Final PLC Topic

Explain the function of the backplane of a PLC rack.

The backplane supplies current and voltage for the modules from the power supply

How do contactors differ from relays?

The contactor is built to handle heavy power loads that are beyond the capacity of control relays.

What is a seal-in circuit?

a method of maintaining current flow after a momentary switch has been pressed and released.

Define the term protocol as it applies to a network.

a set of rules that two or more devices must follow if they are to communicate with each other.

What is a programmable logic controller (PLC)?

a specialized computer used to control machines and process.

How does time proportioning provide analog control?

time proportioning varies the ratio of on time to off. The output is averaged over the amount of time it is on, providing analog control.

What is DeviceNet used for?

to communicate information with, as well as supply power to, the field devices that are connected to the bus

Why should emergency stop circuits be hardwired instead of programmed?

in the event of a total controller failure, means for an independent and rapid shutdown are available.

What operations are performed by the processor during the program scan?

reads all the inputs, takes these values, and energizes or de-energizes the outputs according to the user program.

What information is normally stored in the RAM memory of a PLC?

used as a temporary storage area of data that may need to be quickly changed

What does the instruction set for a particular PLC refer to?

A list of different types of instructions supported.

Why are PLCs installed within an enclosure?

For protection from atmospheric conditions and to minimize the effects of electromagnetic radiation

What methods are used to keep enclosure temperatures within allowable limits?

Proper spacing of components within the enclosure Fan or blower

What is the main characteristic of a closed-loop control system?

Using feedback in which the output of a process affects the input control signal.

What is Ethernet/IP used for?

make plug-and-play interoperability possible among complex devices from multiple vendors. Allows interoperability of industrial automation devices and control equipment on the same network used for business applications and browsing the Internet.

In what way are timers and counters similar?

Timers and Counters both increment (accumulates) counts.

Explain the function of a watchdog timer circuit.

The watchdog timer circuit will time out if a processor hard wire malfunction occurs and immediately halts the operation of the PLC.

How are the retentive timer and the reset instruction related?

They have the same tag name

What is an Up/Down counter?

A CTU counter and a CTD counter are used together with the same address to form an UP/Down counter.

What is Fieldbus used for?

A network for field devices used in process control applications.

What type of tasks function as timed interrupts?

Periodic

What type of protocol does Modbus use?

Serial

How is an internal relay programmed in the ControlLogix system?

by creating a tag (either program or controller type) and assigning a Boolean type to the tag.

Explain how a tachometer is used to measure rotational speed.

small AC or DC generator that develops an output voltage proportional to its rpm whose phase or polarity depends on the rotor's direction of rotation.

What type of access control is used with DH+?

Peer-to-peer communication implementing token passing.

What is the standard programming language used with PLCs?

Relay ladder logic (RLL)

What is ASCII code?

alphanumeric code, which includes letters as well as numbers

Define the term node as it applies to a network.

Each device connected on a network is known as a node.

What does the address assigned to an instruction indicate?

what PLC input is connected to what input device and what PLC output will drive what output device.

Describe the make-up of a module-defined structure.

Module-defined structures are automatically created when the I/O modules are configured for the system. Tags of these types are created to store input, output, and configuration data for the module.

What does the derivative action of a controller respond to?

The speed at which the error signal is changing; that is, the greater the error change, the greater the correcting output

List four factors that enter into the length of the scan time.

The speed of the processor module. The length of the ladder program. The type of instructions executed The actual ladder true/false conditions

Describe three typical modes of operation that can be selected by the keyswitch of a processor.

Run mode, program mode, and remote mode

Why are PLC sequencers easier to program than PLC discrete outputs?

Sequencer instructions simplify your ladder program by allowing you to use a single instruction or pair of instructions to perform complex operations.

Explain the function of the suspend instruction.

It is used to trap and identify specific conditions for program debugging and system troubleshooting.

In what way does internal output differ from a discrete output?

Unlike a discrete output, an internal output does not directly control an output field device.

How does the proportional controller eliminate the cycling associated with on/off control?

allows the final control element to take intermediate positions between on and off, which permit analog control of the final control element to vary the amount of energy to the process, depending on how much the value of the measured variable has shifted from the desired value.

Explain the selectable timed interrupt function.

allows you to interrupt the scan of the main program file automatically, on a time basis, to scan a specified subroutine file.

What information is provided by the cross reference function?

allows you to see each instance of a particular address.

What advantage is there to the nesting of subroutines?

make complex programming easier and program operation faster.

What is ControlNet used for?

positioned one level above DeviceNet. It is used to connect controllers in an industrial network.

How does a bit shift register manipulate individual bits?

serially (from bit to bit) through an array in an orderly fashion.

The programmable controller operates in real time. What does this mean?

that an event taking place in the field will result in an operation or output taking place.

A first in, first out word shift register operates in asynchronous manner. Explain what this means.

two separate shift pulses are required: one to shift into the file (LOAD), and one to shift data out of the file (UNLOAD). These two shift pulses operate independently (asynchronously) of each other.

What information is normally stored in the ROM memory of a PLC?

used to store the programs and data that define the capabilities of the PLC.

What are data compare instructions used for?

compare data stored in two or more words (registers) and make decisions based on the program instructions.

What is the function of the file of a sequencer?

contains the data that will be transferred to the destination address when the instruction undergoes a false-to-true transition.

How are reed switch sensors actuated?

if the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together to actuate the switch.

When are input branch instructions used as part of a ladder logic program?

in your application program to allow more than one combination of input conditions.

What electronic element can be used as the switching device for a 120 VAC discrete output interface module?

Triac

To what value is the accumulated value of a timer normally set?

Zero

What electronic element can be used as the switching device for DC discrete output module?

Transistor

What is the difference between a produced tag and a consumed tag?

A produced tag sends data while a consumed tag receives data.

Summarize the basic grounding requirements for a PLC system.

A properly installed grounding system will provide a low-impedance path to earth ground. A central ground bus bar is provided as a single point of reference inside the enclosure to which all chassis and power supply equipment grounding conductors are connected.

Describe the make-up of a predefined structure.

A structure-type tag is a grouping of different data types that function as a single unit and serve a specific purpose. The controller creates predefined structures for you that include timers, counters, messages and PID types.

Define a tag in the ControLogix system.

A tag is a text-based name for an area of the controller's where data is stored.

What are tags used for?

A tag is created to represent the data and identify areas in the controller's memory where this data is stored.

Explain the function of tasks within the project.

A task is a collection of scheduled programs. When a task is executed, the associated programs are executed in the order listed.

What type of cable is used when connecting a thermocouple to a voltage sensing analog input module. Why?

A twisted shielded pair cable, reduces unwanted electrical noise signals that can be induced in the conductors from other wiring

Explain the purpose of a redundant PLC processor.

Allows transfer of control to the second processor in the event of a processor fault

Define the term sign bit.

An extra digit used to represent the polarity of a binary number

Explain the function of the fault routine file.

It determines how the processor responds to a programming error.

Compare bit level and word level addressing.

Bit level addressing specifies a discrete device that is connected to a specific terminal. Word level addressing specifies an analog device connected to a module that uses a word of information.

In what ways are control relay coils and contacts rated?

Coils are rated for the type of operating current (DC or AC) and normal operating voltage. Contacts are rated in terms of maximum amount of current the contacts are capable of handling at a specified voltage level and type (AC or DC)

Summarize the two main functions of a SCADA system.

Collecting data and structuring the data for generating reports.

What is meant by the tag display style?

Different styles the programming software will display the tag when monitoring.

What is the function of a network gateway?

Gateways make communication possible between different architectures and protocols.

In ladder logic programming into what two broad categories can instruction types be classified?

Input and Output instructions

What term of a PID control is designed to eliminate offset?

Integral

What does a project contain?

RSLogix software stores a controller's programming and configuration information in a file called a project.

Explain how a diode is connected to suppress a DC inductive load.

The diode is connected in reverse-bias across the solenoid load. In normal operation, the electric current can't flow through the diode, so it flows through the solenoid coil. When voltage to the solenoid is switched off a voltage opposite in polarity to the original applied voltage is generated by the collapsing magnetic field. The induced voltage creates a current flow through the diode bleeding of the high voltage spike.

