The APM521 network power meter (hereinafter referred to as the meter) is designed according to IEC international standards, featuring full electrical quantity measurement, energy statistics, power quality analysis, event logging functions, and network communication capabilities. It is primarily used for comprehensive monitoring of power supply quality in power grids. This series of meters is equipped with DI/DO modules and AO modules that offer a wealth of functions, allowing for flexible implementation of full electrical quantity measurement and monitoring of switch statuses in electrical circuits.