Synonymous with echoplex. A primitive error control method in which the receiving device echoes the received data back to the transmitting device, character by character. The transmitting operator can view the data as received and echoed, and make corrections as appropriate, assuming that he hasn't lost his sight or mind due to the ddoouubbllee vviissiioonn effect. As errors also can occur in the transmission of the echoed data, this approach is highly unreliable. See also echo and error control.