Does someone know a SIL3 validated connection between Freelance and a Safety PLC?
Yes, a SIL3 level WRITE connection can be made between Freelance and a Safety PLC using approved serial communications like Modbus RTU and having the proper "mechanical" safe guards in place required by the standards. Reading SIL3 data from the Safety PLC can be done at anytime by many methods.
Sending a new setpoint to a safety function from a DigiVis console can be considered a form of maintenance to the Safety PLC (making a change). The following document link is for a TUV document that outlines the basics for such a maintenance occurance: http://www.tuv-fs.com/modr_3_e.htm. The key information from this document is that a key switch or some form of phsical enable/disable writing functionality signal must exist and that all requirements identified for Maintenance Override from the PLC Safety Manual are met.
One example of such a manual for a Tricon system can found at: http://www.tuv-fs.com/Safety_Considerations_Guide_Tri-GP_v2_Systems.pdf. Information on Maintenance Overrides starts on page 27 (actual document page 37) and provides details on design and operating requirements for using Maintenance Overrides with that system type. In the Additional Recommendations section on page 29, there is a diagram that reasonably well defines the overall design and requirements. Key items within the diagram are the hardwired switch that is used to enable/disable writing from the DCS console, the application programming that is required to be writen within the safety PLC to handle the required maintenance override functions (the changes must pass through this program and not be written directly to the safety functions), and the output warning back to the operator that a maintenance override is active.
As indicated, the sample above is for a Tricon system. Each Safety PLC system should have a corresponding Safety Manual that provides this type of detail for how to implement Maintenance Overrides. You will need the specific safety manual for the system being used especially for identifying the programming requirements for the application programming to manage the maintenance override inside of the safety PLC and identify the available options for creating the warning of a maintenance override to the operator.