Allen-Bradley 1779-KFMR User Manual page 106

Chapter 8
In a command packet, these LINK, NODE, and USER fields combine to
fully specify the network address of the remote station that is to receive
the packet. In a reply packet, these fields specify the network address of
the remote station sending the reply.
CMD (Command — High Nibble) — The high nibble of the CMD
6
byte is a Data Highway field. Bit 6 (2 value) of the CMD byte is the
command/reply indicator. This bit is set to 0 for command messages and
1 for reply messages. Therefore, the high hex digit of the command byte
is 0 for command messages and 4 for reply messages. The low nibble
comes from the application layer and is described later.
Bit numbering starts with 0 and ends with 7 in each byte.
STS (Status — Low Nibble) — The low nibble of the STS byte is a
Data Highway field. In a command message, this field has a value of 0.
In a reply message reporting no error or a remote error, this field also has
a value of 0. The high nibble comes from the application layer and is
described later.
If the network layer of your computer cannot deliver a command to
another node, it writes a local error code into this field to generate a reply
message which it returns to the command initiator in your application
layer. Appendix A lists meanings of status codes you may receive in a
reply packet.
TNS (Transaction) — This two-byte field indicates which Data
Highway message transaction is taking place. A complete transaction
consists of the command packet and the corresponding reply packet.
When the host computer initiates a transaction, it must increment the TNS
value to distinguish the packets of the current transaction from the packets
of the previous transaction. When another station receives a Data
Highway command packet from your host computer, it must copy the
TNS value into the TNS field of the corresponding reply packet. This
helps the command initiator of your host computer match each command
with the corresponding reply.
Application Fields
Figure 8.1 shows the format of the Data Highway message packet,
including the application fields. The application fields are those Data
Highway message fields defined at the application layer. Here, we
describe these application fields.