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I am laying out an RS485 driver and read a few design guides which recommend a 120 Ohm termination resistor. This makes sense since the characteristic impedance of twisted pair cables is in the 100 Ohm to 150 Ohm range.

However what confuses me, is how would the signal be affected by the impedance of the PCB? There is also no mention of the connector impedance?

Should I be routing the RS485 traces as short as possible to mitigate transmission line affects on the PCB?

I understand the basics of transmission line theory, however I don't think I fully understand the implications of transmitting between two mediums. Transmission lines make more sense to me when the source and load share a consistent medium.

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Normally trace width and separation is adjusted so that the traces will have the necessary characteristic impedance, so it looks like another transmission line just like the cable does. If the PCB traces are not very long then the mismatch is minimal even if impedance is not matched. Some connectors have controlled impedanve but not all, in this case the connector shows up as short length of transmission line with a impedance mismatch so it causes reflections that degrade the signal. But for RS485 communications, sometimes it appears to work even without termination, so with proper wiring and termination, minimizing PCB trace length and not worrying too much about the connector are good suggestions. Ideally the connector would have the same impedance as well.

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  • \$\begingroup\$ This really clears it up for me, thanks! \$\endgroup\$ – Shamus MacDonald May 24 '19 at 13:11

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