The same address is used for two coil instructions with the same PLC program. What will happen as a consequence of this?

The monitored rung may be true; but if a rung farther down in the ladder diagram is false, the PLC will keep the output off.

What happens when the processor encounters a temporary end instruction?

The processor resets the watchdog timer, performs an I/O update, and begins running the ladder program at the first instruction in the main program.

Compare the memory storage characteristics of volatile and nonvolatile memory elements.

Volatile: Loses all its stored information if all operating power is lost or removed. Memory is easily altered and usually supported by a battery backup power supply Nonvolatile: Retains stored information when the power is accidentally or intentionally removed. Memory is generally unalterable.

When is the output of a PLC counter energized?

When the accumulated count equals the preset count

Explain the difference between the timed and instantaneous contacts of a mechanical timing relay.

When the coil is energized, the timed contacts are delayed from opening or closing while the instantaneous contacts close or open immediately

When is the output of a programmed timer energized?

When the preset time equals the accumulated time

When does the rung of a TOF timer begin accumulating time?

When the rung of ladder logic the TOF is located on goes from true-to-false.

The value of an OTE instruction as it appears in the Monitor Tags window is 1. Explain what this means as far as the status of a real-world field output and programmed XIC and XIO instructions associated with this tag are concerned.

When true it can be used to energize an output device or simply set a value in memory to one.

What type of operations are sequencers most suitable for?

Whenever a repeatable operating pattern is required

Explain two ways of creating tags.

You may create tags in the tag editor before your program is entered, enter tag names as you program, or use question marks [?] in place of tag names and assign the tags later.

What is a logic gate?

a circuit with several inputs but only one output that is activated by particular combinations of input conditions.

What is the main advantage of the jump instruction?

ability to reduce the processor scan time by jumping over instructions not pertinent to the machine's operation at that instant

Outline the sequence of events involved in a PLC scan cycle.

reads all the inputs, takes these values, and energizes or de-energizes the outputs according to the user program.

Explain the binary principle.

the idea that many things can be thought of as existing in only one of two states, the states are 1 and 0, where 1 represents on or high or true and 0 represents off or low or false.

How does an optical encoder work?

uses a light source shining on an optical disk with lines or slots that interrupt the beam of light to an optical sensor. An electronic circuit counts the interruptions of the beam and generates the encoder's digital output pulses.

What advantage do tag-based addressing systems have over rack/slot and rack/group types?

the allocation of variable names for program values is not tied to specific memory locations in the memory structure, as is the case with rack/slot and rack/group type systems.

Describe the make-up of a user-defined structure.

A user-defined structure supplements the predefined structures by providing the ability to create custom-defined structures to store and handle data as a group.

What timer status bit is set to 1 when the TON timer times out?

Done bit

Which routine is configured to execute first?

Main routine

The current required for the control circuit of a magnetic starter is normally much smaller than that required by the power circuit. Why?

The control circuit requires only a small amount of current to energize the coil, while the power circuit must be capable of handling the much higher motor load current.

In a PLC counter instruction, what rule applies to the addressing of the counter and reset instruction?

They both have the same address.

Why do PLCs execute memory-checking routines?

To be sure that the PLC memory has not been corrupted

Why are parity bits used?

To detect errors that may occur while a word is being moved

Explain how redundant media works.

Two separate cables are installed to guard against failures such as cut cables, loose connectors, or noise

Explain the difference between a unipolar and a bipolar analog input module.

Unipolar modules can accept an input signal that varies in the positive direction only. Bipolar signals swing between a maximum negative value and a maximum positive value

What is the verify results function used for?

Used to verify program logic.

What operating state will cause an inductive load to generate a very high voltage spike?

When current in an inductive load is interrupted or turned off, a very high voltage spike is generated.

in general, what do data manipulation instructions allow the PLC to do?

allows the movement, manipulation, or storage of data that exists in either single or multiple word groups from one data memory area of the PLC to another.

Explain how an OSR (one shot rising) instruction can be used to freeze rapidly changing data.

can be used to freeze rapidly changing data because it will only allow one scan worth of data through the line of logic that the OSR is situated on to the instruction utilizing the data.