Applications involving 2- or 4-wire E&M tie lines are supported in the TSU Multiplexer with the E&M modules. In addition to supporting Type I, II and V E&M signaling, the E&M modules support legacy leased line modems in Transmit Only (TO) mode. Each dual port plug-in and dual port plug-on module supports the 2713 Hz loop back tone. The module installs in the expansion slot of the TSU Multiplexer to provide RJ interface connectivity to the KSU, PBX, switch or leased line analog modem.