RIM-Protocol Converter User Guide - Version 1.0
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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.
Table 4.1 Exception Codes
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).
Code
Name
Description
01
Illegal Function
The function code is not supported
02
Illegal Data Address
Attempt to access as invalid address
03
Illegal Data Value
Attempt to set a variable to an invalid value
