1. A data modem comprising: signal processing means responsive to selected ones of a plurality of sets of instructions and to signals applied to said signal processing means for executing corresponding ones of a plurality of signal converting algorithms appropriate for alternative ones of data modem types to process said applied signals, digital processing means responsive to an external signal source designating individual ones of the data modem types for executing various ones of a plurality of communication line control algorithms, and means responsive to the execution of the communication line control algorithms for selecting the instruction sets corresponding to the signal converting algorithms appropriate for the designated data modem types.