1 Basic Principles of PLC Ladder Diagram Foreword: Background and Functions of PLC PLC (Programmable Logic Controller) is an electronic device, previously called “sequence controller”. In 1978, NEMA (National Electrical Manufacture Association) in the United States officially named it as “programmable logic controller”. PLC reads the status of the external input devices, e.g. keypad, sensor, switch and pulses, and execute by the microprocessor logic, sequential, timing, counting and arithmetic operations according the status of the input signals as well as the pre-written program stored in the PLC.
The generated output signals are sent to output devices as the switch of a relay, electromagnetic valve, motor drive, control of a machine or operation of a procedure for the purpose of machine automation or processing procedure. The peripheral devices (e.g. personal computer/handheld programming panel) can easily edit or modify the program and monitor the device and conduct on-site program maintenance and adjustment. The widely used language in designing a PLC program is the ladder diagram.
With the development of the electronic technology and wider applications of PLC in the industry, for example in position control and the network function of PLC, the input/output signals of PLC include DI (digital input), AI (analog input), PI (pulse input), NI (numeric input), DO (digital output), AO (analog output), and PO (pulse output). Therefore, PLC will still stand important in the industrial automation field in the future. 1.1 The Working Principles of Ladder Diagram The ladder diagram was a diagram language for automation developed in the WWII period, which is the oldest and most widely adopted language in automation.
In the initial stage, there were only A (normally open) contact, B (normally closed) contact, output coil, timer and counter…the sort of basic devices on the ladder diagram (see the power panel that is still used today). After the invention of programmable logic controllers (PLC), the devices displayable on the ladder diagram are added with differential contact, latched coil and the application commands which were not in a traditional power panel, for example the addition, subtraction, multiplication and division operations. The working principles of the traditional ladder diagram and PLC ladder diagram are basically the same.
The only difference is that the symbols on the traditional ladder diagram are more similar to its original form, and PLC ladder diagram adopts the symbols that are easy to recognize and shown on computer or data sheets. In terms of the logic of the ladder diagram, there are combination logic and sequential logic. 1. Combination Logic Examples of traditional ladder diagram and PLC ladder diagram for combination logic: Traditional Ladder Diagram PLC Ladder Diagram X0 Y0 X0 Y0 X1 Y1 X1 Y1 X2 X4 Y2 Y2 X2 X4 X3 X3 Row 1: Using a normally open (NO) switch X0 (“A” switch or “A" contact). When X0 is not pressed, the contact DVP-PLC Application Manual 1-1