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Data Access Service

Table of contents

Introduction

The PLCnext Runtime provides an RSC service, called IDataAccessService, that a client can use to read and write Global Data Space (GDS) variables. From the API documentation for IDataAccessService:

The direct access functionality is a way for reading and writing values from and to variables. This is the fastest way, with a minimum of influence to the real time process, but it is not guaranteed that the data will be read/write in the same task cycle. For task consistent reading the subscription service Arp.Plc.Gds.Services.ISubscriptionService has to be used.

Guide details

Description Value
Created 7.01.2021
Last modified 7.01.2021
Controller AXC F 2152
FW 2021.0.0 LTS
Arpversion 21.0.0.35466
SVN Revision 35466
SDK 2021.0.0 LTS (21.0.0.35466)
PLCnext CLI 21.0.0 LTS (21.0.0.489)

References

A complete description of the Data Access service is available from these sources:

Quick-start example

This example demonstrates features of the Data Access RSC service.

It is assumed that the user has some experience building C++ Components and Programs for PLCnext Control.

Prerequisites:

  • PLCnext Command Line Interface (CLI) tool, version 2021.0. This is included in the "PLCnext Technology C++ tool chain", available on the Phoenix Contact website.

  • A Software Development Kit (SDK) for the AXC F 2152 PLCnext Control, version 2021.0.0 or later. This is also included in the "PLCnext Technology C++ tool chain".

  • (optional) Eclipse IDE, with the PLCnext Technology feature installed.

  • (optional) Visual Studio, with the PLCnext Technology add-in installed.

  • PLCnext Engineer version 2021.0.1 or later.

Procedure:

  • Clone this repository, e.g.

    git clone https://github.com/PLCnext/CppExamples.git

  • Create the GDS variables that will be accessed:

    • Create a new PLCnext Engineer project for your target PLC.
    • Create one cyclic task.
    • Add the User Library "DataAccess.pcwlx", which is included in the Examples/DataAccess directory of this repository. This library contains one Program, called "DataAccess".
    • Create one instance of the "DataAccess" program in the cyclic task.
    • Name the program instance DataAccessInstance.
    • Write and start the project on the PLC.

  • Create a new ACF project using either the PLCnext CLI tool, or Eclipse, or Visual Studio, with the following settings:

    • Project name: DataAccess
    • Component name: DataAccessComponent
    • Project namespace: DataAccess

  • Copy the contents of the Examples/DataAccess/src directory in this repository, to the src directory of the ACF project. Replace the existing source files with the same name.

  • Build the ACF project.

  • Deploy the ACF project to the PLC.

  • Restart the PLCnext Runtime.

  • Check the contents of the file /opt/plcnext/logs/Output.log. It should contain messages showing the values of some OUT port variables from the DataAccessInstance program instance.

  • In the PLCnext Engineer project, in Debug mode, add the IN port variables from DataAccessInstance to the Watch window. The values of some of these variables are being written by the C++ component using the Data Access RSC service.