MODBUS TCP/IP PROTOCOL MANUAL - Maguire Products

Note: One potential source of confusion is the relationship between the reference numbers usedin MODBUS functions, and the ‘register numbers’ used in Modicon PLC’s. For historical reasons,user reference numbers were expressed as decimal numbers with a starting offset of 1. HoweverMODBUS uses the more natural software interpretation of an unsigned integer index starting atzero.So a modbus message requesting the read of a register at offset 0 would return the value knownto the application programmer as found in register 4:00001 (memory type 4 = output register,reference 00001)The checksum specified in the MLAN protocol is NOT used in the Modbus TCP/IPcommunication. Error checking is taken care of automatically by the Modbus TCP/IP software aseach Ethernet frame contains it's own CRC.When the blender controller has executed the command, the Modbus function code is echoed inthe reply Modbus TCP/IP data packet. If an error is encountered the top bit of the reply functioncode will be set and will be followed by a single byte indicating the reason for the error.Response Format with Error(Write/Read)InputByte Modbus Description Name Range Description0 Transaction ID Usually 01 Transaction ID Usually 02 Protocol ID Usually 03 Protocol ID Usually 04 Total Data Length 05 Total Data Length 36 Slave Address MLAN Address7 Function Code131, 132, 143or 1448 Error Code Reason for failureError Code ExplanationsThe Modbus TCP/IP Error Code byte can take the following values:07 Communication error (NAK negative-acknowledge).01 This means that the command is not supported.06 Communication Error (Time-out).03 Invalid parameter passed in the command.11