2 function field, 3 data field, 4 error check (checksum) field – RLE LD2000 User Manual
Page 52: 5 5-1.2 exception responses, Table 5.1, Exception codes
www.rletech.com
52
970.484.6510
5
Modbus Communication
5.1.1.2 Function
Field
The function field is one byte in length and tells the LD2000 which function to perform. The
supported functions are 03 (Read 4xxxx output registers), 04 (Read 3xxxx input registers), 06
(Preset single register) and 16 (Preset multiple registers).
5.1.1.3 Data
Field
The data field of the request is a variable length depending on the function. The data fields for
the LD2000 are 16-bit registers, transmitted high order byte first (big-endian).
5.1.1.4 Error Check (Checksum) Field
The checksum field lets the receiving device determine if the packet has transmission errors.
The LD2000 RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
5.1.1.5 5-1.2 Exception Responses
If a Modbus master sends an invalid command to the LD2000 or attempts to read an invalid
register, an exception response is generated. The response packet will have the high order bit
of the function code set to one. The data field of the exception response contains the exception
error code.
Table 5.1 Exception Codes
Code
Name
Description
01
Illegal Function
The function code is not supported
02
Illegal Data Address
Attempt to access an invalid address
03
Illegal Data Value
Attempt to set a variable to an invalid value