Apparatus and method for selectively companding data on a slot-by-slot basis

A data processing system (10) and associated method (150) of operation selectively compand data on a slot-by-slot basis. The data processing system (10) includes a processor (12), processor bus (14), memory (16), output serial interface (18), and input serial interface (70). The output serial interface (18) includes an output data buffer (20), compression module (22), output shift register (24), and output controller (26). In operation, the output serial interface (18) receives output data (28) and selectively compresses the data on a slot-by-slot basis. The shift register (24) then places the transmit data (32) on the serial link (25). An input serial interface (128) selectively expands data received on the serial link (25) on a slot-by-slot basis to produce input data (82). The associated method (150) includes method steps for companding data on a slot-by-slot basis. An ISDN interface and digital telephone (200) employ the teachings of the present invention.