Rockwell Automation Publication 1756-PM004D-EN-P - September 2012 65
Chapter
4
Data Access Control
Introduction
In the Logix platform, version 18 or later of the application, there are two tag
attributes that allow you to control access to tag data. These attributes are:
• External Access
• Constant
The External Access attribute controls how external applications, such as HMIs,
can access tags. It has possible values of Read/Write, Read Only, and None. See
“Configure External Access”
on page 66.
The Constant attribute value determines if a tag can be modified by controller
logic. Also, by using FactoryTalk Security software, it is possible to control which
users are permitted to change tags designated as constants in the Logix Designer
application. See page 77
for more information on the Constant attribute.
By using these two attributes, you can help safeguard tag data by preventing
unwanted changes to tag values. Also, by reducing the number of tags exposed to
external applications, you can also reduce the time required to develop
HMI screens.
External Access
By using the External Access attribute, you can control how external applications
and devices can access tags.
This process can help you manage the thousands of tags you might have in a
project that have similar names that can get easily confused when referencing
them in applications or devices.
Using this attribute also can help improve system performance by reducing the
number of tags RSLinx has to maintain, scan, and cache. This volume can impact
the performance of the RSLinx data server and other
related applications.
External applications and devices include:
• RSLinx Classic and RSLinx Enterprise software.
• other Logix controllers.
• PanelView terminals.
• PLC/SLC controllers.
• FactoryTalk Historian software.
• other third-party software.
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