CCS/SN65HVD77: How to calculation for termination resister

Typically T(r) is 120 ohm - this is mentioned in the RS485 standard but not specifically required. It may not be necessary to terminate if the propagation delay across the entire length of wire is less than the width of 1 bit. If you do need to use termination, you should not use below 90 ohm.

For further reading see: e2e.ti.com/.../rs-485-basics-when-termination-is-necessary-and-how-to-do-it-properly

Hayashi,

Not a problem.
Typically, RS485 cable has a characteristic impedance of 120 ohms. If your termination is not closely matched, your reflection coefficient will be higher, resulting in a much noisier signal.

I will leave it as a task for the reader, if they wish to understand better, to search "reflections of signals on conducting lines".