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Dual Digital Output Module

The dual digital output (DDO) modules receive output signals from the main processors along a single parallel or series path, and applies a 2-out-of-3 voting process individually to each switch. The switches produce one output signal which is then passed to the field termination. While the quadru-plicated output circuitry on TMR modules provides multiple redundancy for all critical signal paths, dual circuitry provides just enough redun-dancy to ensure safe operation. The dual module is optimized for those safety-critical control programs where low cost is more important than maximum availability.

The dual digital output modules have a voltage-loopback circuit which verifies the operation of each output switch independently of the presence of a load and determines whether latent faults exist. Failure of the detected field voltage to match the commanded state of the output point activates the LOAD/FUSE alarm indicator.

In addition, ongoing diagnostics are performed on each channel and circuit of a dual digital output module. Failure of any diagnostic on any channel acti-vates the Fault indicator, which in turn activates the chassis alarm signal. A dual module operates properly in the presence of most single faults and may
operate properly with some kinds of multiple faults, but stuck-OFF faults are an exception. If one of the output switches has a stuck-OFF fault, the output goes to the OFF state and a glitch may occur during switch-over to a hot-spare module.

The dual digital output modules support hot-spare capability which allows online replacement of a faulty module. Each module is mechanically keyed to prevent improper installation in a configured chassis.

The dual digital output modules require a separate external termination panel (ETP) with a cable interface to the Tricon backplane. Digital outputs are designed to source the current to field devices, so field power must be wired to each output point on the field termina-tion.