• Until the 1970s logic functions for machine control were implemented using relays. A wiring diagram that resembled a ladder was used by electricians to build machine control panels with relays.
• Maintenance of relay-based control panels was very difficult. Also, if a change was required due to changes in the product, the production down time was too long to rewire the panels.
• In a PLC, the control logic (ladder diagram) is implemented in software, making it very easy to modify.
• A PLC contains a CPU, I/O modules, power supply, backplane, and a programming device.
• PLCs are constructed in two ways: (1) fixed I/O and (2) modular.
• In a PLC scan cycle, the controller reads all inputs, runs the ladder diagram once, and energizes or de-energizes the output devices.
• PLC I/O and most field devices can be classified into sinking and sourcing types.
• Sourcing field devices must be connected to sinking PLC I/O. Similarly, sinking field devices must be connected to sourcing PLC I/O.
• The FEC34 PLC in the Distribution Station has 12 inputs and 8 outputs, consisting of 2 relays and 6 transistors. The inputs are the sinking type, and the transistor outputs are the sourcing type.
• Sequence control systems are processes executed in several, clearly separate steps.
• A sequential function chart contains steps, transitions, and actions.
• STL is a PLC programming language which allows the programmer to solve control tasks using simple English-like statements.
• STL programs consist of steps and sentences.
• In an STL program, execution will wait at a step until the conditions are true at which time the actions will be performed. Only then will the program proceed to the next step.
• Festo PLCs are programmed using FST programming software. FST provides an integrated environment with editing, communication, debugging, and online features.
• An FST project contains all necessary components of a PLC program and documentation.
• A program is a list of STL statements for the controller to perform a control task.
FST has various online and debugging features, such as online I/O display, online mode of the STL editor, and compilation for syntax checking.
Programmable Logic Controller
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Student Name: ________________________________ Date: ___________________
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Lesson 5 Review
1. A PLC is:
2. A ladder diagram shows:
3. A PLC consists of the following main parts:
a) b) c) d) e) f)
4. A PLC scan consists of the following steps:
a) b) c)
5. PLCs are constructed in two ways:
6. The difference between them is:
Student Name: ________________________________ Date: ___________________
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7. A normally open switch is to be connected to a PLC as an input device. Complete the wiring diagram for the following PLCs:
Generic PLC FEC32 PLC
8. Explain the difference in the wiring:
1
GRD
Sinking Input module
2 3
E0.0
GRD
FEC34 Input module
E0.1
24V 24VDC
Sensor Supply Eight Sinking Inputs E0.7
E1.0 E1.1 E1.2
E1.3 Four
Sinking Inputs
Student Name: ________________________________ Date: ___________________
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9. A pilot light is to be connected to a PLC as an output device. Complete the wiring diagram for the FEC34 PLC, if the light is connected to:
Transistor-based output Relay-based output
10. Explain the difference in the wiring:
Transistor-based
Student Name: ________________________________ Date: ___________________
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11. Complete the ladder diagram below to implement the following control logic:
RUNG
1: IF S1 is pressed OR S2 AND S3 are pressed THEN activate CR1
ELSE RUNG
2: IF both S2 AND S3 are pressed THEN activate CR2 AND CR3
Where S1, S2 and S3 are normally open switches, CR1, CR2 and CR3 are relay coils.
Power Rail
Neutral Rail
RUNG 1
RUNG 2
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Glossary
Algorithm: A list of well-defined instructions for completing a task. Starting from an initial state, the instructions describe a computation that proceeds through a well-defined series of successive states, eventually terminating in a final ending state.
Allen-Bradley controller: This is one example of a PLC (Programmable Logic Controller). The three most common Allen-Bradley Controllers are: MicroLogix (ML1200, ML1500), Control Logix, and SLC (SLC500). Other manufacturers of PLCs are Festo, Siemens, Mitsubishi, Omron, and more.
Backplane: A circuit board that connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a bus. It is used as a backbone to connect several printed circuit boards together to make up a complete system.
Banana connector: A banana connector (commonly banana plug for the male, banana jack for the female) is a single-wire (one conductor) electrical connector used for joining wires to equipment. The plugs are frequently used to terminate patch cords for electronic test equipment.
