apparatus operable without need for an external power source for conveniently interconnecting data communication equipment , such as a modem , and multiple data terminal equipments , such as terminals and / or printers , which each have an interface conforming to a serial data communications standard , for permitting access to the dce of any of the dtes that may be connected to the apparatus . in the preferred embodiment , intended for the interconnection of interfaces conforming to the e . i . a . rs - 232 - c standard , a circuit interconnecting the data leads of a multiplicity of pluggable cable connectors supported on a chassis includes a transistor connected to be powered by input data signals from the dtes and to operate as a switch under certain signal conditions and to be effectively removed from the circuit under other signal conditions .