PLC Programmer Certification
Examination Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
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Question 1
In a PLC scan cycle, which step occurs immediately after the input scan?
A. Program execution
B. Output update
C. Housekeeping
D. Communication processing
Answer: B. Output update
Rationale: The PLC scan cycle follows a fixed sequence: input scan, program
execution, output update, and housekeeping/communications. Immediately
after the input scan, the CPU executes the user program, but the physical
outputs are not updated until the output update step. However, in the classic
scan cycle, the order is Input Scan → Program Execution → Output Update. The
question asks for the step immediately after the input scan—that is program
execution. Correction: The correct answer is A. Program execution. (Self-
correction: The scan cycle order is Input Scan, then Program Execution, then
Output Update, then Housekeeping. So after input scan comes program
execution.)
(Note: The above is a corrected rationale. For the exam, the final answer is A.)
,Question 2
Which addressing mode is most commonly used in PLC ladder logic to reference a
physical input point?
A. Direct addressing
B. Indirect addressing
C. Indexed addressing
D. Symbolic addressing
Answer: A. Direct addressing
Rationale: Direct addressing uses a fixed memory address (e.g., I:1/0) to refer to
a specific physical input. Indirect and indexed addressing use pointers or offsets,
which are less common for simple I/O. Symbolic addressing assigns names but
still resolves to a direct address. For physical inputs, direct addressing is
standard.
Question 3
A PLC with a 16-bit analog input module has a resolution of 0–32767 counts for a
4–20 mA signal. What is the current value if the count is 16384?
A. 8 mA
B. 10 mA
C. 12 mA
D. 16 mA
Answer: C. 12 mA
Rationale: The range 4–20 mA corresponds to 0–32767 counts. The current = 4
mA + (count / 32767) * (20-4). For count 16384, current = 4 + (16384/32767)16 ≈
4 + 8 = 12 mA.*
Question 4
In RSLogix 5000, which data type is used for a single-bit output?
A. BOOL
B. INT
,C. DINT
D. REAL
Answer: A. BOOL
Rationale: BOOL is the Boolean data type for a single bit (0 or 1). INT and DINT
are integer types, REAL is floating-point. Outputs are typically BOOL in tag-
based PLCs.
Question 5
What is the maximum number of rungs that can be in a single ladder logic routine
in most modern PLCs?
A. 255
B. 1024
C. 65535
D. No practical limit (limited by memory)
Answer: D. No practical limit (limited by memory)
Rationale: Modern PLCs do not enforce a fixed rung count; the limit is the
available user memory. Older PLCs had limits, but contemporary systems allow
thousands of rungs as memory permits.
Question 6
Which instruction is used to perform a bitwise OR operation in ladder logic?
A. XOR
B. OR
C. AND
D. NOT
Answer: B. OR
Rationale: The OR instruction performs a bitwise logical OR. XOR is exclusive OR,
AND is bitwise AND, NOT is inversion. In ladder, OR is often implemented with
parallel branches, but the instruction itself is OR.
, Question 7
A normally open contact in ladder logic is true when:
A. The input is de-energized
B. The input is energized
C. The output is energized
D. The timer is done
Answer: B. The input is energized
Rationale: A normally open (NO) contact closes (conducts) when the associated
input or bit is true (energized). It is false when de-energized.
Question 8
What is the primary purpose of the watchdog timer in a PLC?
A. To time motor start-up
B. To detect CPU faults or scan overruns
C. To control batch processes
D. To synchronize communications
Answer: B. To detect CPU faults or scan overruns
Rationale: The watchdog timer monitors the scan cycle time. If the scan exceeds
a preset value, the watchdog triggers a fault, indicating a potential hardware or
software hang.
Question 9
Which protocol is commonly used for Ethernet-based industrial communication in
PLC systems?
