DataAccess

Data Access Service

Table of contents

• Data Access Service
  • Table of contents
  • Introduction
  • Guide details
  • References
  • Quick-start example

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	05.06.2024
Controller	AXC F 2152
FW	2025.0
SDK	2025.0
PLCnext Technology Toolchain	2025.0

References

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

• PLCnext Technology Info Center
• API documentation

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 Technology Command Line Interface (CLI) tool, version 2025.0.

• A Software Development Kit (SDK) for the AXC F 2152 PLCnext Control, version 2025.0.

• (optional) Eclipse IDE, with the PLCnext Technology add-in installed.

• (optional) Visual Studio, with the PLCnext Technology extension installed.

• PLCnext Engineer version 2025.0.

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/Custom.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.