Installation guide

182 Modbus Plus Transaction Elements

890 USE 100 00

A .2 The Message Format  HDLC Level

Messages appearing on the network contain three levels of protocol to

handle the processes of synchronization, routing, transferring data, and

checking for errors. The message format satisfies the network HDLC,

MAC, and LLC level protocols.

Figure 70 illustrates the Highlevel Data Link Control (HDLC) level

format of a typical message transmitted from a networked controller.

The format at the other levels is shown on the following pages.

HDLC LEVEL:

PREAMBLE

OPENING

FLAG

BDCST

ADDRESS

MAC / LLC FIELD

FCS

CRC16

CLOSING

FLAG

LENGTH: 1 1 1 12

Figure 70 Typical Message Format

A .2.1 HDLC Fields

At the HDLC level, the network protocol defines the beginning and end

of the message frame, and appends a frame check sequence for error

checking. The message contains the following HDLC level fields:

Preamble

One byte, 0xAA (hexadecimal AA, or alternating ONES and ZEROS).

Opening Flag

One byte, 0x7E (one ZERO, six ONES, one ZERO).

Broadcast Address

One byte, 0xFF (eight ONES). These contents specify that all nodes

are to receive the frame. Each node will then parse the frame’s MAC

level contents to recognize its address as the intended destination.