Chapter
8
81
Data Highway Message Set
In this chapter, you will read about the Data Highway message set. This
chapter includes a description of the fields in each protocol layer above
the data-link layer. It also describes each Data Highway message and the
fields each message requires.
Data Highway messages include the messages available on the Data
Highway network (except for PLC-specific messages). The most
common use for these messages is to read from and write to the data
tables of programmable controllers (including uploading/downloading),
and to obtain diagnostic information from the interfaces connected to
programmable controllers.
To enable the interface to send and receive Data Highway messages
initiated by the host computer, you must set the communication-mode
switches on the host board to select the Native mode. (Refer to Chapter 4
for switch-setting procedures.)
Figure 8.1 shows a data frame containing a Data Highway packet in three
layers:
The message fields at the application layer.
The message header fields common to all Data Highway messages.
The fields common to all message transmissions in the Native mode.
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.
Chapter Objectives
Selecting Data Highway
Messages
Packet Format