Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 82)

6–3 Application Layer Message Packets
Publication 1770Ć6.5.16 - October 1996
Most devices send and receive messages using this message
packet format:
DST CMD STS
CommandĆspecific
data packet
SRC
read
write
The bytes are shown from left to right in the order they are transmitted across the link.
SIZE DATA
TNS
ADDR
SRC CMD STS
CommandĆspecific
data packet
DST TNS
SRC CMD STS DST TNS
FNC
Command
Reply
Field Contents See
DST destination node for the message
page 6 4
SRC source node of the message
page 6-4
FNC function code
page 6 5
CMD command code
page 6-5
ADDR address of memory location (2 bytes) page 6-8
STS status code
page 6 6
EXT STS extended status code
page 6-6
SIZE number of bytes to be transferred page 6-8
TNS transaction number (2 bytes) page 6-7
DATA
data values being transferred by the message
The number of data bytes in a message depends on
the command or function being executed.
Chapter 7
The combination of SRC, CMD, and TNS bytes uniquely identifies
every message packet. One of these fields in the current message
must be different than the corresponding field in the last message
received by a command executor. If all fields are the same, the
message is ignored and is considered to be a duplicate. If the
receiving module has the “ignore duplicate messages” option
enabled, the message will be ignored.
Message Packet Format