A concise overview of the Mitsubishi MELSECNET/B module AJ71AT21B, using twisted-pair cable in a single-bus configuration (master/local station). The PLC program design follows a systematic approach: first, to meet system control requirements; then, to implement each control function or subtask; and finally, to gradually refine system functionality. The initialization program, executed upon power-up, performs necessary operations to prevent misoperation, including clearing data areas and counters, restoring required data, setting or resetting relays, and displaying initial statuses.