When a CMOS driver sends a signal over a relatively long transmission line, there is typically a great deal of ringing at the destination due to impedance mismatches and transmission line effects. Adding a series resistor near the transmitter can significantly improve the signal integrity at the distant end without the need for an impedance matching termination.

Example System Diagram With Damping Resistor (R_d)

Simulated Signal Received From an HCS Family Logic Buffer With Series-Damping Resistor