The termination resistance of RS485 depends on the transmission line being used. It is equal to the characteristic impedance of the transmission line, which is typically 110 - 130 Ohm. Whether termination is required depends on the data transmission rate, cable length, and slew rate. RS485 transceivers are typically connected to UART, which samples data at the midpoint of every data bit. As long as the reflected signal is attenuated to a sufficiently low level at the start of sampling, no matching may be required. The following empirical guideline may be used to determine at which data rate and cable length matching is needed: When the signal rise and fall time is slower than 3 times the propagation delay, no matching is required.