Abstract: The physical integrity of a large-scale hardware connection between two modules, such as in the context of high-speed digital printing, is tested by entering a diagnostic mode in which single-instruction handshake routines are repeatedly sent from one module to another through a parallel bus. Each handshake routine activates all of the communication lines in the bus. If a sending module does not receive the expected response to the single-instruction handshake routine within a predetermined time, the sending module causes an audible signal to be emitted. Service personnel can manually manipulate various cables, connectors, and sockets forming a hardware connection while the system is in diagnostic mode, to isolate the location of a hardware fault.