event modbus_exception is a general exception and the 'fc' that is returned here is 'original_request_fc'+128. This means if I send a request with Fc=3 and something goes bad,I will get this exception with fc=131. I thought it would be useful to immediately subtract this value and show in the log exact Fc where the exception was triggered. A small function for this I put before in modbus/utils.bro (on my branch), but its not in the topic/robin/modbus-merge branch.
I suggest to implement this functionality.