The most commonly used PLC programming languages are structured text and ladder diagram. For technicians with limited background, starting with ladder diagrams is the fastest way to learn PLC programming. Regardless of PLC brand, ladder diagrams closely resemble real electrical control circuits. Below are several commonly used control circuits for review.
1. Start, Latch and Stop Circuits
The ladder diagrams below implement starting, latching, and stopping for output Y10. X0 is the start signal and X1 is the stop signal. Figures a and c use Y10's normally open contact to achieve self-holding latching, while figures b and d use SET and RST instructions to implement the latching function.
2. Multi-location Control Circuit
The diagram below shows control of a relay coil from two locations. X0 and X1 are the start and stop buttons at one location. X2 and X3 are the start and stop buttons at the other location.
3. Interlock Control Circuit
The diagram below is an interlock circuit for three output coils. X0, X1, and X2 are start buttons, and X3 is the stop button. Since only one of Y0, Y1, Y2 can be energized at a time, the normally closed contacts of Y0, Y1, Y2 are placed in series into the control circuits of the other two coils to prevent simultaneous operation.
4. Sequential Start Control Circuit
As shown, the normally open contact of Y0 is placed in the control circuit of Y1, so Y1 can only be energized when Y0 is energized. When Y0 is deenergized, Y1 is also deenergized. While Y0 is energized, Y1 can be started and stopped independently. X0 and X2 are start buttons; X1 and X3 are stop buttons.
5. Centralized and Distributed Control
On an automated line composed of multiple single machines, interlocking is used between centralized control at a main console and distributed control at individual machine panels. The ladder diagrams for centralized and distributed control are shown. X2 is a selector switch whose contact interlocks centralized and distributed control. When X2 is ON, distributed start control at individual machines is active. When X2 is OFF, centralized main start control is active. In both modes, individual and main panels can issue stop commands.
