Limitations¶

String Size¶

The RIB only supports fixed size arrays of bytes to store string like texts.
Therefore, ROS 2 message types that contain strings e.g. string frame_id in std_msgs/msg/Header will translate those strings to a fixed size byte array with the size of 254 bytes (as shown in the picture below).
⚠️ As the byte array will be available as a string on the PLC side, it is also converted on the PLC side. The PLC based string data type defines the maximum string length of 254 bytes.

Vector Size¶

As the RIB does not provide dynamic SHM allocation during runtime, all ROS 2 based vector types are currently limited to a fixed size of 64 elements.
⚠️ Additionally, a valid elements counter variable will be added to the PLC side, so that the user can define, how many valid elements are stored in the vector, if the number is smaller than 64.

Shared Memory¶

As the RIB uses shared memory to access data through Industrial OS as well as the Software-PLC (SW-PLC),
The actual size of the used shared memory area is therefore limited and has to be conform to the memory limit of the SW-PLC.

An example usage is shown in the picture above, where the two ROS 2 message types geometry_msgs/msg/Twist and sensor_msgs/msg/BatteryState are used to provide data to the PLC (ros2_to_plc) via shared memory.

Those two definitions are combined to one shared memory area during runtime, that shall not exceed the actual SW-PLC memory limitations:

⚠️ Note: As those limitations are defining the total PLC code memory limit, the used ROS 2 .msg typse that needs to be transfered to the PLC or vice versa should not be too big regarding memory layout.

Number of RIB Clients¶

The RIB and its participant manager RIB_App supports up to 128 client applications.
But as the generated ROS 2 package uses a precompiled RIB API library, currently only one ROS 2 client connection is possible, combined with one SW-PLC based connection.

Number of Data Blocks¶

The SW-PLC supports in total up to 20 Cyclic Interrupt DataBlocks as of TIA Portal v19.
Additionally, the RIB supports only up to 16 Consumer / Provider DataBlocks.
As each topic in the YAML configuration defines one Cyclic Interrupt DB (ROS 2 -> PLC and vice versa), the user needs to make sure that the provided YAML defines less than 20 topics in total or up to 16 topics for each side (ROS 2 -> PLC and vice versa) as shown in the picture below.

⚠️ The user has to keep in mind that the defined topics and therefore Cyclic Interrupt DBs will be added to already existing ones.

Topic Rates¶

At this stage, the RIB connections provided in the YAML config should not exceed a specified rate of 1000 Hz.
⚠️ The jitter for a defined rate highly depends on the complexity of the provided data type. For example, a geometry_msgs/msg/Twist is able to reach a stable rate of 1 kHz, but a more complex data type such as trajectory_msgs/msg/MultiDOFJointTrajectory with its vector of strings and nested structs will cause a loss of data at this rate or exceeding the PLC cycle time causing the PLC to stop!
⚠️ The usage of strings and/or vectors and/or nested structs of them should be avoided at higher frequencies (>100 Hz) as their conversion is time consuming!

Generated ROS 2 Package¶

The generated C++ based ROS 2 package has the following properties:

• The generated ROSie API C++ code is not thread safe (as the RIB connection does not provide thread safe shared memory access).
• The global RIB cycle time for ROS 2 values written to the RIB will be set to the highest of all topic frequencies defined in the provided YAML config file.
• The ROSie API code is based on the C++17 standard.
• The ROSie generated ROS 2 code is only tested with Humble and Jazzy.
• The ROS 2 data type wstring is currently not supported, but all other primitive ROS 2 types.

Type Conversions¶

The following table shows the available standard data types and their corresponding data types in ROS 2, RIB and TIA.

For arrays the conversion is:

For vectors the conversion is:

⚠️ Transferring ROS 2 vectors with more elements than the maximum size (current default is 64) will lead to data loss.