EETC PLC Programming 2 — Questions and Answers
Question 1: In ladder logic, what does a normally closed (NC) contact do when its associated coil is energized?
- Passes power (closes)
- Blocks power (opens) (Correct answer)
- Latches in the open state
- Triggers a timer reset
Correct answer: Blocks power (opens)
A normally closed contact opens (blocks power flow) when its associated coil or bit is energized.
Question 2: Which PLC addressing mode uses a fixed memory location that does not change during program execution?
- Indirect addressing
- Indexed addressing
- Direct addressing (Correct answer)
- Symbolic addressing
Correct answer: Direct addressing
Direct addressing specifies a fixed, absolute memory location in the PLC's data table that remains constant throughout execution.
Question 3: What is the primary function of a PLC's accumulator register in arithmetic operations?
- Stores the program counter value
- Holds intermediate results of math operations (Correct answer)
- Buffers I/O module data
- Stores the scan time value
Correct answer: Holds intermediate results of math operations
The accumulator is a working register that holds intermediate values during arithmetic, comparison, and data manipulation operations.
Question 4: A TOF (Timer Off-Delay) instruction begins timing when its rung condition:
- Goes from false to true
- Goes from true to false (Correct answer)
- Has been true for the preset time
- Is first scanned by the CPU
Correct answer: Goes from true to false
A TOF timer starts its countdown when the rung condition transitions from true to false (de-energized).
Question 5: In Allen-Bradley ControlLogix PLCs, which data type is used to store a 32-bit floating-point value?
- DINT
- INT
- REAL (Correct answer)
- BOOL
Correct answer: REAL
The REAL data type in ControlLogix stores a 32-bit IEEE 754 single-precision floating-point number.
Question 6: What does the term 'scan time' refer to in PLC operation?
- Time to download a new program
- Time required to complete one full execution cycle of the program (Correct answer)
- Time between I/O module updates
- Time to establish Ethernet communication
Correct answer: Time required to complete one full execution cycle of the program
Scan time is the total time for the PLC to read inputs, execute the logic program, and update outputs for one complete cycle.
Question 7: Which instruction is used in ladder logic to compare two values and energize an output only when the first value is greater than the second?
- EQU
- NEQ
- GRT (Correct answer)
- LES
Correct answer: GRT
The GRT (Greater Than) compare instruction passes power when the value in Source A is strictly greater than the value in Source B.
EETC PLC Programming 2
In ladder logic, what does a normally closed (NC) contact do when its associated coil is energized?