Overview of Electronic Signal Termination
In lower frequency (slow edge rate) applications, interconnection lines can be modelled as "lumped" circuits. In this case there is no need to consider the concept of "termination". Under the low frequency condition, every point in an interconnect wire can be assumed to have the same voltage as every other point for any instance in time.
However, if the propagation delay in a wire, PCB trace, cable, or connector is greater than 1/6 of the rise time of the digital signal, the "lumped" circuit model is no longer valid and the interconnect has to be analyzed as a transmission line. In a transmission line, the signal interconnect path is modeled as a circuit containing distributed inductance, capacitance and resistance throughout its length.
In order for a transmission line to minimize distortion of the signal, the impedance of every location on the transmission line should be uniform throughout its length. If there is any place in the line where the impedance is not uniform for some reason (open circuit, impedance discontinuity, different material) the signal gets modified by reflection at the impedance change point which results in distortion, ringing and so forth.
When the signal path has impedance discontinuity, in other words an impedance mismatch, then a termination impedance with the equivalent amount of impedance is placed at the point of line discontinuity. This is described as "termination". For example resistors can be placed on computer mother boards to terminate high speed busses. There are several ways of termination depending on how the resistors are connected to the transmission line. Parallel termination and series termination are examples of termination methodologies.
Read more about this topic: On-die Termination
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