1.1 What is a PLC (Programmable Logic Controller)?
1.2 The PLCs purpose in life
1.3 History of PLCs
1.4 Recent developments
2 Basic Concepts
2.1 How the PLC operates
2.1.1 Scan cycle
3 Basic instructions
4 Wikibooks Links
5 External links
What is a PLC (Programmable Logic Controller)?
A Programmable Logic Controller, or PLC, is more or less a tiny computer with a built-in operating system (OS). This OS is highly specialized to handle incoming events in real time, i.e. at the time of their occurrence. The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and it has output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.) The system is user programmable. It uses a language called "Relay Ladder" or RLL (Relay Ladder Logic). The name of this language implies the fact that the control logic of the earlier days, which was built from relays, is being simulated. The PLCs purpose in life
The PLC is primarily used to control machinery. A program is written for the PLC which turns on and off outputs based on input conditions and the internal program. In this aspect, a PLC is similar to a computer. However, a PLC is designed to be programmed once, and run repeatedly as needed. In fact, a crafty programmer could use a PLC to control not only simple devices such as a garage door opener, but their whole house, including turning lights on and off at certain times, monitoring a custom built security system, etc. Most commonly, a PLC is found inside of a machine in an industrial environment. A PLC can run an automatic machine for years with little human intervention. They are designed to withstand most harsh environments a PLC will encounter. History of PLCs
When the first electronic machine controls were designed, they used relays to control the machine logic (i.e. press "Start" to start the machine and press "Stop" to stop the machine). A basic machine might need a wall covered in relays to control all of its functions. There are a few limitations to this type of control. Relays fail.
The delay when the relay turns on/off.
There is an entire wall of relays to design/wire/troubleshoot. A PLC overcomes these limitations, it is a machine controlled operation. Recent developments
PLCs are becoming more and more intelligent. In recent years PLCs have been integrated into electrical networks i.e. all the PLCs in an industrial environment have been plugged into a network which is usually hierarchically organized. The PLCs are then supervised by a control center. There exist many proprietary types of networks. One type which is widely known is SCADA (Supervisory Control and Data Acquisition). Basic Concepts
How the PLC operates
The PLC is a purpose-built machine control computer designed to read digital and analog inputs from various sensors, execute a user defined logic program, and write the resulting digital and analog output values to various output elements like hydraulic and pneumatic actuators, indication lamps, solenoid coils etc. Scan cycle
Exact details vary between manufacturers, but most PLCs follow a 'scan-cycle' format. Overhead
Overhead includes testing I/O module integrity, verifying the user program logic hasn't changed, that the computer itself hasn't locked up (via a watchdog timer), and any necessary communications. Communications may include traffic over the PLC programmer port, remote I/O racks, and other external devices such as HMIs (Human Machine Interfaces). Input scan
A 'snapshot' of the digital and analog values present at the input cards is saved to an input memory table. Logic execution
The user program is scanned element by element, then rung by rung until the end of the program, and resulting values written to an output...