A diagnostic system to monitor digital rod position indication (DRPI) signals of a DRPI system of a nuclear power plant, including a digital diagnostic unit connected between a DRPI display cabinet and a DRPI data cabinet of the DRPI system to monitor digital rod position signals of the DRPI data cabinet. The digital rod position signals include digital rod address signals and digital rod position data signals such that the digital diagnostic unit detects signal level variation and signal timing variation of the digital rod address signals and the digital rod position data signals to determine rod position errors of the DRPI system.