79 lines
3.0 KiB
Markdown
79 lines
3.0 KiB
Markdown
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# open62541 Realtime OPC UA PubSub Publisher
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This example is a self-contained PubSub publisher over raw Ethernet. It
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showcases the realtime-capabilities of OPC UA PubSub. The core idea is that the
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publisher callback can be triggered from a time-triggered system interrupt and
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sends out the PubSub message within the interrupt.
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The publisher retrieves its configuration and the payload data from the
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information model of an OPC UA server. (It could also be run in a standalone
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mode without an OPC UA server.) Since the publisher interrupt preempts the
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execution of the normal OPC UA server, the information model needs to be
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consistent at every time (reentrant). The specific techniques used to make the
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OPC UA server reentrant are described in this publication:
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```
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@inproceedings{pfrommer2018open,
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title={Open source OPC UA PubSub over TSN for realtime industrial communication},
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author={Pfrommer, Julius and Ebner, Andreas and Ravikumar, Siddharth and Karunakaran, Bhagath},
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booktitle={2018 IEEE 23rd International Conference on Emerging Technologies and Factory Automation (ETFA)},
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pages={1087--1090},
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year={2018},
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organization={IEEE}
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}
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```
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Please cite if you use this work.
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OPC UA PubSub for open62541 is funded by an industry consortium in the context
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of an OSADL project (Open Source Automation Development Lab). Technical
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development is conducted by Fraunhofer IOSB and Kalycito Infotech.
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https://www.osadl.org/OPC-UA-TSN.opcua-tsn.0.html
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## Realtime communication with Time-Sensitive Networking (TSN)
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OPC UA PubSub can be used together with TSN for hard-realtime Ethernet-based
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communication. Vendor-specific APIs are commonly used for TSN capabilities. This
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example only uses the standard Linux API for raw Ethernet. Vendor-specific
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examples may be added at a later time.
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## Building the RT Publisher
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The main open62541 library needs to be built with the following build options
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enabled for the realtime PubSub example. Note that some of the other examples
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supplied with open62541 will not link against the library with these build
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options. For good timings, ensure that the `CMAKE_BUILD_TYPE` is set to
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`Release`.
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- UA_ENABLE_PUBSUB
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- UA_BUILD_EXAMPLES
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- UA_ENABLE_MALLOC_SINGLETON
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- UA_ENABLE_PUBSUB_BUFMALLOC
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- UA_ENABLE_IMMUTABLE_NODES
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The publisher contains some hard-coded values that need to be adjusted to
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specific systems. Please check the top definitions in
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`pubsub_interrupt_publish.c` and `start_rt_publish.sh`.
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## Running the RT Publisher
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The publisher must be run as root for direct access to the Ethernet interface.
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`# ./rt_publisher`
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The example contains a script to be used with RT-Preempt Linux kernel. The
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following command starts the publisher, locks the process to a specific CPU, and
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sets the scheduling policy.
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`# start_rt_publish.sh ./rt_publisher`
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The measurements are written to a file (publisher_measurement.csv) with the
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following fields for every publish callback:
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- Counter
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- Publication Interval
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- Nominal time for the current publish
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- Start delay from the nominal time
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- Duration of the publish callback
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