MSG between 2 ethernet module. Two seperate subnets

[SIS_Brady]

Hello,



I am trying to msg between a 1756-ENBT and a 1756-ENBT. One PLC is on a 192.168.34.xx subnet with a 1756-ENBT in slot 2 with a 192.168.36.xx ip address. The other PLC has a 1756-ENBT with an ip address of 192.163.36.xx as well. I cannot get the message to go Done. I keep getting errors. Could someone help me make sure I am setting this message up properly and the communication path is set up properly.

Thanks,

 

[Ken Roach]

Please clarify: one of your controllers has two 1756-ENBT modules. Is that the controller that has the MSG instruction ?

Slot A: ControlLogix MSG originator
Slot B: 1756-ENBT 192.168.34.x
Slot C: 1756-ENBT 192.168.36.x

Slot D: ControlLogix target of message
Slot E: 1756-ENBT 192.168.36.y

Your CIP Path will be "1, C, 2, 192.168.36.y, 1, D"

CIP Paths always consist of pairs of values: "Port, Address"

This CIP Path has three segments:

Port 1 (backplane), Address (ENBT Slot #)
Port 2 (Ethernet), Address (IP Address)
Port 1 (backplane), Address (CPU Slot #)

 

[SIS_Brady]

That is correct. One controller has 2 1756-enbt.



PLC1:
Slot 0: 1756-L75
Slot 1: 1756-ENBT/A (192.168.34.62)
Slot 2: 1756-ENBT/A (192.168.36.10)



PLC2:

Slot 0: 1756-L75
Slot 1: 1756-ENBT/A (192.168.36.20)

 

[Ken Roach]

It should be straightforward, then:

1,2, 2,192.168.36.20, 1,0


Of course those two 1756-ENBT's need to be on the same physical network, or on network segments that are connected via VLAN or bridges.

Capture the .ERR value and see if this is a connection problem, or one of data access or data type mismatch.

 

[PlastekGroup]

Make sure the Control or Integer you are using for your MSG instruction isn't being used by other things in the program as well. If it's being overwritten anywhere in the program it won't function correctly.

 

[lfe]

Two subnets can not communicate with each other as long as there is not a routing device. In now days usually the router is a level 3 switch, but the router device could be another thing, including a computer with two ethernet interfaces with the proper configuration

This is independent of whether or not they share the same physical network, two subnets can share the same switch but they will not be able to communicate if there is not a routing device.

 

[Ken Roach]

My reading is that OP is trying to communicate between the 1756-ENBT at 192.168.36.10 and the 1756-ENBT at 192.168.36.20, which are on the same subnet.

The 1756-ENBT that's on the 192.168.34.x subnet is not involved (the way I read it).

If that's not the case and they are trying to communicate between subnets in some way, I agree that we will need more information about the network.

 

[Firejo]

You can get two subnet schemes to communicate but you have to set the subnet mask correctly. I think "255.255.0.0" would work but I don't know enough about networking to know for sure. What I do know is that a subnet mask of "255.255.255.0" will not work.

 

[Ken Roach]

Plastek,

In the ControlLogix there's a dedicated MESSAGE datatype that serves as the "Control Block".

I agree that in the PLC/SLC it was common to inadvertently overlap or re-use parts of a control block because you could use ordinary Integer data table elements.

You can still inadvertently use the same MESSAGE control tag on more than one MSG instruction in ControlLogix, so it's worth double-checking.

Also: I just went and checked in Studio 5000 v24 and the software does not flag a Warning if you use the same MESSAGE control tag in more than one MSG instruction. So it's very much worth double-checking !

 

[lfe]

You can get two subnet schemes to communicate but you have to set the subnet mask correctly. I think "255.255.0.0" would work but I don't know enough about networking to know for sure. What I do know is that a subnet mask of "255.255.255.0" will not work.

A larger subnet than /22 is not recommended, this means a 255.255.252.0 mask.

 

[PlastekGroup]

Plastek,

In the ControlLogix there's a dedicated MESSAGE datatype that serves as the "Control Block".

I agree that in the PLC/SLC it was common to inadvertently overlap or re-use parts of a control block because you could use ordinary Integer data table elements.

You can still inadvertently use the same MESSAGE control tag on more than one MSG instruction in ControlLogix, so it's worth double-checking.

Also: I just went and checked in Studio 5000 v24 and the software does not flag a Warning if you use the same MESSAGE control tag in more than one MSG instruction. So it's very much worth double-checking !

I know when we first started setting up communications with our PLCs we had this issue originally, one of the Engineers here liked using N7 for the Control portion of the MSG block and it was getting rewritten and not working correctly with no real indication of why.

Just an easy to thing to check on that can be checked on.

 

[JaxGTO]

For sub-nets 192.168.34.xx and 192.168.36.xx to be able to communicate a mask of 255.255.192.0 can be used. 192 allow the lower 6 bits (or sub nets 0 to 63) to communicate.