EETC PLC Programming 4 — Questions and Answers
Question 1: Which communication protocol is most commonly used for real-time control-level communication between a PLC and I/O modules on a DeviceNet network?
- Modbus RTU
- EtherNet/IP
- CAN-based DeviceNet (Correct answer)
- PROFIBUS DP
Correct answer: CAN-based DeviceNet
DeviceNet is based on the CAN (Controller Area Network) physical and data-link layers and is widely used for device-level PLC-to-I/O communication.
Question 2: What is the effect of placing two coils with the same address (bit) in different rungs of a ladder logic program?
- A compile error prevents the program from running
- Only the first coil controls the bit; the second is ignored
- The last coil scanned determines the final state of the bit (Correct answer)
- Both coils share control equally through OR logic
Correct answer: The last coil scanned determines the final state of the bit
Duplicate output coils are a programming error; during each scan, the final coil instruction for a given address overwrites earlier ones, causing unpredictable behavior.
Question 3: A PLC analog input module receives a 4–20 mA signal from a pressure transmitter. What does a 4 mA signal typically represent?
- Sensor fault or broken wire condition
- 50% of the measurement range
- Maximum (full-scale) process value
- Minimum (zero) of the measurement range (Correct answer)
Correct answer: Minimum (zero) of the measurement range
In a 4–20 mA loop, 4 mA represents the minimum (zero) of the process range, while 20 mA represents the maximum (full-scale) value.
Question 4: What does 'forcing' an I/O point in a PLC do?
- Permanently programs the I/O point in firmware
- Overrides the program logic to fix an I/O bit at a specific state for testing (Correct answer)
- Copies the I/O value to a data file for logging
- Automatically resets all forced bits after one scan
Correct answer: Overrides the program logic to fix an I/O bit at a specific state for testing
Forcing overrides normal program control to manually set an input or output bit to a defined state, typically used during troubleshooting and commissioning.
Question 5: In Allen-Bradley PLC5 / SLC 500 addressing, what does the address 'N7:0' refer to?
- Output image file, word 7, bit 0
- Integer data file 7, element 0 (Correct answer)
- Timer file 4, element 7, accumulated value
- Counter file 5, element 0, done bit
Correct answer: Integer data file 7, element 0
In classic Allen-Bradley addressing, N designates the Integer file type, 7 is the file number, and 0 is the element (word) within that file.
Question 6: What is the purpose of the 'One-Shot Rising' (OSR) instruction in ladder logic?
- Keeps an output energized for exactly one scan after a coil is de-energized
- Produces a single true output pulse for one scan on a false-to-true rung transition (Correct answer)
- Counts rising edges and stores them in an integer register
- Resets a latch coil when a rising edge is detected
Correct answer: Produces a single true output pulse for one scan on a false-to-true rung transition
The OSR instruction generates a one-scan true output pulse only on the rising edge (false-to-true) of its input rung condition.
Question 7: Which PLC operating mode allows the programmer to monitor and edit logic while the machine is running?
- Program mode
- Test mode
- Run mode with online editing (Correct answer)
- Remote program mode
Correct answer: Run mode with online editing
Online editing in Run mode allows a technician to modify and download logic changes to a running PLC without stopping the controlled process.
EETC PLC Programming 4
Which communication protocol is most commonly used for real-time control-level communication between a PLC and I/O modules on a DeviceNet network?