The Mitsubishi expansion base unit QA65B has 5 slots and requires one power module. It is used for installing older large A-series PLC modules. The purpose of switch control is to generate corresponding switch outputs based on current input combinations and historical input sequences, enabling the system to operate in a specific order, often referred to as sequential control. Sequential control includes manual, semi-automatic, and automatic modes, with control principles categorized as decentralized, centralized, or hybrid.

During each scan cycle, inputs are sampled and outputs are refreshed in a centralized manner. During the input refresh process, when the input port is closed, new input states during program execution are not read until the next scan. A scan cycle consists of input sampling, program execution, and output refreshing. The contents of the image registers change as the program executes. The scan cycle length depends on CPU instruction speed, instruction execution time, and instruction count.

Due to centralized sampling and output, input/output delays occur, resulting in response lag. System program memory stores system programs, including management programs, system programs, and compile/interpret programs for user programs. It is composed of read-only memory (ROM), provided by the manufacturer, unchangeable, and retains data after power-off.