38 Rockwell Automation Publication 1756-PM020F-EN-P - January 2019
• Create an alias of the atomic member and access the alias instead
of the structure.
• Create an atomic tag or UDT structure tag with an atomic member,
and then have the user program copy the data to and from the tag
or atomic member. Access the new tag or atomic member instead.
• In the Logix Designer, version 21 and later, and in RSLogix 5000
software, version 18 and later, external access to controller scope tags
is user-selectable. If an External Access tag attribute is set to None, the
tag cannot be accessed from outside of the controller. Therefore,
structures that contain members whose external access is set to None
cannot be accessed as a whole (that is, by reading or writing the entire
structure). Similarly, structures that have one or more members whose
External Access is set to Read Only cannot be written to as a whole
(that is, by reading or writing the entire structure), but the members
that are not restricted in access can be accessed by using symbolic
segment addressing to the specific member.
Further information on data access control and the effect it has on structure
access can be found in
Logix 5000 Controllers I/O and Tag Data
Programming Manual, publication 1756-PM004.
• To improve tag access performance and to simplify the task of
accessing structured tags through a network, create UDTs for the data
that needs to be accessed through the network with members of the
types listed following only, and access the UDTs as a whole.
• Atomic tags
• Arrays of atomic tags
• Other UDTs of atomic tags
• Arrays of UDTs of atomic tags
See also
CIP Services and User-created Tags on page 41
Structures use a Tag Type Service Parameter that is different from the one
used with atomic tags. Like atomic tags, writing to UDT-based tags as a
whole (that is, the whole structure, not just individual members) requires
supplying the proper value for this parameter in the service request to
successfully write to the tag. The value is also returned when the structure is
read.
The Tag Type Service Parameter for structures is a 4-byte sequence. The first
two bytes are the values A0 and 02, followed by the latter two bytes, which
contain a 16-bit calculated field called a Structure Handle. When transmitted
on the wire, it looks like this:
A0 02 Structure Handle Low Byte Structure Handle High Byte
Tag type service parameters for
structure
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