Transfer data between two 1756-L61 PLCs

Hi,
I have two identical RsLogix 5000, 1756-L61 PLCs connected to a Ethernet hub.
Slot[0] 1756-L61 Brain
Slo2[1] 1756-ENBT/A ENet
Slot[2] 1756-DNB DNET

I need to transfer tag data back and forth between these processors via Ethernet. I would like to use the MSG command but have no clue in some of the info I need to enter.
Can you give ma a idiot's guide to set this up?

Thanks ...

Have a look at Chapter 4 of the Logix Instruction Reference, they give a pretty good description of how to set up the MSG Instruction.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf

You can also do a search here, as this subject has come up more than once.

Stu....

That is a big chapter - here's the short version.

Firstly I prefer to always read data from another PLC or controller, it allows for some traceability.

To read from another PLC, the communications path to the target must be specified in the MSG configuration dialog.

There are two ways to achieve this.....

a> "Build" the path to the target PLC in the I/O configuration.

Firstly add your own (Local) Ethernet module to your backplane (hint: right-click->New Module).
Fill in all the white boxes with the correct information.

Then add the remote EtherNet module to the EtherNet network that appears in the tree (Choose Comm Format "None").

Then add the remote controller onto the new "Backplane" that will appear in the I/O Configuration tree. The controller will then become a named choice for the MSG communication path.

OR:

b> Specify the path explicitly in the MSG configuration.

To do this, the path is made up of repeating pairs of data Port,Destination,Port,Destination etc..

Port : the Backplane port is number 1, "Front" ports are numbered sequentially 2,3,.... etc.

Destination : Either a Slot number, or an address....


Example :...

Assume
Local Controller, Slot 0
EtherNet Comms Module : Slot 1
Remote EtherNet Address : 192.168.4.67
Remote Controller : Slot 4

Path to the remote controller is then...

1,1,2,192.168.4.67,1,4

1,1 = Go out the back port to slot 1
2,192.168.4.67 = Go out the front port to ethernet address 192.168.4.67
1,4 = Go out the back port to slot 4


It has to be said the first method is preferable, as it helps to see the route of the comms.

And you say 'hub' but I hope you mean 'switch'. Hubs are bad news these days.

daba said:

Firstly I prefer to always read data from another PLC or controller, it allows for some traceability.

Click to expand...

I used to agree with this - until I had a large network that was clogged up by the communications - The fix was to use event driven writes
i.e. don't send the message until the data has changed. This Reduced the network load 95% loading down to 15% loading

PS in Control logix I use the tag name for traceability eg Tag RX_PLC1 is a user defined data type DINT[60] that is received from PLC1 in PLC1 I use TX_PLC1

Give Produced/Consummed a try.

godfrey said:

Give Produced/Consummed a try.

Click to expand...

+1.
I don't see a need for messages especially when you can set up produced/consumed tags.

bkottaras said:

+1.
I don't see a need for messages especially when you can set up produced/consumed tags.

Click to expand...

Agreed, but would you set up Produced/Consumed tags for data that only needs to be exchanged infrequently ? An example : passing a Recipe for the next PLC to process the batch, which might only need to be done every hour or so.

It would be overkill on network resource to use Produced/Consumed tags for this.

And don't forget that a P/C tag can only contain up to 500 bytes.

And don't forget that a P/C tag consumes a Connection. On a large system you may run short. A message can be set to relinquish its connection after the message is finished with, until the next time it triggers.

Each method has its merits - you choose the best method to fit the application.

daba said:

Agreed, but would you set up Produced/Consumed tags for data that only needs to be exchanged infrequently ? An example : passing a Recipe for the next PLC to process the batch, which might only need to be done every hour or so.

It would be overkill on network resource to use Produced/Consumed tags for this.

And don't forget that a P/C tag can only contain up to 500 bytes.

And don't forget that a P/C tag consumes a Connection. On a large system you may run short. A message can be set to relinquish its connection after the message is finished with, until the next time it triggers.

Each method has its merits - you choose the best method to fit the application.

Click to expand...

I agree on the recipe and such part.
But, as he stated he only has 2 CPU's thus connections/resources aren't an issue with his set up.
For a larger system/network I would investigate and decide.

bkottaras said:

I agree on the recipe and such part.
But, as he stated he only has 2 CPU's thus connections/resources aren't an issue with his set up.
For a larger system/network I would investigate and decide.

Click to expand...

Isn't that what I said

Each method has its merits - you choose the best method to fit the application.

Click to expand...

daba said:

Isn't that what I said

Click to expand...

Yep.
And I also said that for his current setup, I'd scrap the messages and go for produced/consumed tags!