In a sequencer unit that is a PLC unit constituting a programmable logic controller (PLC), a conventional technique has not provided a method of calculating any arithmetic expression created by a user in the inside of the unit. When causing the PLC to implement an arithmetic operation, the user adopts a method of combining ladder commands to create a ladder program in advance such that the arithmetic operation is implemented according to calculation sequence of a desired arithmetic expression and implementing the arithmetic operation within a sequencer CPU (Central Processing Unit). Therefore, to implement a complicated arithmetic operation, it is necessary to create a ladder program corresponding to the arithmetic operation.
As measures against such a problem, there has been proposed a technique that can execute an arithmetic operation within an apparatus by converting an arithmetic expression formed by a combination of parentheses and operators and calculation sequence of the arithmetic expression into ladder commands and registering the ladder commands in the apparatus including a sequence control function in advance (see, for example, Patent Literature 1).
As a function of causing the PLC to implement the arithmetic operation inside the unit, there is an analog input/output unit installed with a simplified “loop control function” for applying, according to an arithmetic expression set in advance, an arithmetic operation to a digital value obtained from an analog value inputted to an analog input channel and performing analog output (see, for example, Patent Literature 2).