Hello

It's my first time to write a topic in forum.

sorry about my bad English skill

My communication system is dead and i done know the reason.

system status is below

CPU 412-5H redundancy with CP443-1

There is a Modbus TCP commnucation between PLC and DCS, and PLC is a server.

They worked very well, but recently PLC modbus tcp server is broken.

I downloaded some programs but i've never touched programs about modbus

i tested with a simulater 'Modscan32'.

It looks working well when i start test communication, but the connection is terminated after few seconds ( 2~ 25 sec randomly).

I can find some error code of modbus 16#8304 A0FF A01A and read all topics in forum.

however they are not helpful

In netpro, connection status is 'connection status is always passive connection establishment in progress'. When I test modbus with modscan32, it turns to 'established', and there is no error code of modbus for a few seconds.

I've never used wireshark

I just followed some manuals on the internet, so i dont know if it is what you want to see

i will attach two files for packet list

thank you for your help