A data transmission system that can controllably transmit data in three different modes between connected data devices. The first mode transmits data at a 64 Kbps rate. The second mode transmits data at a 56 Kbps rate plus control signals at an 8 Kbps rate. The third mode transmits data and control messages at a rate of up to 50 Kbps between connected terminals or other data devices. For all modes, the data is transmitted by inserting it one byte at a time into an information field of successive ones of cyclically reoccurring time multiplexed frames.
