PLC MID-TERM REVIEW
State two ways in which I/O is incorporated into the PLC.
Fixed configurations modular configurations
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.
Explain the function of the backplane of a PLC rack.
The backplane supplies current and voltage for the modules from the power supply
With reference to a PLC: What is a remote rack? (a) Why are remote racks used? (b)
a) A rack that is located away from the processor module near the field devices b) To minimize the amount of wiring required
What are the functions of a PLC programming device? 3
monitor enter change the user program.
Define the term sign bit.
polarity of a binary number
The ______ may perform functions such as timing, counting and comparing in addition to logic processing.
processor
What does the instruction set for a particular PLC refer to?
A list of different types of instructions supported.
The location of a specific input or output field device is identified by the processor by means of its:
Address
List three functions of a PLC programming terminal device. MEC
Allows the user to enter, change, monitor a PLC program
Explain the purpose of a redundant PLC processor.
Allows transfer of control to the second processor in the event of a processor fault
What do PC-based control systems use to interface with field devices?
An input/output PC interface card
How does the processor identify the location of a specific input or output device?
By its address
In a PLC system, there is a physical connection between field input devices and output devices. T/F
False
The programming device must be connected to the controller to run the program. t/f
False
______ are designed to plug into a slot or connector. (True or False)
I/O modules
How many programs can a PLC have stored in memory at any one time?
One
When a new program is loaded into a ______ _______ , the old program that was stored in the same locations is over-written and essentially erased.
PLC's memory
Name the two distinct sections of an I/O module.
Power sections Logic sections
State five other functions, in addition to simple logic processing, that PLC processors are capable of performing. TLC & CMC
Timing latching counting comparing motion control complex math functions
What electronic element can be used as the switching device for DC discrete output module?
Transistor
Which of the following devices can be used for switching the output of a discrete DC output module?
Transistor or Relay
A PLC is basically a computer designed for use in electrical control applications. t/f
True
An analog input or output is a signal that varies continuously within a certain range. (True or False)
True
Removing the programming device from the PLC will not affect the operation of the user program. t/f
True
The instruction set for a particular PLC lists the types of instructions supported t/f
True
The programmable controller operates in real time t/f
True
When dealing with PLC memory, one K of memory represents 1024 bits. t/f
True
The processor module of the PLC is where the: a. ladder logic program is stored. . b. input connections are made. c. output connections are made d. sensors are located.
a. ladder logic program is stored.
Batteries are used in a PLC's processor to a. maintain data in volatile memory when line power is removed from the processor. b. operate the status lights LEDs. c. maintain data in nonvolatile memory when line power is removed from the processor. d. maintain outputs through a power failure.
a. maintain data in volatile memory when line power is removed from the processor.
All servo motors: a. operate in the closed-loop mode. b. operate in the open-loop mode. c. operate without feedback. d. both b and c.
a. operate in the closed-loop mode.
The most commonly used programming device is a: a. personal computer. b. dedicated industrial programming terminal. c. hand-held programmer. d. proprietary programming device
a. personal computer.
one function of a PLC input interface module is to _____ signals from field devices and ____ them into signals that can be used by the processor
accept && convert
Field or real-world devices refer to: a. input devices only. b. output devices only. c. load devices only. d. all devices that are physically wired to the PLC.
all devices that are physically wired to the PLC
Which of the following special I/O modules would be used to operate a seven-segment LED Display? a. Encoder-counter module. b. BCD-output module. c. Stepper-motor module. d. High-speed counter module.
b. BCD-output module.
A human machine interface (HMI) a. allows the user to monitor a process. b. allows the user to control a process. c. can provide a graphical representation of a process . d. all of the above.
d. all of the above.
Unlike personal computers, PLCs are: a. equipped with input and output modules. b. equipped with a control programming language. c. designed for the industrial environment. d. all of the above.
d. all of the above.
Solenoid valves are available to control: a. oil flow. b. air flow. c. water flow. d. all of these
d. all of these
The Output Latch (OTL) instruction: a. can turn an output on, but it cannot turn the output off. b. is used only to turn a bit on and latch it on. c. allows the output to remain on even if the rung changes to false. d. all of these
d. all of these
Electronic components found in PLC modules a. are not effected by electrostatic voltages. b. can be damaged by electrostatic voltages. c. can have their performance degraded by electrostatic voltages. d. both b and c.
d. both b and c.
