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Chapter Objectives
Selecting PAD
Packet Format for Native Mode
PAD Message Set
In this chapter, you will read about the PAD message set. This chapter
includes a description of each field of each protocol layer above the
HDLC data-link layer for PAD messages.
PAD messages let devices communicate over the Data Highway II
network in a packet assembler/disassembler message format. You can
send and receive PAD messages between your host device and the
1779-KFM interface.
With PAD message format, a device with only enough intelligence to
provide the HDLC data-link layer protocol can communicate with other
stations in the Data Highway through the 1779-KFM interface. PAD
format is also useful for devices, such as robots, which have a fixed
message format. In sending a message to a robot, a computer could
encode the message data field with the format that the robot expects. At
the receiving node, the interface disassembles the packet and sends only
the data field to the robot. Since the computer encodes the data field, the
robot sees the format it expects.
To enable the interface to send and receive PAD messages, you must set
the communication mode switches on the host board to select either the
Native or PAD mode. (Refer to Chapter 4 for switch-setting procedures.)
Figure 11.1 shows a Native-mode data frame containing a PAD packet in
three layers:
The message field at the application layer.
The message header fields common to all PAD 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
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Troubleshooting
