Allen-Bradley 1756-CNB/E, User Manual

Page: 81 / 186

Publication 1756-UM523F-EN-P - December 2006
Configure and Program the Controller 81
8. Group your data by how often you need it. To update the secondary controller, the primary controller divides its memory into blocks of
256 bytes. Anytime an instruction writes a value, the primary controller crossloads the
entire block that contained the value. For example, if your logic writes only 1 BOOL value
to a block, the controller crossloads the entire block (256 bytes).
To minimize crossload time, group your data by how often you need it.
Suppose that you have some DINTs that you use only as constants to initialize your logic.
You have some BOOLs that you update every scan. And you have some REALs that you
update every second.
9. Use DINT tags instead of SINT or INT
tags
To keep your logic as efficient as possible, use the DINT data type instead of the SINT or
INT data types.
A ControlLogix controller usually works with 32-bit values (DINTs or REALs). If you use a
SINT or INT value:
•
the controller usually changes a SINT or INT value to a DINT or REAL value before it
uses the value.
•
if the destination is a SINT or INT tag, the controller usually changes the value back to
a SINT or INT value.
•
you do not have to program the controller to change values to or from SINTs or INTs.
The controller does it automatically. But it takes extra execution time and memory.
Action Details
This is better
One user-defined data type for the DINTs.
The controller crossloads these 12 bytes
only once.
One user-defined data type for the
BOOLs. The controller crossloads these
4 bytes every scan.
One user-defined data type for the REALs.
The controller crossloads these 12 bytes
every second.
Than this
One user-defined data type for all the
data. The controller crossloads these
28 bytes every scan.