I have doubt how to length matching (if necessary, idk if it is really necessary). Iam checking a layout board in my company and looking on it I realized theres a mismatch for RS-485 communication using chip MAX485E.
I have a connector which I will read MODBUS using RS-485 protocol and looking on MAX485E datasheet it states that differential driver output R= 27ohm (RS-485).
I think it means I need to do length matching for this impedance, but how to calculate it using Saturn calculator, for instance or any other calculator like Kicad calculator?
I would like to know how to calculate trace width and gap to route differential pair in this situation. I have a 2 layers board, using FR-4 (Er=4.6) ; TanD = 0.02 ; Height of substrate (thickness): 1.6mm ; Rhou = 1.72e-08. Im using Kicad with diff pair width = 0.2mm and width gap = 0.25mm set as default in Software. If someone have any document to help me about theory too gonna be good.
Best Answer
At the typical feature size of a circuit board, that matters for signals in the GHz. RS-485 isn't even close to that fast.