ITMC PLC Operations 3 — Questions and Answers
Question 1: A PLC analog input module reads a 4-20 mA signal. A reading of 4 mA typically represents:
- 50% of the process range
- 100% of the process range
- 0% (minimum) of the process range (Correct answer)
- A broken wire or sensor fault
Correct answer: 0% (minimum) of the process range
In a 4-20 mA loop, 4 mA represents the zero/minimum process value and 20 mA represents full scale.
Question 2: Which PLC memory type retains its data when power is removed WITHOUT a battery backup?
- RAM (volatile memory)
- ROM or Flash EEPROM (non-volatile memory) (Correct answer)
- Data table registers
- Accumulator registers
Correct answer: ROM or Flash EEPROM (non-volatile memory)
ROM and Flash EEPROM are non-volatile memory types that retain data without power or battery backup.
Question 3: In a PLC network using Modbus RTU, the device that initiates all communication transactions is called the:
- Slave
- Node
- Master (Correct answer)
- Gateway
Correct answer: Master
In Modbus RTU, the master device initiates all requests; slave devices only respond when polled by the master.
Question 4: What is the purpose of a 'watchdog timer' built into a PLC's CPU?
- To track machine uptime hours for maintenance scheduling
- To reset the CPU if the scan cycle takes longer than a preset maximum time (Correct answer)
- To delay output activation after power-up
- To synchronize multiple PLCs on a network
Correct answer: To reset the CPU if the scan cycle takes longer than a preset maximum time
The watchdog timer detects program hangs by resetting or faulting the CPU if a scan cycle exceeds the allowed maximum time.
Question 5: During PLC troubleshooting, forcing an output ON while in RUN mode will:
- Permanently change the ladder logic program
- Energize that output regardless of the rung logic condition (Correct answer)
- Only work if the CPU is in PROGRAM mode
- Disable all other outputs on the same module
Correct answer: Energize that output regardless of the rung logic condition
Forcing overrides the program logic and holds the output at the forced state until the force is removed.
Question 6: What does PID stand for in the context of PLC process control?
- Programmed Input Device
- Proportional-Integral-Derivative (Correct answer)
- Process Input Data
- Pulse-Interval-Duration
Correct answer: Proportional-Integral-Derivative
PID (Proportional-Integral-Derivative) is a closed-loop control algorithm that minimizes the error between a setpoint and measured process variable.
Question 7: In Allen-Bradley ControlLogix PLCs, tags are stored in which type of memory organization?
- Fixed address-based memory (file/word structure)
- Tag-based (symbolic) memory structure (Correct answer)
- Sequential register stacks
- Circular buffer arrays only
Correct answer: Tag-based (symbolic) memory structure
ControlLogix uses a tag-based memory structure where engineers assign symbolic names rather than fixed numeric addresses.
ITMC PLC Operations 3
A PLC analog input module reads a 4-20 mA signal.
A reading of 4 mA typically represents: