Allen-Bradley 1779-KFMR User Manual page 143

Chapter 9
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. For a command to read an immediate-access block, this field
specifies the block (00 through 0F hexadecimal) to read.
TNS (Transaction) — This two-byte field indicates which time-critical
message transaction is taking place. Except for a write with no reply
command, 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.
Application Fields
Figure 9.1 shows the time-critical message packet format including the
application fields. The application fields are those time-critical message
fields defined at the application layer. Here we describe these application
fields.
CMD (Command) — This one-byte field specifies the type of action to
be performed by the command executor at the destination station. This
field is only in each time-critical command message; it is not in the
corresponding time-critical reply message. The possible command-field
hexadecimal values are: