Abstract: A diagnostic controller is situated with a pair of duplicated logic units which themselves are remotely situated in relation to a controlling data processing unit. A distinct data path is provided between each of the remote logic units and the data processing unit for the conveyance of control information therebetween. When a remote logic unit appears to malfunction, the data processing unit takes the associated data path out-of-service and transmits commands over the in-service data path to start the diagnostic controller. Responsive to these commands, the diagnostic controller autonomously simulates data received over the out-of-service data path and applies the simulated data to the suspect remote logic unit. Results from this suspect unit are compared by the diagnostic controller with anticipated results wherein a mismatch indicates an actual malfunction in the suspect unit. Any detected malfunctions are reported to the data processing unit over the in-service data path.