How can I communicate between my ROS-pc and a plc from SEW? And how do I install it on my pc? The plc that I use is written in CODESYS

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I don't know what the status of it is right now, but at the ROS-Industrial Europe spring 2018 conference in Stuttgart, INESCTEC presented about "IEC 61131-3 and ROS compatibility".

Presentation: Bridging Automation and Robotics - An Interprocess Communication between IEC 61131-3 and ROS (slides).

You could perhaps send a message to the ROS-Industrial Discourse category to inquire about whether this is something that can be / is distributed, or at least accessible.

Edit: it was also the topic of a MSc thesis: Embedded System Development with CODESYS and ROS Integration

Final edit: you may be interested in the ROBIN FTP in the ROSIN project:

ROBIN aims to address the demand for flexible robotics in industrial environments and the necessity to integrate robots and automation equipment in an efficient manner. For this, the ROBIN project will focus on developing and releasing a bidirectional, reliable and structured communication bridge between ROS and CODESYS, a softPLC that can run on embedded devices and that supports a variety of fieldbuses, and even OPC-UA. The developed software will allow the parametrization of ROS modules through IEC61131-3 programming languages and also streamline the interoperability between ROS and robotic hardware or automation equipment, fully empowering the Industry4.0 paradigm of Plug’n’Produce.

Results have not yet been made available, but this may offer a way to integrate with CODESYS systems in the near future.