Programmable logic controller

The utility model relates to a programmable logic controller, which can be configured into a single controller system or an active-standby redundant controller system. The programmable logic controller comprises a set of programming configuration software, a set of logic operation system software, a plurality of controller modules and controller frames thereof, a plurality of switch modules and a plurality of I/O modules and I/O extension frames thereof, and the core components include redundant Ethernets and double differential, isomerous, full-duplex and time-division multiplexing S buses. The programmable logic controller resolves the problems of the monotonicity of the logic programming language and the bad readability and maintainability of data indexes. The problems of the full access redundancy and data transmission efficiency bottleneck of the programmable logic controller are resolved, and the programmable logic controller improves the reliability of a communication system.