An encoder is used to: a. encode signals from a scanner b. decode signals from a scanner c. change DC to AC d. convert motion into a digital signal
d. convert motion into a digital signal
the main element of an analog output module is
digital to analog converter
_____ I/O modules allow only ON/OFF devices to be connected.
discrete
incoming control signals to a PLC are called a _______
input
Basically, the function of a PLC is to make _____ decisions and control outputs_ based on them.
logical
power required to operate the logic circuits of the processor unit is typically _____ voltage _____
low voltage DC
batteries are used in a PLCs processor to
maintain data and volatile memory when line power is removed from the process
A _____ PLC system is configured using two processors
redundant
the symbol and a ladder logic diagram represents a virtual ______
relay
PLCs were originally designed as replacements for ____ _____ _____
relay control panels
the following statement that does not apply to the optical isolator circuit used in I/O modules is that it
rests lies AC signals
To operate the program, the controller is placed in ____ mode.
run
The ___________ is the process of reading inputs, executing the program, and setting outputs on a continuous basis.
scan
____ time required for one complete execution of the user program
scan
a personal computer communicates with the PLC processor via a ____ or paralell data communication link
serial
A ______ sensor a sensor that actuates an electrical switch/outputs a signal when a light beam is detected.
through-beam photoelectric
E octal number system consists of digits zero, one, two, three, four, five, six, and seven. There are no eights or nines t/f
true
Shielded twisted pair cable is used for connecting to thermocouple inputs to reduce unwanted electrical noise. t/f
true
The scan time is the time required for one complete execution of the user program. t/f
true
a PLC power supply module does not normally supply power to the field device T/F
true
a redundant PLC system is configured using two processors t/f
true
digital modules are also called discrete modules t/f
true
in the PLC system, there is a no physical connection between the field input device and the output device T/F
true
plug-in compartments allow I/O modules to be easily connected and replaced T/F
true
removing the programming device from the PLC will not affect the operation of the user program T/F
true
software installed and run on a personal computer can be used to write a PLC program t or f
true
the base of a number system determines the total number of unique symbols used by that system t/f
true
the octal number system consist of digits zero, one, two, three, four, five, six, and seven there are no8 and 9 t/f
true
high density I/O modules may have up to _____ inputs or outputs per module
64
all digital computing devices operate using the binary number system because digital circuits can be easily distinguished
between two voltage levels
PLCs work on_____ numbers in one form or another to represent various code quantities
binary
With reference to PLC discrete input modules: What types of field input devices are suitable for use with them? List three examples of discrete input devices.
ON/OFF devices such as selector switches pushbuttons limit switches
a non-volatile memory will retain its programmed contents if operating power is lost T/F
true
If you had a hand-held programming terminal from one manufacturer you can program only that manufacture's PLC using it. T/F
T
Information stored in a RAM memory location can be written into or read from. T/F
T
There is more than one correct way to implement the ladder logic for a given control process. t/f
T
7 PLC outputs
Control relays Lights Alarms Motor starters Pumps Fans Valves
A tachometer normally refers to a(n) ____ used for speed measurement. a. load cell b. capacitive proximity sensor c. ultrasonic sensor d. small generator
d. small generator
the decimal system uses the number nine as its base t/f
false
_____ isolation used in I/O modules helps to reduce the effects of electrical noise.
Optical
programmable logic controllers are categorized according to the # of ____ points
I/O
digitl is same as
discrete
_____memory is used by the PLC's operating system. a. RAM b. EEPROM c. Flash d. ROM
d. ROM
Analog & Digital input is a continuous field input to the _____ ______ drives
Variable Frequency
When a field device contact connected to the input module closes: a. a logic 1 is recorded in the memory location of the coil with the same address. b. a logic 1 is recorded in the memory location of the contact with the same address. c. a logic 0 is recorded in the memory location of the coil with the same address. d. a logic 1 is recorded in the memory location of the contact with the same address.
b. a logic 1 is recorded in the memory location of the contact with the same address.
In event of a power interruption, a _____ is used in some processors to provide power to the RAM. a. inductor b. capacitor c. transistor d. resistor
b. capacitor
in the event of a power interruption, a______ is used in some processors to provide power to the RAM
capacitor
Basically, the function of a PLC is to make logical decisions and ____ outputs based on them.
control
Identify four tasks in addition to relay switching operations that PLCs are capable of performing.
Counting calculating comparing processing of analog signals.
____ swappable I/O modules are designed to be changed with the power on and the PLC operating
Hot swappable
The Allen-Bradley SLC-500 address I:2/4 refers to an:
Input module in slot 2, terminal 4.
One advantage of discrete relay contact output modules is that they can be used with ____ or ____ devices.
AC or DC
A ____ (3) is an example of a device that could be used to provide a discrete input to a PLC.
Pushbutton selector switch limit switch.
List three field devices that are commonly controlled by a PLC analog output module.
Control valves chart recorder electronic drives
The maximum current rating for the individual outputs of an AC output module is usually in the 20 to 30 ampere range. (True or False)
False
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.
At the start of the PLC scan the status of all ____ are read.
inputs are read.
