Allen-Bradley 1779-KFMR User Manual page 156

Messages are initiated at the application layer and are placed into packets
in the network layer. For outgoing messages, the packet from the network
layer is passed down to the HDLC data-link layer and is sent out as the
data field of the data frame. For incoming messages, the data field from
the data frame is passed up from the HDLC data-link layer as a packet to
the network layer. In Chapter 6, we described the fields of the frame at
the HDLC data-link layer. Here, we describe the fields of the packet
passed up from and down to the HDLC data-link layer. At each layer
below the application layer of the communication software, the data field
is passed up to the next higher layer where additional fields are defined.
Native Fields
Figure 10.1 shows the format of the RC control message packet including
the Native fields. The Native fields are those fields common to all
message transmissions in the Native mode. Here we describe these
Native fields:
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 RC control
command messages, the hexadecimal value in this byte must be 05. For
RC control reply messages, the hexadecimal value in this byte is 85.
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.
Chapter 10