FINAL REVIEW PLC ACTUAL EXAM
QUESTIONS WITH VERIFIED ANSWERS
What is a programmable logic controller (PLC)?
A programmable logic controller (PLC) is a specialized computer used to control machines and
process.
Identify four tasks in addition to relay switching operations that PLCs are capable of
performing.
Counting, timing, calculating, comparing and the processing of analog signals.
List six distinct advantages that PLCs offer over conventional relay-based control systems.
"Increased Reliability
More Flexibility
Lower Cost
Communications Capability
Faster Response Time
Easier to Troubleshoot"
Explain the differences between open and proprietary PLC architecture.
An open architecture design allows the system to be connected easily to devices and programs
made by other manufacturers. A proprietary architecture makes it more difficult to connect to
another manufacturer's system.
State two ways in which I/O is incorporated into the PLC.
Fixed and modular configurations
Describe how the I/O modules connect to the processor in a modular-type PLC
configuration.
The I/O modules plug into a rack. When a module is slid into the rack, it makes a connection
with a series of contacts called the backplane. The PLC processor is also connected to the
backplane and, therefore, can communicate with all modules in the rack.
Explain the main function of each of the following major components of a PLC: Processor
module (CPU)
Accepts input data from sensing devices, executes the stored user program from memory, and
sends appropriate output commands to control devices.
Explain the main function of each of the following major components of a PLC: I/O
modules
Forms the interface by which field devices are connected to the controller.
,Explain the main function of each of the following major components of a PLC:
Programming device
Used to enter the desired program into the memory of the processor.
Explain the main function of each of the following major components of a PLC: Power
supply module
Supplies DC power to other modules.
What are the two most common types of PLC programming devices?
Laptop computers and personal computers
Explain the terms program and programming language as they apply to a PLC.
A program is a user-developed series of instructions that directs the PLC to execute actions.
A programming language provides rules for combining the instructions so that they produce the
desired actions.
What is the standard programming language used with PLCs?
Relay ladder logic (RLL) is the standard programming language used with PLCs.
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: When do the pressure switch contacts close?
When the pressure reaches the preset value
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: When do the temperature switch contacts close?
When the temperature reaches the preset value
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: How are the pressure and temperature switches connected with
respect to each other?
In series
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: Describe the two conditions under which the motor starter coil will
become energized.
If the pressure switch and temperature switch contacts close or if the pushbutton contacts close.
What is the approximate value of the voltage drop across each of the following when their
contacts are open: Pressure switch
0 VAC
,What is the approximate value of the voltage drop across each of the following when their
contacts are open: Temperature switch
0 VAC
What is the approximate value of the voltage drop across each of the following when their
contacts are open: Manual pushbutton
120 VAC
The programmable controller operates in real time. What does this mean?
A programmable logic controller operates in real time in that an event taking place in the field
will result in an operation or output taking place.
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What do the individual symbols represent?
Instructions
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What do the numbers represent?
Addresses
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What field device is the number 1/2 identified with?
Temperature switch
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What field device is the number 0/1 identified with?
Motor starter coil
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What two conditions will provide a continuous path from left
to right across the rung?
If contacts I/1 and I/2 are closed
If contact I/3 is closed
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: Describe the sequence of operation of the controller for one
scan of the program.
Inputs are examined, and their status is recorded in the controller's memory. The ladder diagram
is evaluated. The output is set to 0 or 1 according to whether or not a complete current path can
be traced from left to right.
, Compare the method by which the process control operation is changed in a relay-based
system to the method used for a PLC-based system.
Relay system is changed by changing the wiring. PLC system is changed by changing the
program.
Compare the PLC and PC with regard to: Physical hardware differences
Unlike personal computers, the PLC comes with its program language built into its memory and
has no permanently attached keyboard, CD drive, or monitor.
Compare the PLC and PC with regard to: Operating environment
Unlike personal computers, PLCs are designed to operate with wide ranges of ambient
temperature and humidity.
Compare the PLC and PC with regard to: Method of programming
Unlike personal computers, the PLC is programmed in relay ladder logic or other easily learned
languages.
Compare the PLC and PC with regard to: Execution of program
PLCs execute a single program in a sequential order, while computers execute several programs
or tasks simultaneously in any order.
What two categories of software written and run on PCs are used in conjunction with
PLCs?
Software that allows the user to write and document a PLC program. Software that allows the
user to monitor and control a PLC program.
What is a programmable automation controller (PAC)?
A PAC is a device that combines PLC-style control and ruggedness with PC functionality.
List four criteria by which PLCs are categorized.
Functionality
List four criteria by which PLCs are categorized.
Number of inputs and outputs
List four criteria by which PLCs are categorized.
Cost
List four criteria by which PLCs are categorized.
Physical size
Compare the single-ended, multitask, and control management types of PLC applications.
QUESTIONS WITH VERIFIED ANSWERS
What is a programmable logic controller (PLC)?
A programmable logic controller (PLC) is a specialized computer used to control machines and
process.
Identify four tasks in addition to relay switching operations that PLCs are capable of
performing.
Counting, timing, calculating, comparing and the processing of analog signals.
List six distinct advantages that PLCs offer over conventional relay-based control systems.
"Increased Reliability
More Flexibility
Lower Cost
Communications Capability
Faster Response Time
Easier to Troubleshoot"
Explain the differences between open and proprietary PLC architecture.
An open architecture design allows the system to be connected easily to devices and programs
made by other manufacturers. A proprietary architecture makes it more difficult to connect to
another manufacturer's system.
State two ways in which I/O is incorporated into the PLC.
Fixed and modular configurations
Describe how the I/O modules connect to the processor in a modular-type PLC
configuration.
The I/O modules plug into a rack. When a module is slid into the rack, it makes a connection
with a series of contacts called the backplane. The PLC processor is also connected to the
backplane and, therefore, can communicate with all modules in the rack.
Explain the main function of each of the following major components of a PLC: Processor
module (CPU)
Accepts input data from sensing devices, executes the stored user program from memory, and
sends appropriate output commands to control devices.
Explain the main function of each of the following major components of a PLC: I/O
modules
Forms the interface by which field devices are connected to the controller.
,Explain the main function of each of the following major components of a PLC:
Programming device
Used to enter the desired program into the memory of the processor.
Explain the main function of each of the following major components of a PLC: Power
supply module
Supplies DC power to other modules.
What are the two most common types of PLC programming devices?
Laptop computers and personal computers
Explain the terms program and programming language as they apply to a PLC.
A program is a user-developed series of instructions that directs the PLC to execute actions.
A programming language provides rules for combining the instructions so that they produce the
desired actions.
What is the standard programming language used with PLCs?
Relay ladder logic (RLL) is the standard programming language used with PLCs.
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: When do the pressure switch contacts close?
When the pressure reaches the preset value
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: When do the temperature switch contacts close?
When the temperature reaches the preset value
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: How are the pressure and temperature switches connected with
respect to each other?
In series
Answer the following with reference to the process control relay ladder diagram of Figure
1-17 of this chapter: Describe the two conditions under which the motor starter coil will
become energized.
If the pressure switch and temperature switch contacts close or if the pushbutton contacts close.
What is the approximate value of the voltage drop across each of the following when their
contacts are open: Pressure switch
0 VAC
,What is the approximate value of the voltage drop across each of the following when their
contacts are open: Temperature switch
0 VAC
What is the approximate value of the voltage drop across each of the following when their
contacts are open: Manual pushbutton
120 VAC
The programmable controller operates in real time. What does this mean?
A programmable logic controller operates in real time in that an event taking place in the field
will result in an operation or output taking place.
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What do the individual symbols represent?
Instructions
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What do the numbers represent?
Addresses
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What field device is the number 1/2 identified with?
Temperature switch
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What field device is the number 0/1 identified with?
Motor starter coil
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: What two conditions will provide a continuous path from left
to right across the rung?
If contacts I/1 and I/2 are closed
If contact I/3 is closed
Answer the following with reference to the process control PLC ladder logic diagram of
Figure 1-20 of this chapter: Describe the sequence of operation of the controller for one
scan of the program.
Inputs are examined, and their status is recorded in the controller's memory. The ladder diagram
is evaluated. The output is set to 0 or 1 according to whether or not a complete current path can
be traced from left to right.
, Compare the method by which the process control operation is changed in a relay-based
system to the method used for a PLC-based system.
Relay system is changed by changing the wiring. PLC system is changed by changing the
program.
Compare the PLC and PC with regard to: Physical hardware differences
Unlike personal computers, the PLC comes with its program language built into its memory and
has no permanently attached keyboard, CD drive, or monitor.
Compare the PLC and PC with regard to: Operating environment
Unlike personal computers, PLCs are designed to operate with wide ranges of ambient
temperature and humidity.
Compare the PLC and PC with regard to: Method of programming
Unlike personal computers, the PLC is programmed in relay ladder logic or other easily learned
languages.
Compare the PLC and PC with regard to: Execution of program
PLCs execute a single program in a sequential order, while computers execute several programs
or tasks simultaneously in any order.
What two categories of software written and run on PCs are used in conjunction with
PLCs?
Software that allows the user to write and document a PLC program. Software that allows the
user to monitor and control a PLC program.
What is a programmable automation controller (PAC)?
A PAC is a device that combines PLC-style control and ruggedness with PC functionality.
List four criteria by which PLCs are categorized.
Functionality
List four criteria by which PLCs are categorized.
Number of inputs and outputs
List four criteria by which PLCs are categorized.
Cost
List four criteria by which PLCs are categorized.
Physical size
Compare the single-ended, multitask, and control management types of PLC applications.
