noise. Twisted pair cables maintain signal integrity, crucial for communication networks.
Signal Integrity: Twisted pair cables reduce crosstalk between adjacent pairs and minimize signal degradation over distance, helping maintain signal integrity.
In practice...
Following the guidelines above, here are some examples of good and bad practices when wiring up your RS485 run when using an Ethernet cable.
You can see how only one twisted pair has been used for the signal, and not 2 pairs. If you feel you need to use 4 wires, either because you don't rely on the connection or you think it is too flimsy with just a single wire, at least use the 3rd option (labelled "OK WIRING"), that way you will at least retain the noise immunity properties.
If you don't already have some Ethernet cable to spare, don't worry, there is another, more practical and less bulky, solution. Just twist 2 cables together (Check out this video to learn how to easily obtain one with the help of a drill).
Notes:
- You may want to consider a shielded cable only for very long runs (+10 meters) or very "electrically" noisy environments.
- If your system works fine with using a twisted pair (2 wires) for each line, that's great! You don't need to change the existing wiring if it's already working, but it is advisable to keep this knowledge in mind while extending the line or rewiring for any reason.
The takeaway is that the RS485 signal doesn't carry any power, so it doesn't require any additional copper beyond what a single wire of a twisted pair already has. Using a twisted pair (2 wires) as a "single wire" will only harm your system's performance and resiliency.
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