Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 86)

6–7 Application Layer Message Packets
Publication 1770Ć6.5.16 - October 1996
TNS
The TNS (transaction) bytes contain a unique 16-bit transaction
identifier. Generate this number by maintaining a 16-bit counter.
Increment the counter each time your command initiator (application
program) creates a new message, and store the counter value in the
two TNS bytes of the new message. In a multi-tasking environment,
you must use only one TNS counter, and the procedure to read and
increment the TNS must be indivisible.
Command initiator checks TNS value of reply and matches it to one of its
command messages. The low byte (least significant bits) of the TNS value
is transmitted before the high byte (most significant bits).
TNS
TNS
command
reply
Command executor copies the TNS field of the command message into
the TNS field of the corresponding reply message.
For command messages
transmitted by
Values are assigned by
a PLC the interface module assigns the TNS values.
computer node
your application level software. The software must
assign a unique 16Ćbit transaction number.
Important: Do not change the TNS value in a reply message.
If you change this value, the command initiator cannot
match its command to the corresponding reply message.