Modbus IP Through A Large Network

[Bee Master]

I am currently working on a project that connects 10 seperate controlers through Modbus IP. This is in a large office building and so the network administrator has provided me with static IP addresses for all the panels that vary depending on what part of the building they are in and what switch they are connected to (ie. 163.194.232.61, 163.194.239.15 etc..). I am able to connect with every panel through Remote Access but my master panel is unable to connect through Modbus IP. I am not a networking guy so I am probably missing something simple. In Modbus IP what is the proper way to set up communications. Does the default gateway value need to match on all the panels or is there something else I am missing?

[Flex727]

Just to keep you occupied until the resident geniuses drop by, take a look at the example projects available under the Help menu. Go to Help/Examples/Project Examples/Communications/Ethernet/Ethernet MODBUS IP for an example each for Master and Slave. The examples are for a v280, but it's the same for all the Vision PLCs.

[Joe Tauser]

Ditto on looking at those examples. The V280 Modbus IP Master example shows that you have to include and keep track of a Socket Connect block in order for everything to work. It's not obvious in the Modbus IP Help that this is needed, but it is mentioned several times in general Ethernet Help that a Connect is required before TCP communications of any kind can take place.

Joe T.

[Simon]

Ditto Ditto on the above,

Another issue to be sure about is the structure of the IP network - as the addresses you have indicated are "Class B" addresses (he first two section of the IP address are the same, the last two sections of the IP address are different). Most of the Unitronics Modbus examples deal with "Class C" addresses (first 3 parts of the IP address don't change, only the last part does).

Did the system admin also give you a subnet mask and a default gateway?

Did you set up the networking on the PC that Remote Access is installed in, or did the IT people do that?

If you feel like you are going in circles, I would grab two PLCs and set up a point to point conneciton with a crossover cable and the basic master and slave example programs. This allows you to test the Modbus TCP comms without having to deal with wider LAN related issues.

[Bee Master]

Thank you for your suggestions. I was finally able to get the communication working. What I found was all my panels that shared a Class C addresses could communicate together without an issue. The panels that had Class B addresses would not communicate. What I had to do was connect the socket to the panel I wanted to talk to and then disconnect the socket and and reconnect it to the next panel I wanted to communicate to. Kind of a pain and probably not the best method but it works.

[Simon]

At least you now know how it is arranged.

I think you will need the help of the IT people if you want to set it up so that all PLCs were visible as a single network.

Possibly a VPN would be the way to go, but that's just a suggestion.