List five common physical quantities/MEASUREMENTS by a PLC analog input module.
speed TEMPERATURE pressure position flow level weight
a modular PLC that has room for several I/O modules is capable of being customized for particular application t/f
true
a thermocouple would be classified as an analog input sensing device t/f
true
modifying relay type process control circuits usually involves changing the ____ ____
circuit wiring
modifying the relay type process control circuits usually involve changing the _____ _____
circuit wiring
The number of I/O points does not affect the memory size required for a PLC installation. T/F
false
when dealing with a PLC memory one K of memory represents _____ bits
1024
All servo motors operate in open loop. T/F
F
What is the most important characteristic of the Gray code?
Only one digit changes as you count up from zero
Which module of the PLC connects directly to field devices such as pilot lights, motor starters, and solenoids?
Output
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
Outline four functions that an HMI interface screen can be configured to perform.
Replace hard-wired pushbuttons and pilot lights with realistic-looking icons alarms, complete with time of occurrence and locations SHOW operations in graphic format change timer and counter presets Display variables as they change over time
Describe three typical modes of operation that can be selected by the keyswitch of a processor.
Run mode program mode remote mode
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 a nd all the 0's are changed to 1's.
Why do PLCs execute memory-checking routines?
To be sure that the PLC memory has not been corrupted
_____ sensors operate using ultrasonic waves.
Ultrasonic
does the programmable controller operate in real time?
Yep
the control plan stored on a PLC is called a ______
a program
Which module of the PLC is responsible for performing logical operations? a. Processor b. Input c. Output d. Power supply
a. Processor
the purpose of lead indicator is to ____ and a fault _____
aid and a fault diagnosis
Analog signals can have ______ states
an infinite number of states or different voltages.
The power required to operate the logic circuits of the processor unit is typically: a. low voltage ac. b. high voltage ac. c. low voltage dc. d. high voltage dc.
c. low voltage dc.
a _____ signal has two states or voltages.
digital
Most PLC electronic components are sensitive to _______ discharge
electrostatic
RAM memory is nonvolatile in PLC's T/F
false
The expression of a number in binary requires fewer digits than in decimal system t/f
false
based 10 - _____ system
hexadecimal system
What is a programmable logic controller (PLC)?
industrial computer control system specialized computer used to control machines and process.
a control management PLC application normally requires a ___ -___ plc
large-size PLC
a control management PLC application normally requires a _____-_____ plc
large-size PLC
the___________ significant bit of a word is the digit that represents the smallest value
least
the ASC2 code include the ____ & ____
letters & numbers
the PC program is stored in the processor modules ______
memory
programmable logic controllers are categorized according to the ____ of I/O points
number
which of the following number systems has a base eight
octal
discrete I/O interfaces allow only___/_____ type devices to be connected
on/off
a personal computer communicates with the PLC processor via a serial or ____ data communication link
parallel
an______bits is used to detect errors that may occur while a word ismoved
parity
the purpose of those in their zener diode capital (ZD) is to set the minimum level of ______ that can be detected
set the minimum level of voltage that can be detected
at the start of the PLC scan the status of all _____are read
status of all inputs are read
base eight - _____ system
the decimal system
A _______ would be classified as an analog input sensing device.
thermocouple
What allows the discrete relay type output module to switch either AC or DC load devices.
uses an electromechanical relay element
the only allowable digits in the binary number system are
zero or one
The PLC chassis comes in different sizes according to the ____ of slots they contain.
#
The decimal system has a base of _________. A. 0 B. 10 C. 100 D. 1000
B. 10
What electronic element can be used as the switching device for a 120 VAC discrete output interface module?
Triac
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
What is the function of the optical isolator circuit?
An optical isolator is used to provide electrical isolation between the field wiring and the PLC backplane internal circuitry.
______ I/O modules allow analog or varying voltage or current devices to be connected.
Analog I/O
What is required for a personal computer to be used as a PLC programming terminal?
Appropriate programming software
The acronym BCD stands for:
Binary Coded Decimal
What type of I/O modules have both inputs and outputs connected to them?
Combination I/O modules
In addition to field devices what other connections are made to a PLC module?
Connections for the power supply
_______ memory modules used to copy a program from one PLC to another usually contain memory
EEPROM
In a magnetic motor starter, the control circuit is required to handle the full load current of the motor. t/f
F
A ____ is an example of an actuator that could be controlled by a discrete output from a PLC.
Motor starter
List five factors affecting the memory size needed for a particular PLC installation.
Number of I/O points size of control program data-collecting requirements supervisory junctions required future expansion.
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.
What is the standard programming language used with PLCs?
Relay ladder logic (RLL) is the standard programming language used with PLCs.
A small current flows through a solid-state proximity sensor even when the output is turned OFF t/f
T
HMI screens are developed using a software package on a PC which is downloaded into the PLC operator interface device. T/F
T
What two categories of software written and run on PCs are used in conjuncture with PLCs?
