CLX to SLC Communication without MSG Instruction?

[jds8086]

I'll start off by saying I'm unfamiliar with communications between PLC's, hence my question here.


We have a machine with a ControlLogix & a SLC 5/04 that communicate over DH+. In the program for the ControlLogix PLC i see the MSG instructions for reading from and writing to the SLC 5/04. What has me confused is there are no MSG instruction in the SLC 5/04. So am i right in assuming that this works simply by having the source & destination elements listed in the ControlLogix program for the memory address in the SLC 5/04 and it just simply reads/writes directly to the SLC 5/04's memory and it's program just uses those memory addresses directly instead of having to use MSG instructions to send/receive data from the ControlLogix?

 

[MasterBlaster]

You are exactly correct in your assumption. I usually made it a practice to annotate those destination memory addresses in the processors in such a way to indicate the SOURCE of the data. That eases troubleshooting in the future.

 

[jds8086]

You are exactly correct in your assumption. I usually made it a practice to annotate those destination memory addresses in the processors in such a way to indicate the SOURCE of the data. That eases troubleshooting in the future.

Okay thank you. Expanding on that just a bit, is there any configurable security involved in this or is it just wide open and security is left to the network itself being secure?


Also, lets say i wanted to change this SLC to a ComapactLogix, focusing on communication, would it still operate in the same fashion or would both ends use MSG instructions?

 

[Ken Roach]

In the late 70's and early 80's there were PLC systems where you had to write pairs of MSG instructions, one to send and one to receive.

But DH+ is the finest technology that 1988 had to offer, and it's a token-passing network. Only the message initiating controller needs a MSG instruction, whether it's reading from or writing to another controller.

ControlLogix (we're getting up on 1999) can do both un-scheduled MSG instruction communication, and also cyclic "Produced / Consumed Tags" over various networks including Ethernet. Those are configured with some background settings and the network modules take care of transporting them, and the data automagically appears in the Consumed Tags.

 

[MasterBlaster]

Okay thank you. Expanding on that just a bit, is there any configurable security involved in this or is it just wide open and security is left to the network itself being secure?


Also, lets say i wanted to change this SLC to a ComapactLogix, focusing on communication, would it still operate in the same fashion or would both ends use MSG instructions?

DH+ has no configurable security of which I am aware. It is a VERY old protocol.

The CL platform operates identically.

You do have the option of putting "read" MSG's at both ends, or "write" MSG's at both ends. Some prefer to do that for ease of troubleshooting. But as I mentioned earlier, proper commenting significantly eases troubleshooting.

 

[jds8086]

In the late 70's and early 80's there were PLC systems where you had to write pairs of MSG instructions, one to send and one to receive.

But DH+ is the finest technology that 1988 had to offer, and it's a token-passing network. Only the message initiating controller needs a MSG instruction, whether it's reading from or writing to another controller.

ControlLogix (we're getting up on 1999) can do both un-scheduled MSG instruction communication, and also cyclic "Produced / Consumed Tags" over various networks including Ethernet. Those are configured with some background settings and the network modules take care of transporting them, and the data automagically appears in the Consumed Tags.

DH+ has no configurable security of which I am aware. It is a VERY old protocol.

The CL platform operates identically.

You do have the option of putting "read" MSG's at both ends, or "write" MSG's at both ends. Some prefer to do that for ease of troubleshooting. But as I mentioned earlier, proper commenting significantly eases troubleshooting.

Thank you two for your help! It's an amazing resource having people out there such as yourselves willing to share your knowledge.

 

[Operaghost]

As to your question on upgrading the SLC to a CompactLogix, the same still holds true. There are obviously going to be some configuration settings that will be different within the MSG instruction setup. But the way it works is unchanged.

OG

 

[daba]

Thank you two for your help! It's an amazing resource having people out there such as yourselves willing to share your knowledge.

Bad practice to both READ (from PLC B to PLC A) and WRITE (from A to B) as jds8086 is possibly suggesting.

I prefer all my messaging to use READ. That way I can look at the message configuration to see where the data is coming from.

There is nothing worse on a large system than to see data being changed, and it is not being changed by this PLC, so it's time to "go hunting".... On a network with 45 PLCs that is a nightmare!!