ITMC PLC Operations 2 — Questions and Answers
Question 1: Which PLC addressing mode directly specifies a memory location by its physical address without any calculation?
- Immediate addressing
- Direct addressing (Correct answer)
- Indirect addressing
- Indexed addressing
Correct answer: Direct addressing
Direct addressing specifies the exact memory location (e.g., I0.0, Q0.1) without any pointer calculation.
Question 2: In a PLC program, a TON timer's accumulated value (ACC) resets to zero when:
- The timer preset is reached
- The enable (EN) bit turns OFF (Correct answer)
- The done (DN) bit turns ON
- Power is cycled to the PLC
Correct answer: The enable (EN) bit turns OFF
A TON timer's ACC resets to zero whenever the rung condition (enable input) goes FALSE.
Question 3: What does the term 'scan time' refer to in PLC operation?
- Time to download a program to the PLC
- Time required to complete one full execution cycle of the ladder logic (Correct answer)
- Time between PLC power-on and first output
- Duration of a single timer instruction
Correct answer: Time required to complete one full execution cycle of the ladder logic
Scan time is the total time for the PLC to read inputs, execute the program, and update outputs in one cycle.
Question 4: A CTU (Count Up) counter's output bit (CU) activates when:
- A false-to-true transition occurs on the count input rung
- The accumulated value equals the preset value (Correct answer)
- The counter resets
- Power is applied to the PLC
Correct answer: The accumulated value equals the preset value
The CTU done bit turns ON when the accumulated count reaches or exceeds the preset value.
Question 5: Which communication protocol is most commonly used for high-speed deterministic communication between PLCs and I/O modules on a local network?
- Modbus RTU over RS-485
- EtherNet/IP or PROFINET (Correct answer)
- HTTP over TCP/IP
- RS-232 serial
Correct answer: EtherNet/IP or PROFINET
EtherNet/IP and PROFINET are industrial Ethernet protocols designed for deterministic, high-speed PLC-to-I/O communication.
Question 6: In ladder logic, an Examine If Closed (XIC) instruction represents:
- A normally closed contact that triggers when the bit is OFF
- A normally open contact that triggers when the referenced bit is ON (Correct answer)
- An output coil that sets a bit HIGH
- A comparison instruction checking two values
Correct answer: A normally open contact that triggers when the referenced bit is ON
XIC (–| |–) passes power when the referenced bit is logic 1 (ON), mimicking a normally open contact that closes when energized.
Question 7: When a PLC's CPU enters FAULT mode, what typically happens to the outputs?
- Outputs hold their last state indefinitely
- Outputs are forced ON to maintain process safety
- Outputs are de-energized and go to a safe/off state (Correct answer)
- Outputs toggle at 1 Hz until fault is cleared
Correct answer: Outputs are de-energized and go to a safe/off state
Most PLCs de-energize all outputs when a CPU fault occurs to prevent uncontrolled machine operation.
ITMC PLC Operations 2
Which PLC addressing mode directly specifies a memory location by its physical address without any calculation?