Allen-Bradley 1779-KFMR User Manual page 142

Command Native Fields:
Reply Native Fields:
SEL (Select) — This one-byte field specifies the message set within the
Native mode and distinguishes commands from replies. For time-critical
command messages, the hexadecimal value in this byte is 0B. For
time-critical reply messages, the hexadecimal value in this byte is 8B.
SSQ (Sub-Sequence Number) — This one-byte value is a reserved byte
in the 1779-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.
Time Critical Fields
Figure 9.1 shows the time-critical message packet format including the
time-critical fields. The time-critical fields are those header fields
common to all time-critical message set transmissions. Here we describe
these time-critical fields.
LINK — Time-critical messages are limited to being sent to other nodes
on the same Data Highway II link. Therefore, always leave the value in
this one-byte field at 000 octal.
Chapter 9