The Mitsubishi expansion base A55B has 5 slots, requires no power supply, and is used for QnA/A series PLCs. Switch control, also known as sequential control, determines output based on current and historical input states, enabling systems to operate in manual, semi-automatic, or automatic modes. Control principles include decentralized, centralized, and hybrid methods. Each scan cycle involves centralized input sampling and output refreshing. During input refresh, if the input port is closed, new input states are not read until the next scan. A scan cycle consists of input sampling, program execution, and output refresh. The image register content changes during program execution. Scan cycle duration depends on CPU instruction speed, instruction time, and instruction count. Centralized sampling and output cause input/output response delays. System program memory, stored in read-only memory, holds management and compilation programs, is factory-set, unchangeable, and retains data when power is off.