Processing Any Communications Requests
During the message-processing phase of the scan cycle, the FX2N processes any messages that were received from the communications port or intelligent I/O modules.
Executing the CPU Self-test Diagnostics
During this phase of the scan cycle, the FX2N checks for proper operation of the CPU and for the status of any expansion modules.
Writing to the Digital Outputs
At the end of every scan cycle, the FX2N writes the values stored in the process-image output register to the digital outputs. (Analog outputs are updated immediately, independently from the scan cycle.)
Programming Concepts, Conventions, and Features
The FX2N continuously executes your program to control a task or process. You use GX-Developer to create this program and download it to the FX2N. GX-Developer provides a variety of tools and features for designing, implementing, and debugging your program.
Guidelines for Designing a Micro PLC System
There are many methods for designing a Micro PLC system. The following general guidelines can apply to many design projects. Of course, you must follow the directives of your own company’s procedures and the accepted practices of your own training and location.
Partition Your Process or Machine
Divide your process or machine into sections that have a level of independence from each other. These partitions determine the boundaries between controllers and influence the functional description specifications and the assignment of resources.
Create the Functional Specifications Write the descriptions of operation for each section of the process or machine. Include the following topics: I/O points, functional description of the operation, states that must be achieved before allowing action for each actuator (such as solenoids, motors, and drives), description of the operator interface, and any interfaces with other sections of the process or machine.
Design the Safety Circuits
Identify equipment requiring hard-wired logic for safety. Control devices can fail in an unsafe manner, producing unexpected startup or change in the operation of machinery. Where unexpected or incorrect operation of the machinery could result in physical injury to people or significant property damage, consideration should be given to the use of electro-mechanical overrides which operate independently of the FX2N to prevent unsafe operations. The following tasks should be included in the design of safety circuits:
(1)Identify improper or unexpected operation of actuators that could be hazardous.
(2)Identify the conditions that would assure the operation is not hazardous, and determine how to detect these conditions independently of the FX2N.
(3)Identify how the CPU and I/O affect the process when power is applied and removed, and when errors are detected. This information should only be used for designing for the normal and expected abnormal operation, and should not be relied on for safety purposes.
(4)Design manual or electro-mechanical safety overrides that block the hazardous operation
independent of the CPU.
(5)Provide appropriate status information from the independent circuits to the FX2N so that
the program and any operator interfaces have necessary information.
(6)Identify any other safety-related requirements for safe operation of the process.


Specify the Operator Stations
Based on the requirements of the functional specifications, create drawings of the operator stations. Include the following items:
(1)Overview showing the location of each operator station in relation to the process or machine
(2)Electrical drawings with the associated I/O of the FX2N CPU or expansion module
Create the Configuration Drawings
Based on the requirements of the functional specification, create configuration drawings of the control equipment. Include the following items:
(1)Overview showing the location of each FX2N in relation to the process or machine PLC可编程控制器英文参考文献和中文翻译(3):http://www.751com.cn/fanyi/lunwen_3792.html