User Manual

September 2013 25 © Intek, Inc. 2013
Revision D
Basic Function Codes - The Modbus packet structure is determined by the function code being
performed. Some of the basic commands that CMS supports are shown in Table 4.
Table 4: Modbus function codes
Function
Hex Code
Read Holding Registers
03
Read Input Registers
04
Read Exception Status
07
Write Multiple Registers
10
Write Holding Register
06
Exception Packet Definition - If an error is detected in one of the following packet definitions a
certain error code is applied and sent back in an exception response.
Table 5: Modbus error codes
Code
Description
01
Invalid function code – function code not supported by device
02
Invalid data address – address defined by the start address and
number of registers is out of valid range
03
Invalid data value – number of registers = 0 or > 125
Table 6: Modbus exception response
Type
Byte Length
Example
Device address
1 Byte (1 – 247)
0x01
Function code
1 Byte (Function code + 0x80)
0x83
Exception code
1 Byte
0x01