Installation guide

184 Modbus Plus Transaction Elements
890 USE 100 00
A .3 The Message Format  MAC Level
At the Medium Access Control (MAC) level, the network protocol
defines the message destination and source nodes, and controls the
passing of tokens.
Figure 71 illustrates the MAC level format of a message containing a
Modbus command. The Modbus command is imbedded in the LLC field
of the frame.
HDLC LEVEL:
PREAMBLE
AA
OPENING
FLAG
7E
BDCST
ADDRESS
FF
MAC / LLC FIELD
FCS
CRC16
CLOSING
FLAG
7E
LENGTH: 1 1 1 12
MAC LEVEL:
DEST
ADDRESS
SOURCE
ADDRESS
MAC
FUNCTION
LLC FIELD (INCLUDES MODBUS COMMAND)
LENGTH: 1 1 1 2
BYTE
COUNT
Figure 71 MAC Level Message Format
A .3.1 MAC Fields
The message contains the following MAC level fields:
Destination Address
The address of the node intended to receive the message, in the range
of 1 to 64. Additional information identifying the message transaction
in the node’s application is contained in the LLC level packet.