CIP Class Code AC

NevisGroup

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Join Date
Jan 2014
Location
Wisconsin
Posts
78
Hello all.
I am in the process of writing some code in Python to chat with a Logix controller. Part of the process has been to watch the conversation between an HMI and the controller with WireShark. As I am going though the dumps I have everything pretty well worked out except I have one request to Class ID 0xAC. I can not find any documentation on what that is.

More detail.
I have a Multiple Service Request Packet.
The first service request is 0x03 (Request Attributes)
The path is 20AC2401. Which translates to Class 0xAC , Instance 01
It then goes on to ask for Attribute 0x01.

The next service is 0x4D, (Write Tag Service) which I have all kinds of documentation about.

Does anybody know what Class AC is? More importantly where is it documented??
(I have ODVA Vol 1 and 3 and various docs from Rockwell)
Thanks
🍺
 
From one of the guides, available here: http://www.rockwellautomation.com/sales-partners/technology-licensing/developer-guides.page we have...

The client application should use the Get_Attribute_List service to periodically
retrieve attributes 1, 2, 3, 4 and 10 of class 0xAC in the controller. If the value of
these attributes changes between reads, then the client application must refresh
the:
• List of symbols
• Association between symbols and templates
• Template information.​

There is some more detail on this but I will let you dig through it.
 
Funny... after I posted I see this warning:

IMPORTANT Do not use the Multi Service Packet service to group the attribute check
with the Tag Write service, as this would allow data to be written before
determining whether the tag information needs to be re-read.

Your description is seems to be exactly what the warning tells us not to do. Can you tell us what the HMI is, so I know to avoid it until they fix their driver?
 
robertkjonesjr:
I hope you are a drink'n man, 'cause I owe you a beer or 17!

I have read PM020 front to back several times and I missed that!!!!
Do you know AC is properly documented or do I just look for changes?
(I am currently digging through the rest of the docs)

Yes I caught the same warning! The HMI I was snooping is from Automation Direct. I want to do some more investigating but I don't think that this actually applies. Since my reads and write are to individual tags.

I am just writing enough to move some large DINT arrays in and out and a hand full of BOOLS.
 
I do not know of any other documentation for this object. I just stumbled across it. I know from talking with some of the Rockwell guys from corporate they get frustrated because third parties are supposed to use these guides to access data, but many people reverse engineer their protocols or extensions that they use. I suspect their specific protocols and extensions are higher performing, but isn't that the point - you get better performance when you use native devices from a vendor. Agree or disagree, that's business... The problem that occurs is that they can make a change to their specific protocol or vendor extension and then all those third-parties no longer work... and it's not really Rockwell's fault.

So my guess is this is not documented, but others may know. It's not an easy protocol to use, so do you have any plans to share your python code?
 
Once I am through this project I will leave a note here for anyone that wants my code.
You are forewarned...It is very purpose written. I intend to only implement what I need.
 

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