In general, the productivity in generating PLC is far behind compared to other domains, for instance, VLSI design, where efficient computer aided design tools are in practice. Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLC-programming requires a simultaneous consideration of hardware and software. The software design becomes, thereby, more and more the major cost driver. In many industrial design projects, more than 50% of the manpower allocated for the control system design and installation is scheduled for testing and debugging PLC programs.
In addition, current PLC based control systems are not properly designed to support the growing demand for flexibility and reconfigurability of manufacturing systems.
PLC Concepts
PLC continuously cycles through the control logic in your program, reading and writing data.
The FX2N Relates Your Program to the Physical Inputs and Outputs. The basic operation of the FX2N is very simple:
（1）The FX2N reads the status of the inputs.
（2）The program that is stored in the FX2N uses these inputs to evaluate the control logic. As the program runs, the FX2N updates the data.
（3）The FX2N writes the data to the outputs.
The FX2N Executes Its Tasks in a Scan Cycle
The FX2N executes a series of tasks repetitively. This cyclical execution of tasks is called the scan cycle. The FX2N performs most or all of the following tasks during a scan cycle:
(1) Reading the inputs: The FX2N copies the state of the physical inputs to the process-image input register.
(2)Executing the control logic in the program: The FX2N executes the instructions of the program and stores the values in the various memory areas.
(3) Processing any communications requests: The FX2N performs any tasks required for communications.

(4)Executing the CPU self-test diagnostics: The FX2N ensures that the firmware, the program memory, and any expansion modules are working properly.
(5) Writing to the outputs: The values stored in the process-image output register are written to the physical outputs.
The execution of the user program is dependent upon whether the FX2N is in STOP mode or inRUN mode. In RUN mode, your program is executed; in STOP mode, your program is not executed.
Reading the Inputs
Digital inputs: Each scan cycle begins by reading the current value of the digital inputs and then writing these values to the process-image input register.
Analog inputs: The FX2N does not update analog inputs from expansion modules as part of the normal scan cycle unless filtering of analog inputs is enabled. An analog filter is provided to allow you to have a more stable signal. You can enable the analog filter for each analog input point.
When analog input filtering is enabled for an analog input, the FX2N updates that analog input once per scan cycle, performs the filtering function, and stores the filtered value internally. The filtered value is then supplied each time your program accesses the analog input.
When analog filtering is not enabled, the FX2N reads the value of the analog input from expansion modules each time your program accesses the analog input.
Analog inputs are updated every scan with the most recent result from the analog-to-digital converter. This converter is an averaging type (sigma-delta) and those values will usually not need software filtering.
Executing the Program
During the execution phase of the scan cycle, the FX2N executes your program, starting with the first instruction and proceeding to the end instruction. The immediate I/O instructions give you immediate access to inputs and outputs during the execution of either the program or an interrupt routine.
If you use interrupts in your program, the interrupt routines that are associated with the interrupt events are stored as part of the program. The interrupt routines are not executed as part of the normal scan cycle, but are executed when the interrupt event occurs (which could be at any point in the scan cycle). PLC可编程控制器英文参考文献和中文翻译(2):http://www.751com.cn/fanyi/lunwen_3792.html