EETC PLC Programming 5 — Questions and Answers
Question 1: In PLC programming, what is 'dead-band' as it applies to analog process control?
- The time delay between a setpoint change and controller response
- A range around a setpoint within which no corrective action is taken (Correct answer)
- The maximum allowable deviation before a safety shutdown
- The signal loss across long analog wiring runs
Correct answer: A range around a setpoint within which no corrective action is taken
Dead-band defines a neutral zone around the setpoint where the controller ignores small deviations to prevent excessive cycling of the output.
Question 2: A PLC program uses a CTU (Count Up) instruction with a preset of 10. The output bit (DN) energizes when the accumulated count:
- Reaches 9
- Reaches 10 or greater (Correct answer)
- Resets to 0
- Exceeds 10 by at least 1
Correct answer: Reaches 10 or greater
The CTU Done bit energizes when the accumulated value equals or exceeds the preset value.
Question 3: What is the role of a watchdog timer in a PLC system?
- Tracks total machine uptime for maintenance scheduling
- Monitors scan time and faults the PLC if a scan exceeds the maximum allowed time (Correct answer)
- Counts the number of times the PLC has been power-cycled
- Synchronizes multiple PLCs on a network
Correct answer: Monitors scan time and faults the PLC if a scan exceeds the maximum allowed time
The watchdog timer automatically faults the PLC if a single program scan takes longer than the configured maximum time, indicating a program loop or CPU hang.
Question 4: In Instruction List (IL) programming (IEC 61131-3), the operator 'LD' performs what action?
- Loads a value into the current result (accumulator) (Correct answer)
- Stores the current result to a variable address
- Performs a logical AND with the current result
- Jumps to a labeled program section
Correct answer: Loads a value into the current result (accumulator)
The LD (Load) operator loads the specified operand value into the current result register, starting or resetting an expression.
Question 5: When configuring a PID instruction in a PLC, the 'derivative' term (D) primarily responds to:
- The total accumulated error over time
- The difference between setpoint and process variable
- The rate of change of the error signal (Correct answer)
- The output clamp limits set by the programmer
Correct answer: The rate of change of the error signal
The derivative term responds to how quickly the error is changing, damping the controller response to prevent overshoot.
Question 6: What is a 'latch' (OTL) and 'unlatch' (OTU) coil pair used for in ladder logic?
- To create a retentive output that holds its state independently of rung conditions (Correct answer)
- To synchronize two separate PLC programs over a network
- To temporarily disable a rung during fault conditions
- To store a bit value in a FIFO data buffer
Correct answer: To create a retentive output that holds its state independently of rung conditions
OTL sets a bit retentively (it stays on even if the rung goes false), and OTU clears it; together they form a retentive memory function.
Question 7: In a PLC network using PROFIBUS DP, what is the role of the 'master' device?
- It supplies DC power to all slave devices on the bus
- It initiates all communications and polls slave devices for I/O data (Correct answer)
- It serves as the primary clock source for synchronizing all nodes
- It stores the backup program for all slave PLCs
Correct answer: It initiates all communications and polls slave devices for I/O data
In PROFIBUS DP, the master device controls bus access, cyclically polls each slave, and exchanges I/O data with them in a token-passing scheme.
EETC PLC Programming 5
In PLC programming, what is 'dead-band' as it applies to analog process control?