E&M module provide a quick and simple upgrade path for TSU multiplexers requiring support for PBX's, KSU's, analog telephones, dial-up modem pools, fax machines and leased line modems. Applications involving 2- or 4-wire E&M tie lines are supported in the TSU multiplexer with the E&M module. In addition to supporting Type I, II and V E&M signalling, the E&M module support legacy leased line modems in Transmit Only (TO) mode. Each dual port plug-on module supports the 2713Hz loop back tone.