At the start of the PLC scan the a) status of all inputs is read. b) status of all outputs is updated. c) program is executed. d) diagnostics and communications tasks are executed.
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.
The Correct Answer and Explanation is :
The correct answers are:
- At the start of the PLC (Programmable Logic Controller) scan: a) the status of all inputs is read.
- The scan time is the time required c) to execute one cycle of the total program.
Explanation:
PLC Scan Cycle:
A PLC operates by continually scanning a program in a repetitive process called a scan cycle. The cycle can be broken down into several key steps:
- Reading Inputs: At the very beginning of the scan cycle, the PLC reads the current status of all input devices connected to it. This is crucial because the logic of the PLC program is dependent on these inputs. The inputs reflect the real-world variables, such as switches, sensors, and other electrical signals.
- Executing Program: After reading the inputs, the PLC executes its user-written logic, which is a sequence of instructions created to control the connected machinery or process. This part of the cycle is where the PLC processes the data it has gathered from its inputs and determines what actions to take. This might involve calculations, comparisons, or other data processing tasks.
- Updating Outputs: Once the program is executed based on the input statuses and the internal program conditions, the PLC updates the outputs accordingly. This step involves sending signals to various output devices, such as motors, lights, relays, etc., to perform actions based on the program decisions.
- Diagnostics and Communication: Lastly, the PLC performs any necessary diagnostic checks to ensure all systems are functioning correctly and handles any communication tasks with other systems, which might include networking with other PLCs or systems, or communicating with a central computer.
Scan Time:
Scan time refers to the total time taken to complete one cycle of the above processes—from reading the inputs to executing the program and finally updating the outputs. This time is critical in control applications because it impacts how quickly a PLC can respond to changes in input conditions and how timely it can control outputs, directly affecting the responsiveness and efficiency of the controlled system or process.
Now, let’s generate an image illustrating the PLC scan cycle.
Here’s an illustration depicting the PLC scan cycle in a factory setting, visually representing each step of the process.
