Triconex 8131X is the I/O expansion chassis for the Tricon CX safety system. It accepts up to six logical pairs of I/O modules and provides the backplane, interface position and field-termination locations needed to exchange as many as 192 points with the 8120X main chassis. Its main responsibility is to organize I/O modules and their bus connection outside the central processor frame.

At the left side of Triconex 8131X, a 4701X or 4703X interface module receives the copper or fiber I/O-bus path and passes processor traffic onto the expansion backplane. Each numbered logical slot contains two module positions associated with the same field-point group, so the bus address combines the chassis number, logical slot and left/right position when the processors identify installed hardware.

Two external 24 V DC sources feed Triconex 8131X through redundant chassis terminals and power the interface and I/O electronics. Field conductors do not land on the removable modules; FET connectors behind the module positions and interface cables carry the point circuits to external termination panels. The chassis address plug sets the expansion number used by the processors when they exchange those point values over the I/O bus.