Can a network sensor see serial fieldbus links such as Modbus RTU?
No. A network sensor sees the Ethernet and IP traffic that is copied to it. Serial links such as Modbus RTU over RS-485, Profibus DP and other fieldbuses never reach Ethernet, so the commands on them are out of its view. What it can see is the gateway that translates between Ethernet and the serial bus: who talks to the gateway, when and how often. In many plants that is a useful signal, because anyone who wants to reach a serial device from the network has to go through that gateway. List your gateways, protect them like controllers, and mirror the segment they sit on. NetSenX reads Modbus/TCP, DNP3, BACnet/IP, EtherNet/IP and S7comm from passive capture, from the Pro plan. For the serial side, rely on the controllers' own diagnostics and on locked cabinets.