open62541/examples/pubsub_realtime

open62541 Realtime OPC UA PubSub Publisher

This example is a self-contained PubSub publisher over raw Ethernet. It showcases the realtime-capabilities of OPC UA PubSub. The core idea is that the publisher callback can be triggered from a time-triggered system interrupt and sends out the PubSub message within the interrupt.

The publisher retrieves its configuration and the payload data from the information model of an OPC UA server. (It could also be run in a standalone mode without an OPC UA server.) Since the publisher interrupt preempts the execution of the normal OPC UA server, the information model needs to be consistent at every time (reentrant). The specific techniques used to make the OPC UA server reentrant are described in this publication:

@inproceedings{pfrommer2018open,
  title={Open source OPC UA PubSub over TSN for realtime industrial communication},
  author={Pfrommer, Julius and Ebner, Andreas and Ravikumar, Siddharth and Karunakaran, Bhagath},
  booktitle={2018 IEEE 23rd International Conference on Emerging Technologies and Factory Automation (ETFA)},
  pages={1087--1090},
  year={2018},
  organization={IEEE}
}

Please cite if you use this work.

OPC UA PubSub for open62541 is funded by an industry consortium in the context of an OSADL project (Open Source Automation Development Lab). Technical development is conducted by Fraunhofer IOSB and Kalycito Infotech.

https://www.osadl.org/OPC-UA-TSN.opcua-tsn.0.html

Realtime communication with Time-Sensitive Networking (TSN)

OPC UA PubSub can be used together with TSN for hard-realtime Ethernet-based communication. Vendor-specific APIs are commonly used for TSN capabilities. This example only uses the standard Linux API for raw Ethernet. Vendor-specific examples may be added at a later time.

Building the RT Publisher

The main open62541 library needs to be built with the following build options enabled for the realtime PubSub example. Note that some of the other examples supplied with open62541 will not link against the library with these build options. For good timings, ensure that the CMAKE_BUILD_TYPE is set to Release.

• UA_ENABLE_PUBSUB
• UA_ENABLE_PUBSUB_ETH_UADP
• UA_ENABLE_PUBSUB_CUSTOM_PUBLISH_HANDLING
• UA_ENABLE_MALLOC_SINGLETON
• UA_ENABLE_IMMUTABLE_NODES

The publisher contains some hard-coded values that need to be adjusted to specific systems. Please check the top definitions in pubsub_interrupt_publish.c and start_rt_publish.sh.

The publisher code is built and linked against the main open62541 library as follows:

gcc -O2 ../examples/pubsub_realtime/pubsub_interrupt_publish.c ../examples/pubsub_realtime/bufmalloc.c -I../include -I../plugins/include -Isrc_generated -I../arch/posix -I../arch -I../plugins/networking bin/libopen62541.a -lrt -o rt_publisher

Running the RT Publisher

The publisher must be run as root for direct access to the Ethernet interface.

# ./rt_publisher

The example contains a script to be used with RT-Preempt Linux kernel. The following command starts the publisher, locks the process to a specific CPU, and sets the scheduling policy.

# start_rt_publish.sh ./rt_publisher

The measurements are written to a file (publisher_measurement.csv) with the following fields for every publish callback:

• Counter
• Publication Interval
• Nominal time for the current publish
• Start delay from the nominal time
• Duration of the publish callback