There are a few different possible causes for this error. Some possible questions that could help diagnose the issue are:

1. Are the two PLCs connected to each other physically? If not, this could be causing the communication connection to be interrupted.
2. Are the PLCs configured to communicate
The solution for the error described as "The communication connection between M84-PLC1 and PLC2 is interrupted" may involve checking the communication protocols, ensuring proper termination of the connection, checking the modems and communication cards,