Allen-Bradley 1779-KFMR User Manual page 105

Chapter 8
SEL (Select) — This one-byte field specifies the message set within the
Native mode and distinguishes commands from replies. For Data
Highway command messages, the hexadecimal value in this byte must be
03. For Data Highway reply messages, the hexadecimal value in this byte
is 83.
SSQ (Sub-Sequence Number) — This one-byte value is a reserved byte
in the KFM.
EXT (Extension) — This one-byte field blocks messages formatted for
Data Highway from being accepted on Data Highway II unless they are
properly reformatted. The hexadecimal value in this byte must always be
FF.
Data Highway Fields
Figure 8.1 shows the format of the Data Highway message packet,
including the Data Highway fields. These fields are common to all Data
Highway message transmissions in Native mode. Here we describe these
Data Highway message fields:
LINK — This one-byte field specifies the destination Data Highway II
link (0 — 377 octal) for this message within the network. With only one
Data Highway II link in the network, enter the LINK-field value 0 octal.
NODE — This one-byte field specifies the destination node
(1 — 376 octal) within the destination link for this message. 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 within the
destination node for this message. This applies to a 1779-KFL interface
as a destination node because it has two channels to host devices. A value
of 1 in this field routes a message to Channel 1. A value of 2 in this field
routes a message to Channel 2. If you are sending a message to an
interface other than a KFL, the value in this field is 1.