Abstract: A multiple block binary synchronous duplex communications system provides for the orderly transfer of data from one location to another utilizing communications facilities. This communications system increases the efficiency of data links with long propagation delays or with long processing times at either end by maintaining a continuous flow of data and responses in both directions. The memory of the communications link is kept full at all times. Data is transmitted in blocks including an identification code. The received data is checked for errors. When errors occur the transmission of data is not stopped; data is continuously transmitted and signals are returned including the block identification of the block containing the error. The block containing the error is then retransmitted when the transmitting station of the communications system is available for this task.