Ringline dual channel digital transmitters are encapsulated in a foot mounted plastic housing with flying leads for circuit connection. There are two versions of transmitter. RLTX2B is a CMOS based unit with a fixed Ringline 'address' and RLTX2P is a microprocessor based unit with electrically programmable addressing. The Ringline system has 64 addresses. Each address has an 'A' and a 'B' channel. Each transmitter has a pair of wires (blue & white) to connect it to the Ringline field bus and three wires for switch contact monitoring (grey - channel A, brown - channel B & black - common). The RLTX2P has an additional wire (green) to facilitate the programming process. The transmitters receive operating power from the Ringline field bus and at the same time encode the status of the interlocks they are monitoring back onto the bus. Contact interlocks being monitored should be high quality - preferably gold plated. Regulations require a minimum housing protection of IP54 and monitoring interlock isolation from earth of 500V for intrinsically safe applications.