Do not use the analog filter with modules that pass digital information or alarm indications in the analog words。 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)。

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