A Virtual Programmable Logic Controller

Principles of Operation

The PLC controls the operation of a system containing electro-mechanical devices. It operates by scanning inputs to the system and noting their state, then scanning the program and turning outputs ON or OFF as dictated by the program. The inputs are then scanned again to see if the state of any of them have changed, after which the program is scanned again and the outputs adjusted accordingly. The PLC also incorporates timers, counters, and many other special functions.

The scanning operation is shown below.

The time taken to complete one scan cycle is usually between 1 mS and 40 mS, but will depend on the length of the program and the PLC instruction execution rate. Once the program has been written, it is simply a case of connecting input and output devices to complete the process control system.

Typical input devices:

- push buttons
- limit switches
- proximity pwitches

Typical output devices:

- solenoids
- motors
- contactors