ControlLogix Data Types

Overview

The ControlLogix driver in ACM supports multiple data types associated with tags in the device. The following sections outline all the types and their descriptions.

String Support

The protocol handles STRING tags within the device as a single tag. The OPC item for a STRING tag will display the concatenated contents of the character array stored in the device using the following rules for termination of the last character:

1. The first NULL character encountered. 

2. The LEN parameter value if 1 has not occurred. 

3. End of the array if 1 or 2 have not occurred first.

The protocol will update the LEN parameter on writes unless disabled by option. The user can also disable use of LEN while reading strings by option. Please refer to Configuration.

Array Support

The protocol supports reading and writing values to indexed arrays. During a tag discovery poll, any array structured tags will be identified with an OPC item placeholder in the following format:

To access the third logical element in a one dimensional array of a tag called MyArrayTag the user would alter the OPC item name to MyArrayTag[2].

Bit Offset Support

Integer bit offsets are supported for read/write operations only on 16, 32 and 64 bit integer data types. To access a bit offset, use the following format:

Requests for index values residing within the same Integer tag are consolidated to minimize traffic by reducing the number of redundant service requests. To invert the display value of a Bit Offset tag, use the following format: