Read Tags using CIP

JEaglestone · Nov 25, 2020

Hello Everyone,

I've been trying to write a python code to read input and output tags from a PLC using CIP. I found the service Get_Instance_Attribute_List(0x55). But I don't know how to use this service code to read the tags. Can anyone please help me with a code example of how I can read the PLC inputs and outputs using the CIP?

Thank you

dmroeder · Nov 25, 2020

JEaglestone said:
Hello Everyone,

I've been trying to write a python code to read input and output tags from a PLC using CIP. I found the service Get_Instance_Attribute_List(0x55). But I don't know how to use this service code to read the tags. Can anyone please help me with a code example of how I can read the PLC inputs and outputs using the CIP?

Thank you

Well in pylogix, you GetTagList() will retrieve a list of the tags, but it does not read the values. Read("MyTag") will read the values of the tag name that you supply. It has limitations, it doesn't handle parsing UDT's for example.

As I stated in your other thread (not good to have multiple threads on the same subject), describe in detail what you are trying to accomplish. In your other thread, you say that you cannot use python...

Mark- · Nov 25, 2020

Hello,

I do not have any Python code to post.
The method format is described in the Logix5000 Data Access manual.

Attached is a spreadsheet breakdown of the request message. I “believe” is it correct.

JEaglestone · Nov 25, 2020

Hello. Here's what I'm trying to accomplish:
I want to use the class code and the service code to directly read or write the tags from the PLC. I'm using a micro850. But I don't know how to do that in python code. I have to use python. But can't use pylogix library. I need to know how to use the service codes and class codes to retrieve the values. Now, I found the service Get_Instance_Attribute_List(0x55). But I coudn't find a proper attribute for this service that can read or write from the PLC. I need to know which service I can use with which class and attribute to read and write to the PLC. And after that how can I use these service code, class code and attribute in Python to read and write to the PLC.

dmroeder · Nov 25, 2020

So you must use python but you cannot use any other existing python library? You must write your own library? This is no trivial task.

0x55 service is not for "reading/writing" values, it is for requesting attributes of tags/structures. You would probably be more interested in services 0x4c an 0x4d (read/write).

https://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm020_-en-p.pdf

Mark- · Nov 25, 2020

Hello,

>But I coudn't find a proper attribute for this service that can read or write from the PLC.

IIRC, the Micro8xx only supports the "Read Tag Fragmented Service".

drbitboy · Nov 25, 2020

dmroeder said:
...In your other thread, you say that you cannot use python...

Not that I understand what is going on, but the OP said they cannot use pylogix, not python.

I wonder if the task is to write some kind of lower-level driver.

JEaglestone · Nov 25, 2020

Thank you. I came across 0x4C and 0x4D (read/write). But I don't know what object and attribute I should use them with to get the result. And how do we use these hex codes in python to get the result?

dmroeder · Nov 25, 2020

drbitboy said:
Not that I understand what is going on, but the OP said they cannot use pylogix, not python.

I wonder if the task is to write some kind of lower-level driver.

I misunderstood what he was saying in another thread, it was cleared it up in post 4.

drbitboy · Nov 25, 2020

dmroeder said:
I misunderstood what he was saying in another thread, it was cleared it up in post 4.

right, crossing posts, I should have kept refreshing.

drbitboy · Nov 25, 2020

dmroeder said:
https://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm020_-en-p.pdf

Is that the closest thing that exists to a protocol document?

dmroeder · Nov 25, 2020

drbitboy said:
Is that the closest thing that exists to a protocol document?

The ODVA spec is the official, it's just not free.

Mark- · Nov 25, 2020

drbitboy said:
Is that the closest thing that exists to a protocol document?

That document describes the CIP "services" to utilize for data access to Logix devices. FYI, the different years of the document do not include all the same data.

As "dmroeder" stated, ODVA controls the Ethernet/IP protocol specification.

There are a few other RA documents with information on data access.

JEaglestone · Nov 26, 2020

Mark- said:
Hello,

>But I coudn't find a proper attribute for this service that can read or write from the PLC.

IIRC, the Micro8xx only supports the "Read Tag Fragmented Service".

That's service code 0x52. Do you remember what attribute and class goes with it?

AlfredoQuintero · Nov 27, 2020

JEaglestone said:
That's service code 0x52. Do you remember what attribute and class goes with it?

Hello JEaglestone:
Service 0x52 is not issued to a class ID, instance ID, Attribute ID (logical segment) type of path. Rather, it is issued to a symbolic segment path, as explained in manual that I provice a link to below, as well as the screenshot of that manual relevant to your inquiry. I hope you find this useful.
https://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm020_-en-p.pdf

Read Tags using CIP

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