An IEC 61131-3-based PLC implemented by means of an FPGA

The paper discusses the design process of a programmable logic controller implemented by means of an FPGA device. The PLC implements on the machine language level a subset of the instruction set defined in the EN 61131-3 norm. Different aspects of instruction list and hardware architecture design are presented, however two aspects are the most important: Central Processing Unit (CPU) and the Arithmetic and Logic Unit (ALU). The ALU can execute 34 operations, which include the basic logic operations, comparators, and the four basic arithmetic operations. The operations can be performed for fixed-point and floating-point numbers. All the operations are implemented fully in hardware, so the solution is fast. The developed PLC is implemented using an FPGA device; however, the HDL models used for synthesis can be easily ported to an ASIC.