User manual

34 NIC Setup and Testing
Exception Response
Server
Address
Function Code
(0x80)
Exception
code
16 bit CRC
1 byte 1 byte 1 bytes 2 bytes
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. These bits indicate the user
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.