Installation guide

186 Modbus Plus Transaction Elements
890 USE 100 00
A .4 The Message Format  LLC Level
At the Logical Link Control (LLC) level, the message contains the data
field to be transferred, such as the Modbus command. It also contains
additional routing and message control fields.
Figure 72 illustrates the LLC level format of a message containing a
Modbus command.
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
LENGTH: 1 1 1 2
LLC LEVEL:
MASTER
OUTPUT
PATH
ROUTER
COUNTER
TRANS
SEQUENCE
NUMBER
ROUTING PATH
LENGTH: 1 1 1
BYTE
COUNT
5
MODBUS FRAME (MODIFIED)
Figure 72 LLC Level Message Format