Addressing Bits with Allen-Bradley Compact Logix; how to link to an asset?

hello all,

Using allen bradly PLC and created a screen in ignition. I can tag most things but the tags provided from a previous contractor are long addresses i cant adress properly in the tag interface.

Take this tag for example = B3[23].15

Its a bit used for a “cooling call” in a compressor system.

When i open the tag tree in the designer (were all the imported tags are) i can only drill so fare down into the tag tree to only reference B3[23].

How do i reference the full tag?

I can reference everything else properly and its working just issues with long tags.

I’m not sure I would call that a long tag name, either way, just add the bit reference to the end after selecting the Word reference: ns=1;s=[YourDevice]B3[23].15

B3[23].15 is a bit address. It’s bit 15 of word 23 in boolean block 3.

Mirologix tags

You should be able to select the individual bits from the tag browser. The screengrab is from a MicroLogix.

By the way, it’s “Allen-Bradley CompactLogix”. It will matter for search engines and legibility.

I checked and the ControlLogix, CompactLogix driver doesn’t allow you to drill down to the bit level, you have to add it manually.



Yea your correct, you cant drill down to the bit level in the browser. how would i add it manually? i have been trying with different a syntax

this is my tag from the explorer in designer


How do i add B3[23].15 to it

something like

[Sample_Tags]Controller:Global/B3/B3_23_/.15 ? << that doesn’t work by the way.


You are showing a tag binding. You need to create an actual tag that has an OPC Item Path to the bit level, just as I have shown it.

Of course there are other ways you “can” check the 15th bit of an integer value, but that doesn’t seem like what you’re looking for.

For instance you could use an expression:


Yea i am showing a tag binding as all of my other tags are done this way. For example

Here is a snippet of the screen i have created that’s running

As you can see the discharge temp here can be selected in the tag browser as it doesn’t drill down as far.

here is the tag screen from the PLC

Is their a way to just address it at the bit level by adding to the string [Sample_Tags]Controller:Global/B3/B3_23_ ?

Cant add a 4th picture to the above post so new post.

Im in the OPC tag menu, take this tag for example “Local:9:O.Data.5”

How do i address it in the item path? I have this “Local:9:O/5” so fare wich doesnt work (tried a few different variations)


I get it now, this worked ns=1;s=[HC Man]Local:9:O.Data.5

Here is a SS for anyone who stumbles across it in the future

Can a mod change the title of the thread from

Long tags with Allen-Bradley CompactLogix, how to link to an asset?


Addressing Bits with h Allen-Bradley CompactLogix, how to link to an asset/?

Probably more relevant cheers

Just to summarize for future readers:

In the modern Logix driver, when constructing a path to a non-browsable item, the part of the OPC Item Path that follows [deviceName] will basically be identical to the syntax used within Rockwell’s Studio 5000.

For legacy ControlLogix and CompactLogix drivers, there are some extra elements inserted when arrays are involved, but the tail of the Item Path will follow RSLogix 5000 syntax.