A. Modbus RTU
B. PROFIBUS DP
C. EtherNet/IP
D. DeviceNet
Examination Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
Download Pdf
Question 1
In a PLC scan cycle, which step occurs immediately after the input scan?
A. Program execution
B. Output update
C. Housekeeping
D. Communication processing
Answer: B. Output update
Rationale: The PLC scan cycle follows a fixed sequence: input scan, program
execution, output update, and housekeeping/communications. Immediately
after the input scan, the CPU executes the user program, but the physical
outputs are not updated until the output update step. However, in the classic
scan cycle, the order is Input Scan → Program Execution → Output Update. The
question asks for the step immediately after the input scan—that is program
execution. Correction: The correct answer is A. Program execution. (Self-
correction: The scan cycle order is Input Scan, then Program Execution, then
Output Update, then Housekeeping. So after input scan comes program
execution.)
(Note: The above is a corrected rationale. For the exam, the final answer is A.)
,Question 2
Which addressing mode is most commonly used in PLC ladder logic to reference a
physical input point?
A. Direct addressing
B. Indirect addressing
C. Indexed addressing
D. Symbolic addressing
Answer: A. Direct addressing
Rationale: Direct addressing uses a fixed memory address (e.g., I:1/0) to refer to
a specific physical input. Indirect and indexed addressing use pointers or offsets,
which are less common for simple I/O. Symbolic addressing assigns names but
still resolves to a direct address. For physical inputs, direct addressing is
standard.
Question 3
A PLC with a 16-bit analog input module has a resolution of 0–32767 counts for a
4–20 mA signal. What is the current value if the count is 16384?
A. 8 mA
B. 10 mA
C. 12 mA
D. 16 mA
Answer: C. 12 mA
Rationale: The range 4–20 mA corresponds to 0–32767 counts. The current = 4
mA + (count / 32767) * (20-4). For count 16384, current = 4 + (16384/32767)16 ≈
4 + 8 = 12 mA.*
Question 4
In RSLogix 5000, which data type is used for a single-bit output?
A. BOOL
B. INT
,C. DINT
D. REAL
Answer: A. BOOL
Rationale: BOOL is the Boolean data type for a single bit (0 or 1). INT and DINT
are integer types, REAL is floating-point. Outputs are typically BOOL in tag-
based PLCs.
Question 5
What is the maximum number of rungs that can be in a single ladder logic routine
in most modern PLCs?
A. 255
B. 1024
C. 65535
D. No practical limit (limited by memory)
Answer: D. No practical limit (limited by memory)
Rationale: Modern PLCs do not enforce a fixed rung count; the limit is the
available user memory. Older PLCs had limits, but contemporary systems allow
thousands of rungs as memory permits.
Question 6
Which instruction is used to perform a bitwise OR operation in ladder logic?
A. XOR
B. OR
C. AND
D. NOT
Answer: B. OR
Rationale: The OR instruction performs a bitwise logical OR. XOR is exclusive OR,
AND is bitwise AND, NOT is inversion. In ladder, OR is often implemented with
parallel branches, but the instruction itself is OR.
, Question 7
A normally open contact in ladder logic is true when:
A. The input is de-energized
B. The input is energized
C. The output is energized
D. The timer is done
Answer: B. The input is energized
Rationale: A normally open (NO) contact closes (conducts) when the associated
input or bit is true (energized). It is false when de-energized.
Question 8
What is the primary purpose of the watchdog timer in a PLC?
A. To time motor start-up
B. To detect CPU faults or scan overruns
C. To control batch processes
D. To synchronize communications
Answer: B. To detect CPU faults or scan overruns
Rationale: The watchdog timer monitors the scan cycle time. If the scan exceeds
a preset value, the watchdog triggers a fault, indicating a potential hardware or
software hang.
Question 9
Which protocol is commonly used for Ethernet-based industrial communication in
PLC systems?
A. Modbus RTU
B. PROFIBUS DP
C. EtherNet/IP
D. DeviceNet
