PLC Ladder Logic
Programmable logic controllers, or PLCs, are specialized, robust industrial computers. They are designed to continuously control equipment and processes based on process inputs and logical control programming. One of the most common ways to program this type of computer is a technique called ladder logic. This is a technique that provides a visual representation of the logic flow which helps with both initial programming and subsequent troubleshooting. This course discusses the background of ladder logic as well as basic instruction types such as examine if closed and examine if opened. This course also illustrates several varying ladder logic examples such as a lamp, motor starter, and garage door.
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Learning Objectives
- Describe the purpose of a PLC
- Discuss the hardware elements that make up a PLC
- Describe the operation of a PLC
- Identify the difference between the different modes of operation of a PLC
- Discuss the advantages of ladder logic
- Identify three common instructions used in ladder logic programming
- Explain the difference between the use of an XIC and XIO instruction
- Describe the purpose of a rung in ladder logic
Specs
| Course Level | Intermediate |
| Languages | English |
| Compatibility | Audio, Video, MobileReady |
| Based on: | Industry Standards and Best Practices |
Key Questions
What does PLC stand for?
PLC stands for Programmable Logic Controller.
How are most PLCs programmed?
Although a PLC can operate without it, most PLCs are programmed from an external source, typically a personal computer.
What is ladder logic based on?
Ladder logic is based on the electrical ladder-like schematic diagrams that were used to describe electromechanical relay logic.
How does the Examined if Close Instruction Work?
The Examine if Closed (XIC) instruction works with a memory address that is associated with the instruction when it is created and placed on the rung. Physically, if a device such as a switch has open contacts, or a relay is not energized, the PLC will read a 0 (not energized) into the input data table.
How is ladder logic evaluated?
Ladder logic is evaluated from left to right, and top to bottom.
Sample Video Transcript
The use of these common ladder logic instructions is illustrated in a typical pushbutton-based motor starter circuit. In this diagram, each instruction is associated with a memory address that is connected to a physical device:
• XIC- IN 1 represents a normally open momentary contact start switch
• XIO- IN 2 represents a normally open momentary stop switch
• OTE- OUT 1 represents a relay coil that starts a motor Ladder logic is evaluated from left to right, and top to bottom. If the start button is pushed, the IN 1 register will contain a 1. Since the first instruction is an XIC, the instruction will evaluate as true. The second instruction is an XIO. Since this data bit comes from a normally open stop switch, which has not been pressed, the IN 2 data value will be 0 and that instruction will also evaluate as true. Because both of the upstream conditions on the rung evaluate as true, the final instruction on the rung also becomes true, which translates into an output data value of 1 and a running motor. The next instruction to be evaluated is the XIC instruction on the partial second rung which is connected in parallel with the IN 1 instruction. The data value associated with this XIC instruction is OUT 1, which has a value of 1. This is the output value of the energized coil that started that motor. Since the second OUT 1 XIC instruction is in parallel with the IN 1 XIC instruction, an active path of true instructions (which can also be looked at as “power”) to the OTE instruction is maintained, even after the switch that started the motor is released. This is a common motor control technique. With this arrangement, a simple push of the stop switch causes the XIO instruction to go false, which temporarily (IN 2 goes false) and permanently (XIC OUT 1 goes false) stops power to the motor.
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