SACA Programmable Logic Controllers 4 — Questions and Answers
Question 1: 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 an exact, fixed memory location in the PLC program that remains constant during execution.
Question 2: In a PLC ladder diagram, what does a normally closed (NC) contact represent in terms of the physical input signal?
- The contact passes current only when the input coil is energized
- The contact passes current when the input coil is de-energized (Correct answer)
- The contact is always open regardless of input state
- The contact toggles state with each scan cycle
Correct answer: The contact passes current when the input coil is de-energized
A normally closed contact in ladder logic passes current (is true) when its associated coil or input is de-energized or off.
Question 3: What is the primary purpose of a PLC's watchdog timer?
- To schedule periodic backups of the program
- To synchronize communication between PLCs
- To detect if the scan cycle exceeds a maximum allowed time and trigger a fault (Correct answer)
- To count elapsed time for timer instructions
Correct answer: To detect if the scan cycle exceeds a maximum allowed time and trigger a fault
The watchdog timer monitors the PLC scan cycle duration and triggers a fault or system reset if the cycle time exceeds a preset maximum, preventing runaway programs.
Question 4: Which IEC 61131-3 programming language uses graphical blocks connected by signal lines to represent program logic?
- Structured Text (ST)
- Instruction List (IL)
- Function Block Diagram (FBD) (Correct answer)
- Sequential Function Chart (SFC)
Correct answer: Function Block Diagram (FBD)
Function Block Diagram (FBD) uses interconnected graphical function blocks and signal lines to represent control logic.
Question 5: A PLC analog input module receives a 4–20 mA signal from a pressure transmitter. What does a 4 mA signal typically represent?
- 50% of the process range
- 100% of the process range
- 0% of the process range (minimum span) (Correct answer)
- A fault or broken wire condition
Correct answer: 0% of the process range (minimum span)
In a 4–20 mA loop, 4 mA corresponds to 0% (minimum) of the process variable range, and 20 mA corresponds to 100%.
Question 6: What type of PLC instruction is used to move data from one memory register to another?
- TON (Timer On-Delay)
- MOV (Move) (Correct answer)
- CTU (Count Up)
- XIC (Examine If Closed)
Correct answer: MOV (Move)
The MOV (Move) instruction copies the value from a source register to a destination register in PLC memory.
Question 7: In PLC network communications, what does the term 'master-slave' topology refer to?
- One device initiates all communications and others respond only when polled (Correct answer)
- All devices transmit simultaneously on a shared bus
- Each device has equal priority to initiate communication
- Two PLCs share a single processor for redundancy
Correct answer: One device initiates all communications and others respond only when polled
In a master-slave topology, the master device controls the network by polling slave devices, which only respond when addressed.
SACA Programmable Logic Controllers 4
Which PLC addressing mode uses a fixed memory location that does not change during program execution?