Siemens PLC basic instructions: touch point and coil instructions

Programming Principles of PLC Ladder Diagram Language

1. The ladder diagram is composed of multiple steps, each coil can form a step, each step has multiple branches, and each step represents a logic equation;

2. The relays, contacts, and coils in the ladder diagram are not physical, they are bits in the PLC memory (1=0N; 0=0FF); the normally open/normally closed contacts can be quoted unlimited times during programming, and the coil output can only be used once;

3. What flows in the ladder diagram is not a physical current but a "conceptual current", which can only flow from left to right;

4. The operation of the user program is based on the contents of the input/output image register of the PLC, and the result of the logical operation can be used by the subsequent program immediately;

5. The internal relay of the PLC cannot be used for control, and can only store the intermediate state of logic control;

6. The output coil cannot directly drive the on-site actuators, but is driven by the power device on the I/module.

Basic logic instructions are based on bit logic operations. In bit logic instructions, unless otherwise specified, the valid areas of operands are: I, Q, M, SM, T, C, V, S, L and the data type is BOOL Contact and coil instructions are further divided into: standard instructions, immediate instructions, negation instructions, positive (negative) jump instructions.