A common intermediate representation of LD and SFC programs for hardware synthesis purposes

W artykule przedstawiono metody reprezentacji pośredniej programu sterowania opisanego językiem LD oraz SFC zgodnie z IEC61131-3, opracowane na potrzeby syntezy sprzętowej kładów sterowania PLC implementowanych w strukturach programowalnych FPGA. W opisie wykorzystano oryginalną implementację grafu skierowanego. Przedstawiono opracowane reguły odwzorowania, zapewniające zachowanie zależności sekwencyjnych przy jednoczesnym uzyskaniu maksymalnego zrównoleglenia działania. Przedstawiono również zarys metod syntezy na podstawie opracowanego odwzorowania pośredniego.The increased performance of a PLC can be achieved by direct implementation of a control program in an FPGA device [3, 6, 7, 8, 12, 13]. The paper presents a methodology of transforming a standard PLC program given by LD or SFC according to IEC61131-3 to the common intermediate form dedicated for logic synthesis. The intermediate form of the control program is represented by a data flow graph (DFG, Fig. 1). The set of nodes is carefully selected to minimize the number of different types of nodes while assuring implementation of PLC behavior. Attributed edges and multiple argument nodes are used to reduce size of DFG (Fig. 2). The developed method for creating a DAG maintains sequential dependencies between variables and revel operations parallelism. In PLC programs the variables pass values between operations and computation cycles. In order to maintain sequential dependencies, value assignment to a variable is observed. If the accessed variable has not been assigned, its value is used for a driving node (Fig. 3). The SFC is based on step, actions and transitions [2]. The step variable in the DFG is represented by a JK flip-flop equivalent. The activation function of a step is based on analysis of its dependencies with preceding and succeeding steps and transitions (Fig. 5). Actions that are bounded with steps are controlled according to their types (Fig. 6). The presented intermediated representation has been successfully applied to synthesize a PLC implemented in an FPGA device