RLE Protocol Converter V.2.4 - Manual (Page 54)

54 Protocol Converter User Guide 800.518.1519
Modbus Communications
4.1.1.2 Function Field
The function field is one byte in length and tells the Protocol Converter which function to
perform. The supported functions are 03 (Read 4xxxx output registers).
4.1.1.3 Data Field
The data field of the request is a variable length depending on the function. The data fields for
the Protocol Converter are 16-bit registers, transmitted high order byte first (big-endian)
4.1.1.4 Error Check (Checksum) Field
The checksum field lets the receiving device determine if the packet has transmission errors.
The Protocol Converter RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
4.1.2 Exception Responses
If a Modbus master sends an invalid command to the Protocol Converter 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.
4.2. Packet Communications for the Protocol
Converter
This section covers the registers with the name and a brief description of each.
4.2.1 Function 03: Read Output Registers
To read the Protocol Converter 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).
Table 4.1 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