Studio 5000 Produced/Consumed tags over Ethernet Radios

szm_us

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Jul 2011
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In the past, I have used Produced/Consumed Tags with CompactLogix PLC Systems over an Ethernet Network and it has worked great.

On a new project, there are nine AB 5370 CompactLogix PLC Systems (Part #: 1769-L24ER-QBFC1B) connected to each other using MDS Ethernet Radios. One of these PLC Systems will be connected to the SCADA Computer and act as a data concentrator sending / receiving data to the eight other PLC Systems. There will be no data communications between all of the other PLC Systems to each other.

Is it possible or even wise to use Produced/Consumed Tags instead of message statements with an Ethernet Radio Network for this system?

In addition, I would like to use Unicast Connections, since there are no managed network switches in this PLC Ethernet Network?

Any advice or wisdom would sure be appreciated.
 
"Produce/Consume" was originally developed as a "real time I/O" communications protocol that was designed around Ethernet connections with latency times of just a few milliseconds. Because of how easy it makes communications it has become the method of choice which is understandable given what it takes to do message instructions (especially when talking about several devices in a network).

As to radio modems, the first thing you need to understand is that the higher the speed, the less distance you will get and the less reliable the connection will be over any sort of distance. By far the most robust and reliable technology is going to be 900MHz Frequency Hopping Spread Spectrum but it will also be (by far) the slowest technology. Basically, to gain speed you have to sacrifice robustness and reliability. To be fair, high speed technology has come a long way in the past 10 years and is, compared to the same basic technology, much more reliable but if your connection is anything over a few hundred feet and/or line of sight poor, FHSS will still be superior (again when talking about reliability).

So, with regards to your application, Produce/Consume requires low latency (high speed) and robustness/reliability which can be achieved with wireless technology but only in ideal conditions and at very short ranges.

In my opinion (having spent more than 20 years in the wireless data communications industry) plan on using messaging (I.E. message instructions) but if the conditions look favorable, and the MDS modems you are going to use are based on WiFi technology, test P/C on the wireless and see if it works. Also remember that if you do setup P/C across the wireless network and you attempt to do programming across the same link, going online with the PLC runs a risk of bringing down the P/C connection(s). If you do go P/C wireless, make the wireless dedicated to the P/C connection.

Last, if again the MDS modems are WiFi type, when evaluating the RF environment you must take into consideration that EVERYTHING in today's world uses that technology and the spectrum is crowded to say the least. This can cause all kinds of issues not the least of which is the system may run just fine where it is developed and tested but crashes when installed on site (I've seen that one more than once). It also changes over time for a wide variety of reason and might work one day but fail the next (I've seen that one more than once as well). You also need to check with the IT department for the facility where the application will be located. Depending on a lot of factors they may not even allow it to be installed.
 
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Thank-you for your reply.
I believe that I will set it up for messaging.

One question with the messaging, I have always set it up to sequence the messages so that either the .DN Bit is set or the .ER Bit is set before it goes onto the next message. Is there a better way to do this?

Thank-You.
 
State-and-MSG.png
 
I don't know if this is what you are looking for or not
I have done jobs using radio ethernet extenders to connect the remote I/O like Point I/O
I have not seen any problems with it
depending on the radio you can get over 20 miles the even have repeaters that can extend it beyond that if needed
 
We had used Prosoft for wireless communication between ground PLC and moving crane plcs. We had major problems of network time out. We had done many changes in the software. Finally, we installed radiant cables along with the motion of the crane which acts are the movable antenna. The cables are expensive though
 
We had used Prosoft for wireless communication between ground PLC and moving crane plcs. We had major problems of network time out. We had done many changes in the software. Finally, we installed radiant cables along with the motion of the crane which acts are the movable antenna. The cables are expensive though

Radiant cable is also known as "leaky feeder" and it is very expensive to purchase and install.

It is basically coax with holes in the shielding so that the center conductor becomes the radiating element of an antenna. You install it along a "path" that the moving device follows so that the two antennas are never more than a few feet from each other. The other problem you will run into, Leaky feeder has limited run lengths which largely depends on the transmitters output power as well as the receivers receive sensitivity and to complicate it just a little more some radios will work better than others depending on how well they deal with RF over-saturation.

Long and short is it is certainly a technology that can solve problems but it's complicated at best to deal with and pretty much requires someone who is an expert at using it be consulted to determine if it's even doable in any specific application.
 
This question is to BachPhi's response:

I noticed that you move a different number to the "State" register depending if the message is done (Value = 20) versus errors out (Value = 100). Is this so that if a message is done, you activate another message to the same device and if it errors out, you jump to the next device message?

Also, why did you put a "XIO" with the MSG_Read.EN in series with the message statement?

You also have a "TriggerRead" that starts this messaging. What activates this "TriggerRead"?

Do you have any logic to reset the comm if it hangs up?

Thank-You.
 
This question is to BachPhi's response:

I noticed that you move a different number to the "State" register depending if the message is done (Value = 20) versus errors out (Value = 100). Is this so that if a message is done, you activate another message to the same device and if it errors out, you jump to the next device message?

After a read then maybe you want to do a write, same device or different device, it is up to you as programmer

Also, why did you put a "XIO" with the MSG_Read.EN in series with the message statement? Why not

You also have a "TriggerRead" that starts this messaging. What activates this "TriggerRead"? That's up to you, when do you need to read the data?

Do you have any logic to reset the comm if it hangs up? check link below

Thank-You.

You can find out more with another technique using just one MSG to read to multiple controllers:

https://www.advancedhmi.com/forum/index.php?topic=2682.0
 
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