Sharing a Tag between 2 Compact Logix

HI all! I have two Compact Logix PLCs connected through Ethernet. An HMI gets/sets data from PLC1. Now the question is:
Which would be an elegant way to share a tag between both controllers? Eg, let's say I hav an INT that is a set point (let's say speed). So the HMI can write the SP in PLC 1. if I need to write the SP also from PLC2, how should I proceed?
I know that for instance using MSG I can transfer data between them, but what a bout a unique TAG (speed setpoint in PLC1) being able to be written both from HMI and from PLC2?
Any suggestions?
Thanks!

a_racioppi said:

HI all! I have two Compact Logix PLCs connected through Ethernet. An HMI gets/sets data from PLC1. Now the question is:
Which would be an elegant way to share a tag between both controllers? Eg, let's say I hav an INT that is a set point (let's say speed). So the HMI can write the SP in PLC 1. if I need to write the SP also from PLC2, how should I proceed?
I know that for instance using MSG I can transfer data between them, but what a bout a unique TAG (speed setpoint in PLC1) being able to be written both from HMI and from PLC2?
Any suggestions?
Thanks!

Click to expand...

There are a couple of options, I haven't had an opportunity to impliment it yet, but the Logix family supports Produced/Consumed Tags.

Logix Produced/Consumed Tags Manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm011_-en-p.pdf

Logix Message Manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm012_-en-p.pdf

I haven't reviewed this one, but it seems pretty involved.

Logix Instruction Reference:

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

The MSG Instruction is in Chapter 4.

Stu.....

HI SThompson, thanks for that. What is not clear for me it's if the Consumer can modify the tag Produced by someone else? Do you knwo if that's possible? I cannot test that yet, as I'll be getting the hardware next week! Thanks again!

Hi, the consumer can modify the tag but as soon as the tag is re-read by the consumer at the update rate (you set that) then it will be back to what the producer sent out.

What you are wanting to do is set a tag in PLC 1 from either a HMI or PLC 2.
A little bit harder. The HMI is OK because it will only write once to the tag in PLC 1 then give up. A produced tag from PLC2 will keep trying to update tag in PLC 1 which is not what you want. (How will the HMI ever get a go to change the tag.

I would message the new value from PLC2 to PLC1 and write to the tag in PLC1 if the received value is greater than -2000. Then I would change the received value to -2000 (this makes it a one shot write and allows the HMI to overwrite the value if required.)
You will need a bit more error checking etc.
The number -2000 can be anything outside your required speed range.
ie if you are sending the speed as hertz from -50 to +50 the use -100.

Regards Alan Case

How is the HMI connected to PLC1? Why not connect it to PLC2 instead, then PLC 2 can do what it wants....

HI Alan, thanks for that, it makes sense, I will try that

HI lost control. HMI1 polls PLC1 and HMI2 polls PLC2, they are all in the same ethernet network. The problem I have is that we are using Panel View Plus Compact, that can only speak to one controller. I've been asked that the operator can do exactly the same from both HMIs, as HMI2 cannot see PLC1 directly, I though of creating this link through the PLCs instead. Thanks!