The Yokogawa ST5*A PLC module card, also identified as AS-S9221AQ-0 or S9771AJ-0, performs control processing and signal interface duties inside distributed control systems. It mounts as a plug-in card in Yokogawa control racks and CENTUM DCS cabinets, handling internal communication and I/O coordination in process automation setups. Maintenance teams and integrators install the dual-layer PCB design to repair or replace legacy boards in oil and gas, chemical, and manufacturing plants, where it interfaces with modules such as the EA9A, EA1A, or AIP413.
