RLE LD5200 V.2.3 - Manual (Page 87)

rletech.com LD5200 User Guide 87
5 Modbus Communication
5.3.1.4 Error Check (Checksum) Field
The checksum field lets the receiving device determine if the packet has transmission errors.
The LD5200 RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
Exception Responses
If a Modbus master sends an invalid command to the LD5200 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.
5.4 Packet Communications For The LD5200
This section covers the registers, their names, and a brief description of what they refer to.
5.4.1 Function 03: Read Output Registers
To read the LD5200 parameter values, the master must send a Read Output Registers request
packet. The Read Output Registers request packet specifies a start register and the number of
registers to read. The start register is numbered from zero (40001 = zero, 40002 = one, etc).
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
Table 5.1 Exception Codes
Read Registers Request Packet Read Registers Response Packet
Slave Address (1 byte) Slave Address (1 byte)
03 (Function code) (1 byte) 03 (Function code) (1 byte)
Start Register (2 bytes) Byte count (1 byte)
# of registers to read (2 bytes) First register (2 bytes)
Crc Checksum (2 bytes) Second register (2 bytes)
…
Crc Checksum (2 bytes)
Table 5.2 Read Output Registers Packet Structure