PLC logic ladder programming features - Database & Sql Blog Articles

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The plc ladder programming language is a programming language developed from the relay contact control circuit diagram. The structure of the two is very similar, but the program execution process is essentially different. Therefore, as the basic components of the relay contact control system and the ladder diagram, the three elements - contacts, coils, and wiring, are essentially different.
1. The nature and characteristics of the contacts The "normally open" and "normally closed" contacts of the programming elements such as inputs, outputs, internal relays used in the ladder diagram are essentially the data "bit" state of a certain internal memory of the PLC. The "normally open" contact in the program is processed directly using the state of the bit; the "normally closed" contact is processed using the "logical NOT" state of the bit. It differs from the relay control circuit in that:
1 The contacts in the ladder diagram can be used indefinitely in the program. Unlike physical relays, they are limited by the number of actual mounting contacts.
2 At any time, the state of the "normally open" and "normally closed" contacts in the ladder diagram is unique. It is impossible to have both "1" and "normally open" and "normally closed" contacts. There is a strict "non-" relationship.
2. Properties and Features of the Coil The programming elements such as internal relays and outputs used in ladder programming use the same "coil" as the relay control circuit, but they are not actual physical relays. The program controls the output of the above coils, but only assigns the state of the data "bit" of a certain internal memory of the PLC. The data "bit" is set to "1" corresponding to the "power" of the coil; the data "bit" is set to "0" corresponding to "power off". Therefore, it differs from the relay control circuit in that:
1 If necessary, the “output coil” in the ladder diagram can be assigned multiple times in the program, ie the so-called “repetitive coil” can be used in the ladder diagram.
The execution of the 2PLC program is strictly in accordance with the timing of the ladder diagram "from top to bottom" and "from left to right". In the same PLC program execution loop, the output status of the executed instruction cannot be changed (the executed instruction output has been executed). Status can only be changed in the next cycle). By effectively utilizing the program execution characteristics of the PLC, it is possible to design a number of special logics that are different from the relay control lines, such as "edge" processing signals.
3. The nature and characteristics of the connection "connection" in the ladder diagram only represents the processing order relationship of the instruction in the PLC ("top to bottom", "left to right"), it does not have the actual current like the relay control line Therefore, each output coil in the ladder diagram should have its own independent logic control "circuit" (ie, a clear logic control relationship), and the "bridge type connection" often used in relay control lines cannot be used between different output coils. In this way, an attempt is made to change the output of the instruction that has been executed by the subsequent execution condition.