Abstract: A test apparatus is adapted to accept a manually entered binary data message, to transmit the message serially to a remote unit, to receive a responsive serial message from the remote unit, and to display the responsive message so that it can be compared to the desired response. In addition the test apparatus is adapted to generate an error code which is transmitted to the remote unit at the end of each message for use by the remote unit in detecting an error in transmitted information. The tester also generates a similar error code upon receipt of a return message from the remote unit and compares the error code thus generated with an error code transmitted by the remote unit to determine whether there is an error in transmission of the return message. In addition, the test device can transmit a message and the associated error code from its own output to its own input to provide an internal test of its operation.