Central processing unit (CPU): An electronic circuit that can execute computer programs which are actually sets of instructions. The CPU is essentially the “brains” of a system.
Coil element: An electromagnetic coil (or simply a "coil") is formed when a conductor (usually a solid copper wire referred to as the coil element) is wound around a core or form to create an inductor or electromagnet. One loop of wire is usually referred to as a turn, and a coil consists of one or more turns. For use in an electronic circuit, electrical connection terminals called taps are often connected to a coil. Coils are often coated with varnish and/or wrapped with insulating tape to provide additional insulation and secure them in place. A completed coil assembly with taps, etc., is often called a winding.
Control console: The control unit of a mechanical, electrical, or electronic system.
Control logic: This is the part of a software architecture that controls what the program will do. This part of the program is also called the controller. Before the
Appendix
©2010, Festo Didactic Page 32 instruction reaches the control logic, it is translated into binary through an instruction decoder or "decode unit."
Digital I/O: The jack where an input/output device is physically connected to a PLC.
The input relays transfer signals to the internal relays. The output relays signals to external output devices.
Ethernet: A family of frame-based computer networking technologies for local area networks (LANs). It is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology.
Festo controller: Festo Programmable Logic Controller.
Field bus: A family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158. A complex automated industrial system — such as a manufacturing assembly line — usually needs an organized hierarchy of controller systems to function. In this hierarchy there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non time critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus which links the PLCs to the components which actually do the work, such as sensors, actuators, electric motors, console lights, switches, valves, and contactors.
Fixed I/O PLC: A PLC (Programmable Logic Controller) that has the processor and a fixed amount of Inputs and Outputs all combined in one unit. An example would be a Festo FEC or an Allen-Bradley ML1500.
FST programming software: Festo PLC Software.
Function block diagram: A diagram that describes a function between input variables and output variables. A function is described as a set of elementary blocks. Input and output variables are connected to blocks by connection lines. An output of a block may also be connected to an input of another block. Function block diagram is one of five languages for logic or control configuration supported by standard IEC 61131-3 for a control system such as a Programmable Logic Controller (PLC).
GXIEC programming software: Mitsubishi PLC Programming Software.
IEC 61131-3: The third part of the open international standard IEC 1131. Part 3 of IEC 1131 deals with programming languages and defines two graphical and two textual PLC programming language standards:
• Ladder diagram (LD), graphical
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• Function block diagram (FBD), graphical
• Structured text (ST), textual
• Instruction list (IL), textual
• Sequential function chart (SFC) has elements to organize programs for sequential and parallel control processing.
Input module: The part of the PLC that allows input signals to be connected to the PLC’s processor.
Ladder diagram: A graphical diagram based on the circuit diagrams of a relay-based logic.
Ladder logic: A programming language that represents a program by a graphical diagram based on the circuit diagrams of relay-based logic hardware. It is primarily used to develop software for Programmable Logic Controllers (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble ladders, with two vertical rails and a series of horizontal rungs between them.
Logic diagram: A graphical representation of a program using formal logic.
Mitsubishi/MELSEC controller: Mitsubishi manufactured processor driven device that uses logic-based software to provide electrical control to machines.
Modular PLC: A PLC configuration in which each component is split into a separate NOP: No Operation (computer processor instruction) NPN controller.
NPN controller: With NPN logic, switching is from – via the load to +. The reason for this is partly historical but also safety related. With NPN logic, there is a large number of terminals connected directly to the + conductor. If there is a short circuit from one of these terminals to a housing or –, no output will then work. If the same thing happens with PNP logic, the output transistor concerned will be destroyed; but all the other inputs and outputs will continue to work.
Operand: An operand is a quantity on which an operation is performed.
Operating: To act effectively; produce an effect; exert force or influence.
Output module: A device that performs a mechanical action after receiving the electrical signal to do so from the PLC output modules.
Pascal: An imperative and procedural programming language designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to
©2010, Festo Didactic Page 34 encourage good programming practices using structured programming and data structuring.
PLC board: Festo PLC board with DIN rail, terminal block sheet metal for mounting the PLC and cables.
