14. Programmable Logic Controllers

• Considering their popularity, PLCs will be used as the basis for exploring discrete event control systems.

• A Programmable Controller (PC) is an input/output processing computer.

• Advantages of PLCs,

cost effective for complex systems


computational abilities

trouble shooting aids


• Ladder logic was originally introduced to mimic relay logic.

• A thorough review of the research literature is available in Sobh et. al. [1994].

• The PLC can be programmed like other computers using specialized “languages”.

Ladder Logic: A programming technique using a ladder like structure. This was originally adopted because of its similarity to relay logic diagrams to ease its acceptance in manufacturing facilities. The ladder approach is somewhat limited by the lack of loops, etc. (Uses JIC standards)



Mnemonic: instructions and opcodes, similar to assembly language. More involved to program, but also more flexible than ladder logic.


Sequential Function Charts (SFC): an alternative to a flowchart method for concurrent operations. Currently this is not widely used, but it (or a similar version) should become more prevalent in the upcoming years.


• Structured text allows programs that are similar to high level languages such as BASIC.


14.1 PLC Connections


• The PLC continuously scans the inputs, and changes the outputs.

• The Process can be anything: a large press, a car, a security door, a blast furnace.

• As inputs change (e.g. a start button) the outputs will be changed. This will cause the process to change, and new inputs to the PLC will be received.


14.2 Ladder Logic

• As mentioned before, ladder logic has been developed to mimic relay logic. This was done originally to make the computer more acceptable to companies and employees. Original efforts resisted the use of computers because they required new skills and approaches, but the use of ladder logic allowed a much smaller paradigm shift, therefore making them more acceptable.

• Original Relay Ladder Logic diagrams show how to hook-up inputs to run outputs.

Relay: An input coil uses a voltage/current to create a magnetic field. As the coil becomes magnetic it pulls a metal switch (or reed) towards it, and makes an electrical contact. The contact that closes when the coil is energized is normally open. There is a contact that the reed touches without the coil energized is called the normally closed contact. Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated.


A Circuit: A mix of inputs and outputs allows logical selection of a device.



• many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown below is an example of this. It is called a seal in circuit.


14.2.1 Ladder Logic Outputs

• The outputs allow switches to close that supply/cut-off power to control devices.

• Ladder logic indicates what to do with the output, regardless of what is hooked up -- The programmer and electrician that connect the PLC are responsible for that.

• Outputs can go to electrical outputs, or to memory.

• Output symbols,



14.2.2 Ladder Logic Inputs

• Contact coils are used to connect the PLC power lines to drive the outputs.

• The inputs can come from electrical inputs, or memory locations


14.3 Problems

Problem 14.1 Develop a simple relay based controller so that three switches in a room could turn the light on or off.

Problem 14.2 Write the mnemonic program for the Ladder Logic below.

Answer 14.2 XIC 100, XIC 101, OR, OTE 201

Problem 14.3 A PLC can be used effectively to,

a) replace a few relays.

b) replace customized electronics.

c) replace a highly skilled human operator.

d) control a well understood process.

Answer 14.3 d

Problem 14.4 Why would relays be used in place of PLCs?

Answer 14.4 for simple designs

Problem 14.5 Give an example of where a PLC could be used.

Answer 14.5 to control a conveyor system

Problem 14.6 Give a concise description of a PLC.

Problem 14.7 List the advantages of a PLC over relays.

Problem 14.8 Explain the trade-offs between relays and PLCs for control applications.

Problem 14.9 In the figure below, if a limit switch is NO, the power the power flow to the output will normally be___________________.

Answer 14.9 off

Problem 14.10 In the figure above, if a limit switch input is NC, the power indicator in the input module should be __________________.

Answer 14.10 on

Problem 14.11 TRUE or FALSE: The left-most side of the PLC’s ladder diagram simulates the power line.

Answer 14.11 true

Problem 14.12 In ladder diagram programs, all outputs are represented by:

a) Contact symbols

b) Coil symbols

c) a or b

d) None of the above

Answer 14.12 b

Problem 14.13 List the advantages of PLCs over relay logic.