The 531 provides automatic PID control backup for critical loops controlled by PLC, DCS or PC systems. Unlike a single loop controller configured to sense and retransmit a control signal, which adds liability and complexity; the 531 has a simple operator interface, special parameters for control states upon host transfer or failure, and a special pass-through circuit that provides redundancy in the control loop. In HOST mode,
the control signal (4-20mA) is hard wired to pass through the 531 without degradation. In LOCAL mode, the 531 generates the CV (control) signal in AUTO or MANUAL mode. Host CV tracking enables a bumpless transfer to LOCAL mode in either LOCAL AUTO or LOCAL MANUAL mode. A special CV line sense function triggers HOST to LOCAL transfer automatically if host CV signal fails. LOCAL/HOST transfer may also be initiated by a dedicated front panel key or a contact input. The 531’s setpoint may be set locally or remotely from the host, ensuring bumpless changeover to LOCAL AUTO mode. The 531 may also be operated as a manual loading station in the LOCAL MANUAL mode.