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Subscription Service

Table of contents

Introduction

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

The subscription functionality is a more elegant way reading values from [Global Data Space (GDS)] variables, in contrast to permanently reading (polling). A client can subscribe a selection of variables of interest and the subscription will copy the data values to a internal buffer. This is the recommended mechanism to “read” variable values from the PLC system. All read data are always task consistent, because the data is written by the task itself.

Guide details

Description Value
Created 10.12.2020
Last modified 4.01.2021
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 Subscription service is available from these sources:

Quick-start example

This example demonstrates features of the Subscription RSC service, including:

  • Setting up a simple subscription to read a value from a single GDS variable.
  • Retrieving time stamps with subscription data.
  • Reading complex variable data (structures and arrays) from a subscription.
  • Using "Recording" type subscriptions.

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 subscribed to:

    • Create a new PLCnext Engineer project for your target PLC.
    • Create two cyclic tasks, with interval values of 100ms and 500ms.
    • Add the User Library "SubscriptionData.pcwlx", which is included in the Examples/Subscriptions directory of this repository. This library contains one Program, called "RealTimeProgram".
    • Create two instances of "RealTimeProgram", one in each of the two cyclic tasks.
    • Name the two program instances RealTimeProgram100ms and RealTimeProgram500ms.
    • 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: Subscriptions
    • Component name: SubscriptionsComponent
    • Project namespace: Subscriptions

  • Copy the contents of the Examples/Subscriptions/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 value of variable varUInt16 from the program instance RealTimeProgram100ms.

  • Change the .acf.config file to instantiate SubscriptionsComponent2.

  • Restart the PLCnext Runtime.

  • Check the contents of the file /opt/plcnext/logs/Output.log. It should contain messages showing timestamps and values of the variables varUInt16 from the two program instances RealTimeProgram100ms and RealTimeProgram500ms.

  • Change the .acf.config file to instantiate SubscriptionsComponent3.

  • Restart the PLCnext Runtime.

  • Check the contents of the file /opt/plcnext/logs/Output.log. It should contain messages showing the values of all the elements of the variable varSampleStruct from the program instance RealTimeProgram100ms, including all the elements of the array-type struct element.

  • Change the .acf.config file to instantiate SubscriptionsComponent4.

  • Restart the PLCnext Runtime.

  • Check the contents of the file /opt/plcnext/logs/Output.log. It should contain messages showing timestamps and values of the variables varUInt16 from the two program instances RealTimeProgram100ms and RealTimeProgram500ms. In this case, data is recorded for every execution of the corresponding program instance, regardless of how often the subscription client reads the data.