In the context of functional safety, a diagnostic function is a logical function that is implemented with software or hardware and is used to monitor the operation of the safety function for the purpose of detecting faulty operation.
The quality of a diagnostic is measured by its ability to detect a failure of the safety function that is being used to monitor the machine performing a task. This metric is called diagnostic coverage (DC). For most functional safety certifications, a prescriptive diagnostic coverage metric is required.
A diagnostic can be implemented using various techniques, which are suggested by which principle is used when analyzing the safety function to be monitored. For the purposes of datapath analysis, both internal and external diagnostic methods are covered. Additionally, diagnostics implemented in hardware must be tested periodically when the component is in operation.
When a diagnostic detects a fault, there must be a means to indicate the result of the diagnostic to the system so the system integrator can make sure the equipment being monitored takes the appropriate action based on the functional safety standard.
Based on several safety standards, a diagnostic for a diagnostic is not required. This means a failure in the diagnostic does not affect the operation of the safety function being monitored. However, the interface signaling between the diagnostic and the safety function being monitored might cause the safety function to fail and must be evaluated. In this case, the failures in time (FIT) of the interfering logic belongs to the safety function, not the diagnostic.