Allen-Bradley 1779-KFMR User Manual page 177

Chapter 12
SEL (Select) — This one-byte field specifies the message set within the
Native mode and distinguishes error replies from commands and other
replies. For station-management commands, their transaction-completed
replies, and their command-error replies, the hexadecimal value in this
byte is 08. For station management replies to indicate format errors, the
hexadecimal value in this byte is 88.
SSQ (Sub-Sequence Number) — This one-byte field specifies the
sub-sequence number which must be incremented by one for each packet
transmitted. This allows duplicate message detection at this level.
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.
Command Header Fields
Figure 12.1 shows the station-management command packet format
including the command header fields. The command header fields are
common to all station-management commands. Protocol error replies do
not contain header fields. The other error replies contain the command
header fields except for the password.
Here is an example showing the format of command header fields:
All values we show in the fields are hexadecimal unless otherwise stated.
Notice that the message format shown contains several size (SIZ) fields.
These SIZ fields precede fields that can vary in length. Use each SIZ
field to specify the number of bytes the field following the SIZ field will
contain. For example, the SIZ field preceding the PWD field has a value
of 04 hex. This means the PWD field must contain four bytes. Also
notice the SIZ field that has a value of 00 hex followed by another SIZ
field. If you specify 00 hex in a SIZ field, the field that usually follows it
becomes non-existent. In this case the entity (ENT) field is non-existent
because the SIZ field preceding its position contains 00 hex.