RLE LD1500 V.3.1 - Manual (Page 38)

38 LD1500 User Guide 800.518.1519
5 Modbus Communication
5.1.1.2 Function Field
The function field is one byte in length and tells the LD1500 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 LD1500 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 LD1500 RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
5.1.2 Exception Responses
If a Modbus master sends an invalid command to the LD1500 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