12b40 Inputoutput Units

12.B4.1 Input/output units shall be used to connect the data environment to the system and contain all necessary input/output functions to read field sensors and operate controlled equipment based on instructions from the central processing unit.

12.B4.2 Input/output units shall be fully supervised to detect failures. The input/output units shall report the status of all points in its data environment at the rate of at least once every 60 seconds for temperature control. Slightly longer times may be acceptable for non-critical loads on an exception basis.

12.B4.3 Upon failure of the input/output unit (including transmission failure), the input/output unit shall automatically force outputs to a predetermined state.

12.B4.4 For system voltages below 95 VAC the input/ output unit shall totally shut down and de-energize its outputs.

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