Introductory PLC Programming


What is a PLC (Programmable Logic Controller)?Edit

A Programmable Logic Controller, or PLC, is more or less a small computer with a built-in operating system (OS). This OS is highly specialized and optimized to handle incoming events in real time, i.e., at the time of their occurrence.

The PLC has input lines, to which sensors are connected to notify of events (such as temperature above/below a certain level, liquid level reached, etc.), and output lines, to which actuators are connected to effect or signal reactions to the incoming events (such as start an engine, open/close a valve, and so on).

The system is user programmable. It uses a language called "Relay Ladder" or RLL (Relay Ladder Logic). The name of this language implies that the control logic of the earlier days, which was built from relays, is being simulated.

Some other languages used include:

  1. Sequential Function chart
  2. Functional block diagram
  3. structured Text
  4. Instruction List

PLC usage scenariosEdit

A PLC is primarily used to control machinery. A program written for a PLC consists basically of instructions to turn on and off outputs based on input conditions and the internal program. In this respect, it is similar to how a standard computer application is used. Once a PLC program is activated, however, it will typically run continuously as a loop for an indefinite period. PLC-based systems are frequently used not only to control simple devices such as a garage door opener, but also for solving complex application scenarios, such as controlling a whole house, including switching lights on or off at certain times, monitoring custom built security system, and so on.

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.

History of PLCsEdit

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 developmentsEdit

PLCs are becoming more and more intelligent. In recent years PLCs have been integrated into electrical communications such as Computer network(s) 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 centre. There exist many proprietary types of networks. One type which is widely known is SCADA .

Basic ConceptsEdit

How the PLC operatesEdit

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 cycleEdit

Exact details vary between manufacturers, but most PLCs follow a 'scan-cycle' format.

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 memory table.
Diagnosis and communication 
is used in many different disciplines with variations in the use of logics, analytics, and experience to determine "cause and effect". In systems engineering and computer science, it is typically used to determine the causes of symptoms, mitigations, and solutions. it is communicate to input module and send message to output module for any incorrect data files variations.
Output scan 
Values from the resulting output memory table are written to the output modules.

Once the output scan is complete the process repeats itself until the PLC is powered down.

The time it takes to complete a scan cycle is, appropriately enough, the "scan cycle time", and ranges from hundreds of milliseconds (on older PLCs, and/or PLCs with very complex programs) to only a few milliseconds on newer PLCs, and/or PLCs executing short, simple code.

Basic instructionsEdit

Be aware that specific nomenclature and operational details vary widely between PLC manufacturers, and often implementation details evolve from generation to generation.

Often the hardest part, especially for an inexperienced PLC programmer, is practicing the mental ju-jitsu necessary to keep the nomenclature straight from manufacturer to manufacturer.

Positive Logic (most PLCs follow this convention)
True = logic 1 = input energized.
False = logic 0 = input NOT energized.
Negative Logic
True = logic 0 = input NOT energized
False = logic 1 = input energized.
Normally Open
(XIC) - eXamine If Closed.
This instruction is true (logic 1) when the hardware input (or internal relay equivalent) is energized.
Normally Closed
(XIO) - eXamine If Open.
This instruction is true (logic 1) when the hardware input (or internal relay equivalent) is NOT energized.
Output Enable
(OTE) - OuTput Enable.
This instruction mimics the action of a conventional relay coil.
On Timer
(TON) - Timer ON.
Generally, ON timers begin timing when the input (enable) line goes true, and reset if the enable line goes false before setpoint has been reached. If enabled until setpoint is reached then the timer output goes true, and stays true until the input (enable) line goes false.
Off Timer
(TOF) - Timer OFF.
Generally, OFF timers begin timing on a true-to-false transition, and continue timing as long as the preceding logic remains false. When the accumulated time equals setpoint the TOF output goes on, and stays on until the rung goes true.
Retentive Timer
(RTO) - Retentive Timer On.
This type of timer does NOT reset the accumulated time when the input condition goes false.

Rather, it keeps the last accumulated time in memory, and (if/when the input goes true again) continues timing from that point. In the Allen-Bradley construction, this instruction goes true once setpoint (preset) time has been reached, and stays true until a RES (RESet) instruction is made true to clear it.

Latching Relays
(OTL) - OuTput Latch.
(OTU) - OuTput Unlatch.

Generally, the unlatch operator takes precedence. That is, if the unlatch instruction is true then the relay output is false even though the latch instruction may also be true. In Allen-Bradley ladder logic, latch and unlatch relays are separate operators.

However, other ladder dialects opt for a single operator modeled after RS (Reset-Set) flip-flop IC chip logic.

Jump to Subroutine
(JSR) - Jump to SubRoutine
For jumping from one rung to another the JSR (Jump to Subroutine) command is used.

External linksEdit