PLC chapter 6

List three functions of a PLC programming terminal device.

enter, change or monitor a PLC program

Most PLC modules use plug-in wiring terminal strips. Why?

For ease of wiring and replacing modules

What makes the binary system so applicable to computer circuits?

The binary system uses only the digits 0 and 1, and computer circuits can easily distinguish between the two voltage levels that correspond to the binary digits 0 and 1.

Identify two matrix limitations that may apply to certain PLCs.

a maximum of seven parallel lines and ten series contacts per rung is possible.

What is a seal-in circuit?

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

What is a programmable logic controller (PLC)?

a specialized computer used to control machines and process.

State the base used for each of the following number systems: Octal Decimal Binary Hexadecimal

a) Base 8 b) Base 10 c) Base 2 d) Base 16

What information is stored in the output image table file?

stores the status of the output terminals for the controller. Information in the output image table is stored in the form of bits.

What does the memory map for a typical PLC processor consist of?

program files and data files.

What information is stored in the input image table file?

stores the status of the input terminals for the controller. Information in the input image table is stored in the form of bits.

What is the most common approach taken with regard to the measurement of fluid flow?

to convert the kinetic energy that the fluid has into some other measurable form.

What is the function of the optical isolator circuit?

used to provide electrical isolation between the field wiring and the PLC backplane internal circuitry.

What is required for a personal computer to be used as a PLC programming terminal?

Appropriate programming software

Explain the function of the backplane of a PLC rack.

supplies current and voltage for the modules from the power supply

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.

How are reed switch sensors actuated?

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.

Give an explanation of how a scanner and a decoder act in conjunction with each other to read a bar code.

A light source within the scanner illuminates the bar code symbol; those bars absorb light and spaces reflect light. A photo detector collects this light in the form of an electronic-signal pattern representing the printed symbol.

A normally closed push button is connected to a PLC discrete input. Does this mean it must be represented by a normally closed contact in the ladder logic program? Explain why or why not.

A normally closed pushbutton connected to a PLC discrete input does not have to be represented by a normally closed contact in the ladder logic program. The action of the field devices should be separated from the action of the PLC bits.

Explain how a tachometer is used to measure rotational speed.

A tachometer generator is a 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 cable is used when connecting a thermocouple to a voltage sensing analog input module. Why?

A twisted shielded pair cable is used in wiring the circuit to reduce 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

What information is normally stored in an EEPROM memory module?

An EEPROM memory module is used to store, back up, or transfer PLC programs

What is ASCII code?

An alphanumeric code, which includes letters as well as numbers

Explain the basic operating principle of an electromagnetic control relay.

An electromagnetic control relay consists of an electromagnet or coil used to switch contacts. When the coil is energized, an armature moves to open or close contacts.

What is the construction and operation of an electromechanical latching relay different from a standard relay?

An electromechanical latching relay differs from a standard relay by it having two coils, one to latch the contacts and one to unlatch the contacts, and a latching relay does not have to constantly energize its coil to keep its contacts closed.

Define the term sign bit.

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

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.

How does the processor identify the location of a specific input or output device?

By its address

How does an ultrasonic sensor operate?

By sending sound waves toward the target and measuring the time it takes for the pulses to bounce back.

Compare discrete and analog I/O modules with respect to the type of input or output devices which with they can be used.

Discrete I/O modules allow only ON/OFF devices to be connected. Analog I/O modules allow analog or varying voltage or current devices to be connected.

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)

Give one advantage and one limitation to the use of a hand-held programming device.

Handheld programmers are compact, inexpensive, and easy to use but they have limited display capabilities.

A normally open limit switch is to be programmed to control a solenoid. What determines whether an Examine -on or Examine-off contact instruction is used?

If the contacts need to close to energize the output, then the EXAMINE IF CLOSED is used.

In what way does tag-based addressing differ from rack/slot-based addressing?

In tag-based addressing you use a tag (alphanumeric name) to address data (variables). In rack/slot-based addressing a fixed numeric format is used to identify the address data.

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

Input branch instruciton are used in your application program to allow more than one combination of input conditions.

What do the abbreviations NO and NC represent when used to describe switch contacts?

NO stands for a normally open pushbutton switch that completes a circuit when it is pressed and returns to its open position when it is released. NC stands for a normally closed pushbutton switch that opens a circuit when it is pressed and returns to the close position when it is released.

What is the operating difference between a normally open and normally closed relay contact?

Normally open contacts are open when no current flows through the coil but close as soon as the coil is energized. Normally closed contacts are closed when the coil is de-energized and open when the coil is energized.

How many programs can a PLC have stored in memory at any one time?

One

What is the most important characteristic of the Gray code?

Only one digit changes as you count up from zero

Compare the operation of open-loop and closed-loop control.

Open loop is control without feedback. Closed-loop control compares speed or position feedback with the commanded speed or position.

Name the two distinct sections of an I/O module.

Power and Logic sections

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

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

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

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

Explain the purpose of Windows based programming software such as RSLogix.

RSLOGIX is used to develop and monitor ladder logic programs.

What is the standard programming language used with PLCs?

Relay ladder logic (RLL)

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

Run mode, program mode, and remote mode

Explain the principle of operation of a strain gauge.

Strain gauges are based on the principle that the resistance of a conductor varies with length and cross-sectional area. The force applied to the gauge causes the gauge to bend and change its resistance

Explain the difference between the 1's complement of a number and the 2's complement.

The 1's complement of a binary number is the number that results when all the 1's are changed to 0's and all the 0's are changed to 1's.

Compare the function of the CPU and memory sections of a PLC processor.

The CPU section executes the program and makes the decisions needed by the PLC to operate and communicate with other modules. The memory section electronically stores the PLC program along with other retrievable digital information

What does the address assigned to an instruction indicate?

The address assigned to an instruction indicates what PLC input is connected to what input device and what PLC output will drive what output device.

Compare the operation of the reflective-type and through-beam photoelectric sensors.

The reflective-type detects the light beam reflected from the target. The through-beam type measures the change in light caused by the target crossing the optical axis.

Explain the principle of operation of a thermocouple.

The temperature difference between the measuring and reference junction of a thermocouple causes a small DC voltage to be generated.

What are the advantages of a processor that utilizes a flash memory card?

They are extremely fast at saving and retrieving files

What is the function of a PLC interface module?

To accept signals from the machine or process devices and to convert them into signals that can be used by the controller

Outline the method used to actuate inductive and capacitive proximity sensors.

To actuate inductive sensors, we need a conductive material. Capacitive sensors may be actuated by both conductive and nonconductive materials.

Why do PLCs execute memory-checking routines?

To be sure that the PLC memory has not been corrupted

What is the function of a PLC output interface module?

To convert controller signals into external signals that are used to control the machine or process

Why are parity bits used?

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

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

Transistor

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

Triac

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.

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.

Explain the function of the analog-to-digital (A/D) converter circuit used in analog input modules.

Used to convert analog signals to an equivalent digital value

Explain the function of the digital-to-analog (D/A) converter circuit used in analog output modules.

Used to convert digital signals to an equivalent analog value

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.

Name the two general sensing classifications for analog input modules.

Voltage sensing and current sensing

What is a logic gate?

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

Briefly describe each of the following PLC modes of operations: Program Test Run

a) Used to enter a new program or update an existing one in the internal RAM memory b) Used to operate or monitor the user program without energizing any outputs c) Used to execute the user program with input devices being monitored and output devices energized accordingly

In what manner are data files organized?

by the type of data they contain.

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.