Programmable Logic Controllers Flashcards
7 cards from real SACA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Programmable Logic Controllers flashcards as text
Which PLC scan phase occurs AFTER the program logic is executed?
Answer: Output image table update and write to outputs
After program logic execution, the PLC writes the output image table values to the physical output modules to update field devices.
What is the function of a PLC's retentive coil (or latching coil)?
Answer: It maintains its state even when its enabling condition goes false
A retentive (latching) coil maintains its energized state even after the enabling rung condition goes false, until explicitly unlatched.
In an Allen-Bradley ControlLogix PLC, what does the tag-based addressing system offer compared to traditional register-based addressing?
Answer: Descriptive named variables that improve program readability and maintainability
Tag-based addressing uses descriptive named variables instead of cryptic register numbers, greatly improving program readability and long-term maintainability.
What is the purpose of the 'force' function available in most PLC programming software?
Answer: To override normal program logic and manually set an I/O point to a desired state for testing
The force function allows technicians to manually override I/O points to a specific state for troubleshooting, bypassing normal program logic.
A CTD (Count Down) instruction in a PLC has a preset of 10 and an accumulated value of 3. How many more count pulses are needed to set the Done bit?
Answer: 3
A CTD instruction counts down from the accumulated value; the Done bit sets when accumulated reaches zero, so 3 more pulses are needed.
Which PLC network protocol is commonly used for device-level communication with sensors and actuators in a factory floor environment?
Answer: DeviceNet or PROFIBUS DP
DeviceNet and PROFIBUS DP are fieldbus protocols designed for device-level communication between PLCs and field devices such as sensors and actuators.
What is 'structured text' (ST) in the context of IEC 61131-3 PLC programming?
Answer: A high-level textual language similar to Pascal used to write PLC logic
Structured Text (ST) is a high-level, Pascal-like textual programming language defined in IEC 61131-3 for writing complex PLC control logic.