Upon power-up they will still have the same contents as before power was removed. They can also typically be used to store data when power is removed from the PLC. They are usually used as temporary storage for math or data manipulation. DATA STORAGE : Typically there are registers assigned to simply store data.They can be transistors, relays, or triacs depending upon the model chosen. They physically exist and send on/off signals to solenoids, lights, etc. OUTPUT RELAYS-(coils): These are connected to the outside world.Others include off-delay and both retentive and non-retentive types. The most common type is an on-delay type. They come in many varieties and increments. TIMERS : These also do not physically exist.Typically these counters can count up, down or both up and down. They are simulated counters and they can be programmed to count pulses. COUNTERS : These again do not physically exist.Some are on only once during power-on and are typically used for initializing data that was stored. Some are always on while some are always off.

There are also some special relays that are dedicated to performing only one task. They are simulated relays and are what enables a PLC to eliminate external relays. INTERNAL UTILITY RELAYS-(contacts) : These do not receive signals from the outside world nor do they physically exist.Typically they are not relays but rather they are transistors. They physically exist and receive signals from switches, sensors, etc. INPUT RELAYS-(contacts) : These are connected to the outside world.We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations.Appropriate circuits to receive input/output data.The primary reason for designing PLC was eliminating the large cost involved in replacing the complicated relay based machine control systems.Simply program the PLC to count its inputs and turn the solenoids on for the specified time.The bigger the process the more is need for a PLC.What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters. But what if the process included 10 switches and solenoids? We would need 10 external timers. We can do this with a simple external timer.Let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for.PLC LADDER Logic Examples PLC Study Material The user enters a program, usually via software, that gives the desired results.Ĭlick below to get plc examples. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. Programmable Logic Controller ) is a device that was invented to replace the necessary sequential relay circuits for machine control.

Plc ladder logic program