I want to put a firewall hardware product between a RTU and PLC to block some Modbus function codes (e.g. write coil, read coil). (This firewall product is able to do deep packet inspection on Modbus protocol.)

This firewall hardware has two ethernet (RJ45) ports, IN and OUT. But the Modbus uses serial buses (should be RS485) between RTU and PLC. Hence, to solve this issue, can I use two ethernet - RS485 converters to connect firewall hardware between RTU and PLC? Is there any possible issue?

  • \$\begingroup\$ Is this an Ethernet firewall? If so, it probably only works with Ethernet, and other protocols built on top of Ethernet. \$\endgroup\$ – user253751 Jan 19 '16 at 8:18
  • \$\begingroup\$ This product may provide a solution to your question sequi.com/portbloque-s.htm \$\endgroup\$ – Ted Victor Apr 26 '17 at 23:55

Modbus is a serial communications protocol, a link layer standard. In your situation, it is being transported over RS-485, a physical layer standard. So, you might say that you have "Modbus over RS-485".

Ethernet/RS-485 converters take "Ethernet over copper", and turn it into "Ethernet over RS-485". RS-485 is the physical layer standard, Ethernet is the link layer standard. On the other end, the converter does the reverse.

What you are proposing uses the converters in reverse. That is, you would have a converter that expects "Ethernet over RS-485" and is instead receiving "Modbus over RS-485".

Unless you have a very smart converter (and without more details about your hardware, I'm assuming it's not), I think the answer will be "no".


It isn't clear what do you mean with firewall device. Ethernet/Rs485 converters are product specific and usualy require a virtual com port driver on PC to be able to work, therefore you can get PC->ethernet->serial device.
You will need embedded Modbus RTU to Modbus TCP/IP converters and somehow pair them together, not a simple serial to ethernet converter.


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