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A process control interface system having a network of distributed triply redundant input/output field computer units. The system includes a plurality of self-contained remotely located triply redundant field computer units connected to decision making redundant process control computers through a b

A process control interface system having a network of distributed triply redundant input/output field computer units. The system includes a plurality of self-contained remotely located triply redundant field computer units connected to decision making redundant process control computers through a bi-directional communication network having at least two concurrently active communication channels. Each of the field computer units include a set of at least redundant field computers for arbitrating both input and output signals. The input arbitration method enables a plurality of selectable default input conditions, such as select HIGH and select LOW, in the event that a majority agreement cannot be reached among valid input signals. The output arbitration method includes a plurality of selectable default output conditions, such as fail SAFE and fail LAST. Each of the default input and output conditions may be rapidly adjusted through software selection. The field computer units also include individual abort circuits for each output signal to be transmitted to a device which affects the operation of the physical process. These abort circuits effectively enforce the output value signals arbitrated independently through each of the three redundant field computers using a voting procedure.

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[ What is claimed is:] [1.] A method of conducting passive non-intrusive testing of a set of corresponding digital output circuits in a field computer unit having at least three redundant computers, where each of said digital output circuits having a plurality of output channels, comprising the step

[ What is claimed is:] [1.] A method of conducting passive non-intrusive testing of a set of corresponding digital output circuits in a field computer unit having at least three redundant computers, where each of said digital output circuits having a plurality of output channels, comprising the steps of:providing a predetermined period of time in which to conduct apassive test of a plurality of said output channels; andconducting passive testing of a plurality of said output channels during said predetermined period of time,said passive testing for each of said plurality of output channels including the steps of comparing the magnitude of a first signal with a first predetermined high test level when said channel is On, and comparing the magnitude of a second signal with a predetermined low track signal when said channel is Off,said first and second signals being different signals associated with said digital output circuit and each of said comparing steps providing a determination of a different error condition.