The scan time is the time required:
A. to record the status of all input devices.
B. to record the status of all output devices.
C. to execute one cycle of the total program.
D. for the information to pass from input to output.
Answer: C
Share your understanding of this question with the correct explanation.
Explanation:
The scan time is the time required to execute one cycle of the total program in a programmable logic controller (PLC).
In a PLC, the program consists of a set of instructions that define the logic and control operations for the system. During each scan cycle, the PLC performs the following main tasks:
Read Input Phase: The PLC reads the status of all input devices connected to it, such as sensors, switches, and other input modules. It records the current states of these input devices to use them in the program’s logic.
Execute Logic Phase: The PLC executes the program’s logic based on the input statuses recorded in the previous step. The logic typically involves evaluating conditions, performing calculations, making decisions, and determining the states of output devices.
Write Output Phase: The PLC writes the appropriate control signals to the output devices, such as actuators, solenoids, and other output modules, based on the results of the program’s logic.
The time taken to complete these three phases and complete one full cycle of the program is known as the scan time. The scan time can vary depending on the complexity of the program, the number of inputs and outputs, and the processing speed of the PLC.
Option A is incorrect because the scan time is not specifically about recording the status of all input devices; rather, it includes all the steps mentioned above.
Option B is incorrect for the same reason as Option A. The scan time involves not just recording the status of all output devices but also executing the program logic and writing outputs.
Option D is incorrect because the scan time is not solely about the time for information to pass from input to output; it includes the execution of the entire program logic.