Ethernet Discovery

[5liter]

Does anyone know the technique used by network configuration tools (such as RSNetworx) for discovering devices on an ethernet based network? I'm interested in knowing if a general mechanism that could be used for all industrial ethernet networks such as EtherNet/IP and Modbus TCP exists, or if some type of network specific "who's out there" payload is embedded in a protocal specific message.

 

[Peter Nachtwey]

There isn't a standard way.

Rockwell Software, Automation Direct ( Host Engineering ) and Delta Computer Systems have ways of detecting their own devices but not others. The remote devices must be intelligent and have this feature programmed in to them. Automation Direct is an odd case because they don't use TCP/IP for this. They have a program called NetEdit that sends out raw Ethernet packets and waits for a responce. Automation Direct only cares about the MAC ID.

Rockwell and Delta Computer Systems have registered ports the the Ethernet gods back in Switzerland or wherever. In our case we send a message out to port 1324 and all of our devices will resond regards less of the IP address or mac address. This allows us to easily find our controllers and change the IP address if it is required.
The Rockwell scheme is a little different, but basically works the same way. Rockwell also has its own registered port. Our products support the Automation Direct and the Rockwell Ethernet scanning. This way our controllers are easily recognized on either network and our controller looks like another AB or AD PLC on the network.

I don't know any scheme lke this for Modbus/TCP. I don't think it is part of the specifications. The method we use is indepent of the protocol stack being used.

 

[5liter]

That's more information than I've been able to find in any documentation on the AB web site. I did find a paragraph on ARP in the RSNetworks for EtherNet/IP manual, but it is vague, and I don't completly understand ARP. I was hoping the solution might be universal so that at least all the devices could be seen on the wire. From what I have been to figure out (never heard of ARP before last week Friday), a gratuitous ARP message would be enough to at least see what IP addresses are in use, which would be a good first step toward a network who, but my understanding is that there is no ARP service that says "anyone out there start talking". It's all based on timeouts or device power-up only.

I'd like to get me hands on some demo software and use a packet sniffer to watch what is going on with some of these packages.

Do you know what Sixnet, Wago, Opto, or any other manufactures do? Do you have any example documentation on the use of the ports you talk of?

 

[Peter Nachtwey]

No code, it is top secret.

Do a search for Ethereal and Winpcap. This is the packet sniffer we use. Look for the Ethernet RFCs (Request For Comments). This has the information about Ethernet in general.

I have no idea what the I/O makers do about finding ports.

Certainly I have code, but it is a competitive advantage to keep it secret.