SN65HVD1781-Q1: Routing Impedance

In general, RS-485 buses are designed for a differential impedance of 120 Ω.

You can deviate from that, but the impedances of the traces, the cable, and the termination resistors should match.

Hi Mayur,

As Clemens said it is normally designed for 120Ω - as in RS-485 there are 2 120Ω resistors, one at each end of bus. The differential impedance is designed to match the termination resistors so that signal reflections are mitigated. 

However in practice there can be some deviation - there are two major factors to consider here.

1. The Impedance between A and B cannot be less than 54Ω - which is the minimum value of two 120Ω +/-10% resistors in parallel - this specification must be upheld in the system. 

2. Impedance matching - ideally the terminations, trace impedance, and cabling would all be equal to 120Ω to make reflections ~0%; however if there is mismatch between termination and traces/cables there will be signal reflections. The larger the deviation the larger the reflections at the same electrical length of the bus.

In real systems there will be some mismatch due to component tolerances - a very general guideline is to keep mismatches to <20% but I'd try to design for as low as possible -as the higher the mismatch the more risk the system has of comms failures due to corrupted data. 

Please let me know if you have any other questions!

Best,

Parker Dodson