What is scan in PLC?

by JeniPatel

In a Programmable Logic Controller (PLC), a “scan” refers to the process by which the PLC executes its program. During each scan, the PLC reads the state of all input devices, processes the logic of the program, and updates the state of all output devices.
The scan cycle typically consists of three main steps:
1. Input scan:
During the input scan, the PLC reads the state of all input devices, such as sensors and switches, and stores their values in memory.
The “input scan” is the initial phase of the scan cycle in a Programmable Logic Controller (PLC), during which the PLC reads the status of all input devices and saves their values in memory. Sensors, switches, buttons, and other devices that send signals to the PLC informing it of a process or system’s status can be included in the input devices.
The Allen Bradley 1766-L32BXBA MicroLogix 1400 PLC examines each input device’s condition during the input scan, usually one at a time, and logs the results in a memory location known as an input register. The value of each input device, such as whether a sensor is picking up a specific condition or whether a switch is open or closed, is stored in the input register, a digital storage location.
The input scan, which provides the current status of the process or system being controlled, is an important step in the scan cycle. During the program execution step of the scan cycle, the PLC makes decisions and implements its program logic using this data. During the program execution step, if any input device changes its state, the PLC will notice this change during the subsequent input scan and update its memory appropriately.
The type, quantity, and PLC processing power of the input devices used determine the speed of the input scan. When designing their control systems, PLC programmers must take the scan time into account and make sure that it is quick enough to provide precise and timely control of the process or system.
2. Program execution:
During the program execution step, the PLC processes the logic of the program, using the input values stored in memory to determine the state of the output devices. The program may also include data processing, mathematical calculations, and communication with other devices.
“Program execution” refers to the second scan cycle step in a Programmable Logic Controller (PLC) where the PLC processes the logic of the program based on the values stored in memory during the input scan step. The program logic is made up of a set of instructions that specify how the process or system being controlled will be controlled and operated. These instructions are frequently written in ladder logic or another programming language.
The Rockwell Automaion PLC uses the input device values that are read from memory during program execution to make decisions based on the preprogrammed logic. Logical operations, mathematical calculations, data processing, communication with other devices, and other tasks required for process or system control may be included in the program logic.
During the output scan step of the scan cycle, the PLC uses the output of the program execution step to determine the state of the output devices, such as motors, valves, or other devices. Additionally, the program may communicate with external devices like Human Machine Interfaces (HMIs) or Supervisory Control and Data Acquisition (SCADA) systems, update internal registers, set or clear alarms, and set or clear alarms.
The complexity of the program logic and the PLC’s processing power both affect how quickly a program is executed. To ensure that their program logic can execute within the allotted scan time and provide precise and timely control of the process or system being controlled, PLC programmers must carefully design their program logic.
Overall, program execution is a critical step in the scan cycle as it determines the operation and control of the process or system being controlled by the PLC.
3. Output scan:
During the output scan, the PLC updates the state of all output devices based on the results of the program execution step. This may include activating or deactivating motors, valves, or other devices.
Once the output scan is complete, the scan cycle starts again from the beginning, with the input scan step. The entire scan cycle typically takes only a few milliseconds to complete.
The scan cycle is a fundamental concept in PLC programming, as it determines the speed and reliability of the control system. PLC programmers must carefully manage the execution time of their programs to ensure that they can complete within the available scan time and provide accurate and timely control of the equipment.

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