US Patent 7287184 High speed synchronization in dual-processor safety controller

A safety controller executes a control program in two processing units to detect processor failure by comparison of the execution in each unit. This comparison is made rapid by synchronizing the input variables at the beginning of the task and comparing output variables at a conclusion of the task, avoiding line-by-line comparison of input and output variables. Intermediate variables, that are neither input nor output values, are compared at a less frequent interval.