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Programmable Logic Controller (PLC) Questions Answers

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Herein, we have covered several Programmable Logic Controller (PLC) related topics such as Basic Rules of Line DiagramsLine Diagrams—Signals, Decisions, and Action, and Logic Functions in detail.

Basic Rules of Line Diagrams

  1. State the basic rules that determine how and where two or more loads are connected into a control circuit.
  • No more than one load should be placed in any circuit line between the power lines L1 and L2.
  • Two loads must not be connected in series in one line of a line diagram.
  • Loads must be connected in parallel when more than one load must be connected in the line diagram.
  • Control relay coils, solenoids, and pilot lights are loads that are connected directly or indirectly to L2.
  • The number of NC overload contacts between the starter coil and L2 depends on the type of starter and power used in the circuit.
  • One to three NC overload contacts may be shown between the starter and L2 in all line diagrams. One to three NC overload contacts are shown because starters may include one, two, or three overload contacts, depending on the manufacturer and motor used.
  1. State the basic rules that determine how and where two or more switches are connected into a control circuit.
  • A circuit may contain as many control devices as are required to make the operating coil function as specified. These control devices may be connected in series or parallel when controlling an operating coil.
  • Two control devices (a flow switch and a temperature switch) can be connected in series to control a coil in a magnetic motor starter.
  • Two control devices (a pressure switch and a foot switch) can be connected in parallel to control a coil in a magnetic motor starter.
  • Regardless of how the control devices are arranged in a circuit, they must be connected between L1 and the operating coil (or load).
  • The contacts used and the way the control devices are connected into a circuit (series or parallel) determines the function of the circuit.

     3. Explain how to add manufacturer’s terminal numbers to any given control circuit drawn in line (ladder)    diagram format.

  • Manufacturers of electrical relays, timers, counters, etc., include numbers on the terminal connection points. These terminal numbers are used to identify and separate the different component parts (coil, NC contacts, etc.) included on the individual pieces of equipment.
  • Manufacturer’s terminal numbers are often added to a line diagram after the specific equipment to be used in the control circuit is identified.

Line Diagrams—Signals, Decisions, and Action

  1. Identify the components in an electrical circuit as being a part of the signal, decision, or action section of a control circuit.
  • A circuit must respond as designed, without any changes. To accomplish this consistency, all control circuits are composed of three basic sections: the signals, the decisions, and the action sections.
  • A signal starts or stops the flow of current by closing or opening the contacts of the control device. Pushbuttons, limit switches, flow switches, foot switches, temperature switches, and pressure switches may be used as the signal section of a control circuit. All signals depend on some condition that must take place. This condition can be manual, mechanical, or automatic.
  • The decision section of a circuit determines what work is to be done and in what order the work is to occur. The decision section of a circuit adds, subtracts, sorts selects, and redirects the signals from the control devices to the load. The decision section of the circuit accepts informational inputs (signals), makes logical decisions based on the way the control devices are connected to the circuit and provides the output signal that controls the load.
  • Once a signal is generated and the decision has been made within a circuit, some action (work) should result. In most cases, it is the operating coil in the circuit that is responsible for initiating the action. This action is direct when devices such as motors, lights, and heating elements are turned on as a direct result of the signal and the decision. This action is indirect when the coils in solenoids, magnetic starters, and relays are energized.

Logic Functions

  1. Define and give an example of switches connected for AND circuit control logic.
  • AND logic is used in industry when two NO pushbuttons are connected in series to control a solenoid.
  • With AND logic, the load is ON only if all the control signal contacts are closed.
  • Any control device such as limit switches, pressure switches, etc., with NO contacts, can be used in developing AND logic.
  • A simple example of AND logic takes place whenever an automobile that has an automatic transmission is started. The ignition switch must be turned to the start position and the transmission selector must be in the park position before the starter is energized. Before the action (load ON) in the automobile circuit can take place, the control signals (manual) must be performed in a logical manner (decision).
  1. Define and give an example of switches connected to OR circuit control logic.
  • OR logic is used in industry when a NO pushbutton and a NO temperature switch are connected in parallel.
  • The logic of a circuit with a load that is a heating element controlled by two control devices is OR logic because either the pushbutton or the temperature switch energizes the load.
  • In OR logic, the load is ON if any one of the contacts of the control signal is closed. The control devices are connected in parallel.
  • An example of OR logic is in a dwelling that has two pushbuttons controlling one bell. The bell (load) may be energized by pressing (signal ON) either the front or the back pushbutton (control device).
  1. Define and give an example of how a switch is connected for NOT circuit control logic.
  • NOT logic has an output if the control signal is OFF. For example, replacing NO contacts on a pushbutton with NC contacts energizes the solenoid and pilot light without pressing the pushbutton.
  • An example of NOT logic is the courtesy light in a refrigerator. The light is ON if the control signal is OFF. Any time the door is open (signal OFF), the load (courtesy light) is ON.
  1. Define and give an example of switches connected for NOR circuit control logic.
  • NOR logic is an extension of NOT logic in which two or more NC contacts in series are used to control a load.
  • In a circuit with NOR logic, additional operator safety is provided by adding several emergency stop pushbuttons (NOT logic) to the control circuit.
  • By incorporating NOR logic, each machine may be controlled by one operator, but any operator or supervisor can have the capability of turning off all the machines on the assembly line to protect individual operators or the entire system.
  1. Define and give an example of switches connected for NAND circuit control logic.
  • NAND logic is an extension of NOT logic in which two or more NC contacts are connected in parallel to control a load.
  • Every NOT logic must be open (signal OFF) to stop the filling process based on the input of the float switches.
  • An example of a NAND circuit is the courtesy light in an automobile. In an automobile, the courtesy lights are ON if the control signal (door switches) is OFF (normally closed). This circuit is different from a refrigerator door in that an automobile may have two or more door switches, any of which will turn on the courtesy lights.
  1. Define and give an example of switches connected to develop memory control logic.
  • Today, many of the industrial circuits require their control circuits to not only make logic decisions such as AND, OR, and NOT, but also be capable of storing, memorizing, or retaining the signal inputs to keep the load energized even after the signals are removed.
  • A switch that controls house lights from only one location is an example of a memory circuit. Memory circuits are also known as holding or sealing circuits. When the memory circuit is on, it remains on until it is turned off, and remains off until it is turned on.
  • In industrial control circuits, it is more common to find pushbuttons with return spring contacts (momentary contacts) than those that mechanically stay held in one position (maintained contacts). Auxiliary contacts are added to provide memory to circuits with pushbuttons.
  1. List the four most common gates used in digital electronics.
  • An AND gate is a device with an output that is high only when both of its inputs are high.
  • An OR gate is a device with an output that is high when either or both inputs are high.
  • A NAND gate is a device that provides a low output when both inputs are high. A NAND (NOT-AND) gate is an inverted AND function.
  • A NOR gate is a device that provides a low output when either or both inputs are high. A NOR (NOT-OR) gate is the same as an inverted OR function.
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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.