Session 08 - PLC Programming Introduction
- 1. Control System Training – Session 08 PLC Programming Introduction
- 2. Page 2Classification: Restricted Agenda • PLC programming languages • Ladder logic • Ladder diagram programming • PLC programming
- 3. Page 3Classification: Restricted PLC PROGRAMMING There are 5 programming languages available, but the commonly used language is ladder diagram for PLC.
- 4. Page 4Classification: Restricted LADDER LOGIC Ladder logic is the main programming method used for PLC's. Ladder logic has been developed to mimic relay logic. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. The first PLC was programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program - so this programming method has stuck and it is the most common technique for programming in today's PLC. INSTRUCTION LIST There are other methods to program PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal.
- 5. Page 5Classification: Restricted PLC PROGRAMMING SEQUENTIAL FUNCTION CHARTS (SFC) SFC have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. This method is much different from flowcharts because it does not have to follow a single path through the flowchart. STRUCTURED TEXT (ST) Programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC and Pascal. Structured Text (ST) is a high level textual language that is a Pascal like language. It is very flexible and intuitive for writing control algorithms. FUNCTION BLOCK DIAGRAM (FBD) FBD is another graphical programming language. The main concept is the data flow that start from inputs and passes in block(s) and generate the output.
- 6. Page 6Classification: Restricted LADDER DIAGRAM PROGRAMMING Introduction to Ladder Logic Ladder logic uses graphic symbols similar to relay schematic circuit diagrams. Ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines between these two verticals.
- 7. Page 7Classification: Restricted PLC PROGRAMMING • Ladder diagram elements • Power flows from left rail to right rail. • The Inputs are placed on the left side of the rung and represents the beginning of the circuit. • The Outputs are placed on the right side of the rung and can not be connected directly with the left rail. • Contact can not be placed on the right of output. • Each rung has to contain one output at least. • The output address can be used only once in the program. • The inputs can appear in more than one rung of a ladder. The inputs and outputs are all identified by their addresses, the notation used depends on the PLC manufacturer.
- 8. Page 8Classification: Restricted PLC PROGRAMMING
- 9. Page 9Classification: Restricted PLC PROGRAMMING
- 10. Page 10Classification: Restricted LADDER DIAGRAM PROGRAMMING
- 11. Page 11Classification: Restricted Topics to be covered in the next session: • PLC Programming Questions