Combinatorial Circuits with PLC

Abstract

In the case of logic switching elements, it was clear without any special mention that the output (more precisely, the output signal) depends on the inputs (the input signals). The relationship, i.e. the function of the circuit, is clearly defined. You get control when you connect several switching elements together. The signals at the inputs cause the signals that can be tapped at the outputs. The function of the overall circuit is determined by which switching elements are connected to each other and how. The function can be changed at any time by inserting or removing switching elements or changing the connections. In a PLC, as in digital circuits, we also have inputs and outputs. The dependence of the output signals on the input signals is not determined by the wiring, but it is determined by a “program” which signals are to be linked together. This program can be written like an ordinary text with the PC. The advantage of this technique is the standardization of the controls. Because only a few universal types of control devices are required, they can be produced in large quantities. Only the buyer decides by programming which of the “inner building blocks” are to be used and how they are to be “wired”.

For the practical application, one can initially assume that an “infinite number” of individual modules are available. The “wiring” is done by the program. Another advantage is that the function can be changed at any time by changing the program. A screwdriver or soldering iron is not required for this.

We will first practice programming using the “Instruction List”. In this instruction list (IL), the individual links are described by short texts. Each line of the IL contains one instruction. Each instruction consists of an operator and an operand. The operator determines the type of link (e.g. OR link), the operand determines the input/output terminal whose signal is to be linked.