Vital processor implemented with non-vital hardware

Abstract

The invention describes a method and apparatus for effecting vital functions notwithstanding the fact that non-vital hardware is employed. A vital processor is implemented using non-vital hardware in the form of a digital computer which may for example be a microprocessor. The vital processor accepts binary input values and, based on a series of logical expressions relating output values to input values, determines the appropriate output values. Rather than employing a single bit to represent the condition of a particular input or output, unique multibit binary values or names are used. Each input or output has assigned to it at least two unique multibit values, each satisfying the code rules of a different code. Thus rather than representing a closed contact as a single 1 bit, and an open contact as a single 0 bit, in accordance with the invention the closed contact is represented by a unique multibit name which satisfies the code rules of a first code. At any point in the processing the value representing the contact can be checked to see if it satisfies the code rules, and if it does not a potential error is detected and handled. Although it is highly unlikely that a hardware failure would result in generating one of the few multibit names satisfying the code rule, that occurrence is not unlikely enough to be considered vital. Before actually controlling output devices in accordance with the processing, further tests are implemented which ensure that the multibit value computed for a particular output not only satisfies the predetermined code rule which is required, but is also correct bit for bit. Logic equations describing the relationship between output and input are actually computed using the multibit values as opposed to single bit values.