Diplexers are three port devices that are used to combine or split signals that are at different frequencies. A diplexer is designed with two fixed bandpass filters that are connected to a common port. Multiplexers are passive devices used to split or combine signals, usually of different frequencies. Once several frequencies are combined, a single transmission medium is used to send or receive the signals.
For information on Corry Micronics' diplexer and multiplexer offerings, download the datasheet.