1. Contact and Coil Instructions
Programming Principles of PLC Ladder Diagram Language
1. The ladder diagram consists of multiple rungs, each coil can form a rung, and each rung has multiple branches, representing a logical equation;
2. The relays, contacts, and coils in the ladder diagram are not physical but are bits in the PLC memory (1=ON; 0=OFF); normally open/closed contacts can be referenced infinitely in programming, while coil outputs can only be referenced once;
3. The flow in the ladder diagram is not physical current but “conceptual current”, which can only flow from left to right;
4. The calculations of the user program are based on the contents of the PLC’s input/output image registers, and the results of logical operations can be immediately used by subsequent programs;
5. The internal relays of the PLC cannot be used for control but can only store intermediate states of logical control;
6. Output coils cannot directly drive field actuators; they must drive through power devices on I/O modules. Basic logic instructions mainly focus on bitwise logical operations. In bit logic instructions, unless otherwise specified, the valid area of operands is: I, Q, M, SM, T, C, V, S, L and the data type is BOOL. Contact and coil instructions are divided into: standard instructions, immediate instructions, negation instructions, and positive (negative) transition instructions.
2. Timer Instructions
Includes: On-Delay Timer (TON), Retentive On-Delay Timer (TONR), Off-Delay Timer (TOF). The S7-200 has 256 timers (T0~T255).
3. Counter Instructions
Includes: Count Up CTU, Count Down CTD, and Count Up/Down CTUD, with a total of 256 (C0~C255). The counter counts the internal clock pulses of the PLC, while the counter counts external or program-generated counting pulses. The current value of accumulated counts (16-bit signed integer) is stored in the counter’s 16-bit current value register.
Each counter has only one 16-bit current value register address. In a program, do not reuse the same counter number, nor assign it to several different types of counters.
(Content sourced from the internet, copyright belongs to the original author)
Disclaimer: If copyright issues arise, please contact for deletion!No person or organization shall bear relevant legal responsibilities.
