Allen-Bradley 1779-KFMR User Manual page 109

Chapter 8
STS (Status) — The high nibble of the STS (status) byte is supplied by
the application layer. In command messages the STS byte is set to 0.
In reply messages, the STS field is used for reporting either application or
network error codes. A value of 0 means the message was delivered and
executed successfully. Non-zero status can be divided into two
categories—remote errors and local errors.
A remote error means a command was successfully delivered by the
network, but the remote node was unable to execute the command. The
remote node then placed an error code in the high nibble of the STS byte.
A local error means your network layer was unable to deliver the message
to the remote node. Your network layer then turns the command around,
stuffs the low nibble of the STS byte with the appropriate error code, and
returns it to your application.
When you receive a reply message, check the STS byte at the application
layer. If the STS byte is non-zero, refer to Appendix A for an explanation
of the type of error that has occurred.
If your application layer receives a command message and detects an
error, it should format a reply message with a remote error code in the
high nibble of the STS byte.
ADDR (Address) — This variable-length field specifies the starting
address of the block of memory affected at the destination station which
executes the command. You may not have to supply this field, depending
on the message you are sending. The address field is only in command
message. L ADDR indicates that a logical address is required. P ADDR
indicates that a physical address is required.
SIZ (Size) — The one-byte size field specifies the number of data bytes
to be transferred by a message. This field appears only in read
commands, where it specifies the number of data bytes the responding
node must return in a reply message. The allowed value for size will vary
with the type of command you are sending.
DATA — This variable-length field contains binary data from the
application layer. You may not use this field depending on the message
you are sending. A data field can only appear in a command or
success-reply message.