A testing facility for a data communications network including modulators and demodulators, or modems, at central sites, at intermediate, or hubbing, sites, and at remote sites. Hubbing site testing units connect to respective host modems at the intermediate sites. The central testing unit transmits control words containing line address, drop address, and command characters. A configuration control circuit in each hubbing site testing unit generates signals that designate the particular location of the testing unit at the hubbing site. A validation circuit in each testing unit enables the testing unit to respond to the command character if two valid control words are received in succession. Thereafter the testing unit transmits back to the central testing unit signals in response to the test defined by the command character. If the line and drop address characters do not designate that specific hubbing site testing unit, it repeats the command to other units as an outbound request and repeats inbound responses to the central testing unit. Circuitry for substituting command characters under special conditions and for altering the inbound responses are also included.