Process of a scan cycle


Process of a scan cycle
The 5 main steps in one complete scan cycle are reading the inputs, executing the program, processing communication requests, executing CPU diagnostics, and writing to the outputs. To read the inputs, the user writes values to the input image table which are reserved in bytes. When executing the program, the ladder is read from right to left, and top to bottom. When processing communication requests, the user processes any message received from the communications port. To execute the CPU Self-Diagnostic test, check the firmware, check the program, and check the status of the modules. 
System scale
A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O.
Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules are customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. Either a special high speed serial I/O link or comparable communication method is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants. Options are also available to mount I/O points directly to the machine and utilize quick disconnecting cables to sensors and valves, saving time for wiring and replacing components.
User interface
PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.
Communications
PLCs have built-in communications ports, usually 9-pin RS-232, RS-422, RS-485, Ethernet. Various protocols are usually included. Many of these protocols are vendor specific.
Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.
PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.
Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.

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