Modbus Packet Formats - Liebert OpenComms Installation & User Manual

Exception Response
Server Function Code
Address
1 byte
Exception code is one of the following:
01: Unsupported Function code
02: Out-of-range data addresses
03: Out-of-range data value (write to holding
registers)
04: There is currently no LMD device connected to
the server
05: Server is composing response but cannot
response to the client within the time limit required
by Modbus specification.

Modbus Packet Formats

Each Modbus packet consists of the following fields:
•
Device Address
•
Function Code
•
Data Field(s)
•
Error Check Field
Device Address:
The address field immediately follows the beginning of the
frame and consists of 8-bits.
assigned address of the slave device that is to receive the
message sent by the attached master device.
Each slave OpenComms NIC must be assigned a unique address
and only the addressed slave will respond to a query that
contains its address.
34
(0x80)
1 byte
Exception
code
1 bytes
These bits indicate the user
16 bit CRC
2 bytes
NIC Setup and Testing