The maximum signal levels discussed in the previous section place bounds on what is achievable at an interface. However, those bounds are based on a fixed source adjustable load. In practice, there are circumstances in which the source or the load, or both, can be adjusted to produce prescribed interface signal levels. Sometimes it is necessary to insert a circuit between the source and the load to achieve the desired results. Figure 3-52 shows the general situations some examples of resistive interface circuits. By its nature, inserted circuit has two terminal pairs, or interfaces, at which voltage and current can be observed or specified. These terminal pairs are also called ports, and the interface circuit is referred to as a two-port network. The port connected to the source is called the input, and the port connected to the load is called the output. The purpose of this two-port network to make certain that the source and load interact in a prescribed way