Reading single bit from ControlLogix (via CIP)?

Hi,

I'm talking to a ControlLogix PLC using CIP over E/IP, and I'm having trouble accessing single bits inside another type (such as an integer).

For example, I can access B30[4] or N2:I.SlotStatusBits, but when I ask for tagnames like B30[4].2 or N2:I.SlotStatusBits.0 I get an error back from the PLC (0x05, "The particular item referenced could not be found"). I can see these are valid tagnames under RSLogix, but nothing I try will get them back via CIP.

Is this a syntax problem on my end? If not, is there some other way to reference individual bits on ControlLogix, or am I out of luck?

Here's the IOI I'm using for "B30[4].2":
4C 06 91 03 42 33 30 00 28 04 91 01 32 00

Thanks for any advice you can give me!

I can't try it right now, but I think you need to specify number of elements at the end adding 01 00

If somebody wants to take a look how to decode it, manual is here:
http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm020_-en-p.pdf

Edit:
May be try to replace 32 with 02?
4C 06 91 03 42 33 30 00 28 04 91 01 02 00

Thanks for the reply!

I'm including the "01 00" for the number of elements at the end -- it just wasn't printed in the output, sorry about that. I also just tried using 02 instead of 32 in the last segment, and that didn't help. I still get "the particular item referenced could not be found".

Unfortunately there's no example of single-bit addressing in the manual you linked to. The closest thing is "str1Array[8].travel", which looks similar to what I'm doing (building an ANSI symbolic segment starting with 0x91, appending the array subscript as a Member ID with 0x28, and then adding another ANSI symbolic segment starting with 0x91 for the stuff after the '.'). The manual says this should work for structure members, and it does, but it doesn't seem to work for bits.

I also just tried adding the bit number as a Member ID (0x28), Element ID (0x28), Class ID (0x20), Instance ID (0x24), and Attribute ID (0x30) instead of an ANSI symbolic segment, just to see if any of those would work. They all give me a different error ("the IOI was malformatted").

Is there some other tagname format I could try? Please post if you have any other suggestions!

Hi,

I "think" the problem is the bits are not declared structure members like EN or ACC or PRE or travel. RSLogix makes it appear that way but I do think they are declared structure members. The base type is int. Int is not a structure.

My 2 cents. I would LOVE to be wrong!

I spoke with some people who know more than I know:
While I did not get a final answer both of them said that most likely it is not possible directly.
The proper way in this case get a complete integer (or DINT) and extract correct bit in your application.
If I hear an additional infromation then I will post it here.

Aw, I was afraid of that... guess I'll have some parsing to do.

Thanks again, I appreciate the quick answer!