The Mitsubishi A1SJ71QBR11 is a network module designed for use in MELSEC-Q series PLC systems. It supports communication via a single-bus architecture using 3C-2V or 5C-2V coaxial cables, enabling peer-to-peer networking between PCs configured as control stations or normal stations, as well as remote I/O network configurations where the module acts as a remote master station. This module is specifically intended for use in Q mode.

When a program is input from a programming device (such as a GX Works engineering tool) and stored into the user program memory, the CPU proceeds to interpret and translate this user-written program. This translation is performed based on the system’s inherent functions, which are defined by the system program stored in read-only memory (ROM). The system program includes a management program and an interpreter/compiler that converts the user program into a machine-readable compiled format recognized internally by the PLC.

Input statuses and input information are received via the input interface. The CPU stores these values either in the working data memory or in the input image register. Subsequently, the CPU integrates the stored data with the translated user program through logical processing. The results of this processing are then written to the output image register or the working data memory. Finally, these results are output through the output interface to control external actuators or drives.

The PLC system incorporates semiconductor memory circuits with non-volatile characteristics where necessary. The memory is divided into two main categories: system program memory and user memory. The system program memory stores the system firmware, including the management program, the sequence interpreter, and the compiler that processes user programs. This memory is composed of read-only memory (ROM) and is pre-installed by the manufacturer. Its contents are fixed and cannot be altered by the user; they also remain intact even when power is lost.

User memory is further subdivided into the user program storage area and the working data storage area. This memory is implemented using random access memory (RAM) and is fully accessible to the user. However, the contents of RAM are volatile and will be lost when power is removed. To preserve the user program and critical data during power-off periods, a lithium battery is commonly used as a backup power source. The typical lifespan of such a battery is between three and five years, after which replacement is recommended to maintain data integrity.