We have a ring MB300 network for our plant, which connects 4 PLCs ABB 450 using a repeater in between. Since the last few days, one part of the system was not able to communicate with the other. It's a MB300 bus & TCP/IP for each operator station. We have checked the end terminators & found resistance ok. We reset all PLCs but still the system is the same. All transceivers were also replaced but no improvement. Also OS is displaying MAC checksum error. Kindly help.