WDMC Software to write and document a PLC program. Software to monitor and control a PLC program.
When placed in the ___mode, the processor does not scan/execute the ladder program. a) program b) run c) test d) remote
a) program
A ______ module is used to establish connections for the exchange of data. a. thumbwheel b. communication c. servo d. PID
b. communication
A contactor a. is another name for a relay. b. is designed to handle heavy power loads. c. always has an overload relay physically and electrically attached. d. is a physically small relay.
b. is designed to handle heavy power loads.
The following statement that does not apply to the optical isolator circuit used in I/O modules is that it: a. separates high voltage and low voltage circuits b. rectifies AC signals. c. prevents damage caused by line voltage transients. d. reduces the effect of electrical noise.
b. rectifies AC signals.
The force applied to a strain gauge causes it to bend and change its: a. temperature. b. resistance. c. voltage. d. current.
b. resistance.
The sum of the _____ current drawn for all modules in a chassis is used to select the appropriate chassis power supply rating.
backplane
A discrete output interface module is designed to provide: a. output voltages only in the 5 VDC range. b. varying AC or DC voltages depending on the type of module selected. c. ON/OFF switching of the output field device. d. binary-coded outputs.
c. ON/OFF switching of the output field device.
Which of the following is not a function of a PLC programming device? a. To enter the user program b. To change the user program c. To execute the user program d. To monitor the user program
c. To execute the user program
Programmed latching circuits are retentive. This means that if power is interrupted the output will ____ when power is returned, a. switch to the ON state. b. switch to the OFF state. c. remain in its original ON or OFF state. d. flash ON and OFF.
c. remain in its original ON or OFF state.
A(n) ____ converts electrical pulses applied to it into discrete rotor movements. a. tachometer b. solenoid c. stepper motor d. electronic magnetic flowmeter
c. stepper motor
The scan time is the time required: a. to record the status of all input devices. b. to record the status of all output devices. c. to execute one cycle of the total program. d. for the information to pass from input to output.
c. to execute one cycle of the total program.
The PLC power supply module normally is rated to provide the power for: a. all field devices. b. input field devices only. c. output field devices only d. PLC backplane and I/O modules.
d. PLC backplane and I/O modules.
Field or real-world devices refer to: a. input devices only. b. output devices only. c. load devices only. d. all devices that are physically wired to the PLC.
d. all devices that are physically wired to the PLC.
PLC software that runs on a personal computer can be used to: a. write a PLC program. b. document a PLC program. c. monitor the control process. d. all of the above.
d. all of the above.
The main difference between a PLC and relay control system is that: a. different types of input devices are used. b. different types of output devices are used. c. different input and output voltage levels are used. d. one uses hardwired relay control logic and the other uses programmed instructions.
d. one uses hardwired relay control logic and the other uses programmed instructions
______ means that each input or output has two states: true (on) or false (off).
Discrete
Why are parity bits used?
To detect errors that may occur while a word is being moved
In what two ways can the loop power for current sensing input modules be supplied?
supplied by the sensor analog output module
Three PLC Inputs
switches sensors PB's
What is ASCII code?
An alphanumeric code, which includes letters as well as numbers
A/An ___ input or output is a continuously variable signal within a designated range.
Analog
_______ modules are also called discrete modules
Digital
______ memory module store, back up, or transfer PLC programs
EEPROM
Most PLC programming software will allow you to develop programs on another manufacturers PLC. (True or False)
False
What are the two most common types of PLC programming devices?
Laptop computers personal computers
________ is used in I/O modules to seperate the real-world electrical signals from the PLC internal electronic system
Light
With reference to PLC output modules: What type of field output devices are suitable for use with them? List three examples of discrete output devices.
ON/OFF devices such as motor starters lights solenoids
The Allen-Bradley SLC-500 address O:3/0 refers to an:
Output module in slot 3, terminal 0.
The resolution of an analog input channel is specified as 0.3 mV. What does this tell you?
The analog input channel is capable of sensing voltages down to 0.3V increments.
What are the advantages of a processor that utilizes a flash memory card?
They are extremely fast at saving and retrieving files
Explain the difference between a unipolar and a bipolar analog input module.
UNIP accept an input signal that varies in the positive direction only. BIP swing between a maximum negative value and a maximum positive value.
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 current sensing
Define the following as they apply to the binary memory locations or registers: a) Bit b) Byte c) Word d0 LSB e) MSB
a) Each digit of a binary number b) A group of 8 bits c) A group of 1 or more bytes d) The digit that represents the smallest value e) the digit that represents the largest value
Volatile memory elements can be classified as those that: a. do not retain stored information when the power is removed. b. retain stored information when the power is removed. c. do not require a battery backup. d. both b and c.
a. do not retain stored information when the power is removed.