Programs for microprocessor-based systems have to be loaded into them in Machine code, that is in a sequence of binary code numbers to represent the program instructions. We can also use assembly language based on the mnemonics, e.g. LD is used to indicate the operation required to load the data that follows the LD, and a computer program called an assembler is used to translate the mnemonics into machine code.
- The power lines, or rails as they are often termed, are like the vertical sides of a ladder with the horizontal circuit lines like the rungs of the ladder.
- The horizontal rungs show only the control portion of the circuit, in the case shown in the figures above, it is just the switch in series with the motor.
- Circuit diagram often show the relative physical location of the circuit components and how they are actually wired.
- With ladder diagrams no attempt is made to show the actual physical locations and the emphasis is on clearly showing how the control is exercised.
Ladder Diagram Example
- In the normal state, push button 1 is open and push button 2 closed.
- When button 1 is pressed, the motor circuit is completed and the motor starts. Also, the holding contacts wired in parallel with the motor close and remain closed as long as the motor is running.
- Thus when the push button 1 is released, the holding contacts maintain the circuit and hence the power to the motor.
- To stop the motor, button 2 is pressed. This disconnects the power to the motor and the holding contacts open.
- Thus when push button 2 is released, there is still no power to the motor.
Thus we have a motor which is started by pressing button 1 and stopped by pressing button 2.