PLC program: Any component of a PLC software ladder program. Programming components do not physically exist but are representations used by the PLC software.
PLC relay: Hard-wired physical devices that transfer electrical signals from input devices to output devices. PLCs use software to digitally simulate these connections.
PNP controller: DC input modules allow us to connect either PNP (sourcing) or NPN (sinking) transistor type devices to them. In a regular switch (i.e., toggle or pushbutton, etc.) it typically does not matter whether it is wired as NPN or PNP. We should note that most PLCs are using a sensor is used (photo-eye, prox, etc.), the output configuration is important. The difference between the two types is whether the load (the plc) is switched to ground or positive voltage. An NPN type sensor has the load switched to ground whereas a PNP device has the load switched to positive voltage.
Profibus-DP: This is the Siemens Brand name for their industrial network. Profibus (Process Field Bus) is a standard for field bus communication in automation technology and was first promoted in1989 by BMBF (German department of education and research).
Profile plate: The anodized aluminum profile plate is used for mounting all components of the MPS® stations. Both sides are slotted so, if necessary, parts can be mounted on both sides. The slots are compatible with the ITEM profile system. The board is supplied with caps for the sides. The profile plate 350/700 has a hole with a diameter of approximately 5 cm for the I/O cable that connects the PLC board to the station.
Programmable logic controller (PLC): A processor driven device that uses logic-based software to provide electrical control to machines.
RS232 cable: PC Serial Port Communications standard for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment).
Relay coil: A relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double-throw (changeover) switches.
©2010, Festo Didactic Page 35 Reset sequence: This is the process that is followed to reset the memory on a PLC.
RS Logix programming software: Allen Bradley Proprietary PLC programming software.
RS422 programming cable: Allen Bradley PC Communications cable.
Scan: One complete cycle of a PLC checking inputs, executing its programs, and updating the status of its outputs.
Scan cycle: The time it takes to determine the status of input devices (update input image table), interpret the logic (solve ladder logic), and update output devices.
Scan time: The time it takes for a controller to scan all the logic before updating its input (and output) image table.
Sequential function chart: Sequential function chart (SFC) is a graphical programming language used for PLCs. It is one of the five languages defined by IEC 1131-3 standard. Main components of SFC are steps with associated actions, transitions with associated logic conditions, and directed links between steps and transitions.
Siemens controller: A Siemens manufactured processor driven device that uses logic-based software to provide electrical control to machines.
Sinking I/O circuits: A sinking digital I/O circuit provides a ground.
Sourcing I/O circuits: A sourcing digital I/O circuit provides a voltage source.
Statement List Language (STL): STL corresponds to the Instruction List language defined in the IEC 61131-3 specification. It is utilized in Siemens PLCs.
STEP 7 programming software: Siemens PLC programming software.
Step instruction: A step instruction is a method of executing a computer program one step at a time to determine how it is functioning. This might be to determine if the correct program flow is being followed in the program during the execution or to see if variables are set to their correct values after a single step has completed.
Structured text (ST): Structured text is one of the 5 languages supported by the IEC 61131-3 standard. It is designed for programmable logic controllers (PLCs). It is a high level language that is block structured and syntactically resembles Pascal.
©2010, Festo Didactic Page 36 SysLink connector: Festo’s brand name for IEEE-488 Connectors. IEEE-488 is a short-range, digital communications bus specification that has been in use for over 30 years. Originally created for use with automated test equipment, the standard is still in wide use for that purpose.
Terminal block: The terminal block connects the PLC cable to individual functions on the station.
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Function block diagram ...9
I IEC 1131-3 ...9
L Ladder diagram ... 9, 11, 12, 18, 25 N NOP instruction ... 20
P Power supply ... 25
Program execution ... 14
Programmable Logic Controller ... 11–14, 25 S Safety ... 2
Scan cycle ... 14
Sentence ... 18
Sequence control systems ... 15–17, 25 Sinking I/O ... 15
Sourcing I/O ... 15
Statement list ... 10
Statement List Language ... 17
Step execution rules ... 21
STEP instruction... 18
STL... 19, 25 Structured text ... 10