Principle and implementation of software redundancy in Siemens PLC - Database & Sql Blog Articles

Soft redundancy, also known as software redundancy, is a solution for Siemens to implement redundancy functions, and can be applied to control systems that require less switching time between active and standby systems. Soft redundancy can be realized: mainframe power supply, backplane bus and other redundancy; plc processor redundancy; PROFIBUS network redundancy (including communication interface, bus connector, bus cable and other redundancy); interface module IM153-2 redundancy. When working in a software redundant system, the primary and secondary control systems (processor, communication, I/O) operate independently, and the PLC of the primary system masters the I/O control rights in the redundant ET200M slaves. The standby system synchronizes data through a dedicated software redundancy program. The PLC program consists of a non-duplicated user program segment and a redundantbackup user program segment. The main system PLC executes all user programs. The standby system PLC skips the redundancy program by judging the redundancy state and only executes Non-redundant user program. When a component in the primary system fails, the standby system automatically switches to the primary system, performs redundant procedures, and the control task does not interrupt.

Principle of soft redundancy and system configuration requirements

The CPU that can implement the software redundancy function has specific requirements. In the S7-300, the CPU of the 315-2DP type (including the 313C-2DP, 314C-2DP) can be supported. All the S7-400 CPUs support the software redundancy. The CPU models of the main and standby systems can be different, such as one system using the S7-400 series and the other system using the S7-300 series. Data synchronization can be through three different network modes: MPI, PROFIBUS and Ethernet. The I/O signals that need to be controlled by two systems need to be collected by the ET200M slave. Since two CPUs are to be connected, the ET200M should use the redundant IM153-2 interface module and adopt the active bus module. The software needs to install the software redundancy package, and the redundant function library will appear only when the software redundancy package STEP7 is installed.

Programming, debugging and troubleshooting of software redundant systems

The redundant software package in STEP7 contains several function blocks. Each function block has specific functions and calling methods, and the function blocks in different libraries are used differently, and need to be distinguished when used. Since redundancy is achieved through software programs, some features of the software redundancy system should be noted during programming and debugging. If the program block FC100 is initialized in OB100, the parameters for defining the system operation should be correct, and the address of the redundant output image area should be continuous. After modifying the system parameters, reset the CPU and re-download all the items to the PLC.

Soft redundant system connected to other systems

In the practical application, the software redundancy system also involves how to establish data communication with other systems and how to establish redundant links when connecting the upper monitoring software WinCC.