Abstract: To perform a loopback condition for testing purposes, the present apparatus supplies a null signal from an end terminal on a communication link through a set of repeater stations to reinitialize the repeater stations. It then sends N+1 pulses of a frequency different than the data frequency through the repeater stations with each repeater station that receives more than two pulses disgarding one pulse and forwarding the rest. The repeater station or remote end terminal that receives only two pulses reconfigures itself to a loopback condition, returns a null signal back to the end terminal originally transmitting the N+1 pulses and then sends an acknowledgement pulse. Data signal testing can then be performed until a further null condition is sent thereby reinitializing the repeater stations to the data transmission configuration.