PLC Programming

In PLC Programming, Programmer writes a set of instructions/ commands informs the PLC to complete the task.  Various types of PLC Programming languages are used but most user friendly and mostly used languages is Ladder diagram. This graphical language is near to general electrical symbols and easily understood by engineers.

• Ladder Diagram (LD): Mostly used PLC programming language which looks like a ladder..
• Functional Block diagram (FBD): PLC programming language that uses large function block for writing program.
• Structured text (ST): This PLC programming language, a series of steps and descriptions are used to write a program. Structure text uses a lots of IF, Else, and TRUE or FALSE notions.
• Instruction List (IL): This PLC programming language that consists of series of sequential statements to write a program.
• Sequential Functional Chart (SFC): This language uses vertical and parallel connected blocks and gates for PLC programming.

What is sourcing and Sinking in PLC system

When positive side of the power source is at Field devices, field device is source of power to the module, it is called sourcing. Field devices on the negative side of the power supply are sinking field devices. Generally Sourcing field devices are connected to sinking I/O cards and Sinking field devices to Sourcing Cards.

Ladder diagram

• Ladder diagram closely resembles to electrical symbols
• Ladder logic diagram is developed in PLC programming software.
• In electrical circuit the devices are described as on or off, where as in Ladder diagram, instructions are either TRUE or FALSE.
• Ladder Logic Program has at least one output and generally contains one or more inputs.

Hardly there are institutions which focus on practical aspects of PLC programming. Sofcon India Pvt Ltd in its fully equipped labs across the country imparts PLC SCADA training, PLC Programming courses in order to equip budding engineers with practical programming skills.

What is Scan Time in PLC Programming

In a PLC system, processor always scans the status of input devices, whether they are true or false.

Scan cycle in PLC is: reading the input, implementing the logic and updating the output. First the PLC checks status of each input if it is on or off. It records this status into its memory to be used during the next step. Then PLC executes the program. Suppose you have written a program which says when input 1 is high, switch on output1. Finally PLC updates the status of the outputs. After the third step is done, PLC goes back to the first step, thus repeating the steps continuously. Time take to complete one cycle is called scan time of the PLC. Generally the scan time of small PLCs is 1-2 milliseconds. In many fast moving manufacturing applications PLC scan time is very critical.

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