Best Way to Communicate Data Between Two Identical Logix PLCs

Maximuse

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I have two cells. They are identical (almost). Each cell has a Contrologix 1756-L73 processor and a 1756-ENBT addressed at 192.168.1.10.

Each of those PLC networks contain several E/IP nodes for I/O. Between the two cells these are almost identical as well. The only difference being "Cell B" has an additional single I/O point on one of the E/IP nodes (let's say it's a flow meter). I need to be able to read this flow meter value from both PLCs. What is the best method to accomplish this without physically connecting the networks and assigning new IPs to half of the devices?

Thanks!
 
1st produced and consume tags between plc.
2nd Also if both plcs can see same device on its IO tree. One plc is owner and the other just listens to device.
 
I sounds like you want to get to two PLCs, both with IP address 192.168.1.10, on cells 192.168.1.0/24 (=192.168.1.x with netmask 255.255.255.0 probably, a.k.a. Local Area Networks orLANs) from the same host, e.g. a PC host, that is on neither LAN.


Is that correct?


Are either of these LANs connected to any other network?

Can you buy any new equipment i.e. two routers (less than a few hundred each, I think)?

If yes, it will require some chicanery, but it may be possible.
 
Originally posted by widelto:

If your PLCS are identical please take into account that all IP addresses used on both PLCS are to be unique. I mean, all of them have to be different.
Click to expand...

Unless, as Operaghost and drbitboy point out, you put a router or equivalent between them. How much access you want between both networks will probably determine which device you want to go with. My general preference would be the router but either way could work.

Keith
 
widelto said:
Maximuse:
If your PLCS are identical please take into account that all IP addresses used on both PLCS are to be unique. I mean, all of them have to be different.
Click to expand...

That's why we use NAT. A standard router will not work as it requires each network connected to it to have unique IP address ranges. A NAT router is designed for this purpose. Or again, a switch that supports NAT like some Stratix switches.

OG
 
Operaghost said:
You need either a switch that can perform NAT (Network Address Translation) or a NAT device. Rockwell's 1783-NATR can fit that bill.

http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/enet-pp011_-en-p.pdf

OG
Click to expand...

I've tried doing something very much like this before with a 1783-NATR. It won't allow the public and private IPs to be on the same subnet. It would have been great if I could leave PLC A to be 192.168.1.10 and then map PLC B to be 192.168.1.10-->192.168.1.11 but it won't allow it.
 
drbitboy said:
^^^ OP: read this; apparently you are not the first to ask this question.
Click to expand...

Thank you for this, it's a perfect diagram for what I'm trying to describe. The issue for me (and maybe I'm just too dumb to interpret this properly) is that in that diagram I need MACHINE 1 PLC to be able to access MACHINE 2 PLC via produced/consumed tags OR one of it's I/O nodes. I totally understand that the ControlLogix rack in that diagram would be able to access everything. That's unfortunately not what I need though.

As I mentioned, I've run into a roadblock when trying to use a NAT to bridge the two networks, it seems to only allow the creation of a "new" 3rd network (ControlLogix/Cisco 3750/PC in that diagram).
 
Maximuse said:
Thank you for this, it's a perfect diagram for what I'm trying to describe. The issue for me (and maybe I'm just too dumb to interpret this properly) is that in that diagram I need MACHINE 1 PLC to be able to access MACHINE 2 PLC via produced/consumed tags OR one of it's I/O nodes. I totally understand that the ControlLogix rack in that diagram would be able to access everything. That's unfortunately not what I need though.

As I mentioned, I've run into a roadblock when trying to use a NAT to bridge the two networks, it seems to only allow the creation of a "new" 3rd network (ControlLogix/Cisco 3750/PC in that diagram).
Click to expand...


The NAT-fu, and port-forwarding-fu hides the actual IP that the target device thinks it is:



[PLC 192.168.1.10 on cell (LAN) 2] is available as 10.10.2.10 to the [PLC 192.168.1.10 on cell (LAN) 1].


[PLC 192.168.1.10 on cell (LAN) 1] is available as 10.10.1.10 to the [PLC 192.168.1.10 on cell (LAN) 2].


Am I missing something here?
 
Last edited:
drbitboy said:
The NAT-fu, and port-forwarding-fu hides the actual IP that the target device thinks it is:



[PLC 192.168.1.10 on cell (LAN) 2] is available as 10.10.2.10 to the [PLC 192.168.1.10 on cell (LAN) 1].


[PLC 192.168.1.10 on cell (LAN) 1] is available as 10.10.1.10 to the [PLC 192.168.1.10 on cell (LAN) 2].


Am I missing something here?
Click to expand...

Nah, you've got the gist of it.

PLC1(Lan1) would have its gateway address set to the private side IP of the NAT device(NAT1) it is connected too. It would setup produced consumed tags with PLC2(Lan2) using the Public IP of PLC2 according to NAT2).

Vice versa for PLC2.

You would need 2 NAT capable 'things' and a way to connect the 2 NAT things. The public side IPs of the NAT devices should be on a subnet that can talk to either public side range.
 
Dravik said:
Nah, you've got the gist of it.
Click to expand...


thought so, it's essentially the same tech that allows my son to run a server for the video game we play together.


"There is no software problem that cannot be solved with another layer of indirection."


i.e. stolen from



"All war is deception." - Sun Tzu
 
Last edited:

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