Rockwell Automation 1797-OE8H FLEX Ex HART Analog Modules User Manual - Manual (Page 187)

Publication 1797-6.5.3 - March 2006
FLEX Ex HART Modules Network Messaging F-3
Differences Between
Attributes and Assembly
Indexes The two Host Access Groups on the module let two different hosts
communicate at the same time to the module and its associated field devices.
The Attribute used by MSG or CIO instructions send the attribute number to
the adapter module. The I/O modules use Assemblies. The adapter
cross-references the requested Attribute to the corresponding Assembly and
forwards it to the associated FLEX Ex HART module for processing.
Messages are sent and received through the multiple HART
Request/Response buffers in the same Host Access Groups. To maximize data
throughput, these buffers are different sizes.
If you attempt to get the response from Attribute 74 hex, an error message
would be reported. All messages containing HART commands and responses
to and from the FLEX Ex I/O modules are enhanced while standard HART
messages are used between the I/O module and the field device. The Error
message is an example of these enhancements.
The Preamble syncs the field devices to the I/O module. Once the HART
message is received in the I/O module, the Preamble is no longer needed. The
FLEX Ex HART modules can queue up to four HART commands, meaning
that the module needs a method to identify the HART response to the
associated command.
EXAMPLE
If a message from the module was expected to have
23 bytes, the message would fit into Attributes 6B to
70 hex for Host Access Group 1. Therefore, the
response could be obtained by reading any of these
attributes.
If you read the Attribute, 100 bytes would be
returned containing 23 expected bytes and 77
zero-filled bytes. Sending these extra 77 bytes takes
additional time and slows down the response time.
Therefore, use the Attribute that best fits the
expected message size.
Table F.2 Standard HART Message
Preamble Start
Character Address Command Byte Count (Response
Code) Data Checksum
Table F.3 Write HART Command Contained in a MSG or CIO Instruction
Handle
(1 Byte) Start
Character Address Command Byte Count (Response
Code) Data Checksum