Allen-Bradley 1779-KFMR User Manual page 171

Chapter 11
PAD Message Set
11 4
TNS (Transaction) — This two-byte field indicates which PAD
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 PAD 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 the command packets with the
corresponding reply packets.
LINK — This one-byte field specifies the destination Data Highway II
link (000 — 377 octal) for this packet within the network. With only one
Data Highway II link in the network, enter the value in the LINK field to
be 000 octal.
NODE — This one-byte field specifies the destination node
(001 — 376 octal) for this packet within the destination link. The value
of this field corresponds to the octal value you select with the
LINK-ADDRESS thumbwheel switches at each node interface.
USER — This one-byte field specifies the destination channel for this
packet within the destination node. This applies to a 1779-KFL interface
as a destination node because it has two channels to host devices. For the
destination to be Channel 1, enter a USER-field value of 1. For the
destination to be Channel 2, enter a USER-field value of 2. For a
destination node that has only one possible channel, enter a USER-field
value of 1.
In a command packet, these LINK, NODE, and USER fields combine to
fully specify the network address of the destination station. In a reply
packet, these fields specify the network address of the remote station
sending the reply.
Application Fields
Figure 11.1 shows the PAD message packet format including the
application fields. The application fields are those PAD message fields
defined at the application layer. The application fields contain a
maximum of 238 bytes of data in any format. These application fields are
only in the PAD command packet.