Modbus troubleshooting Part II

            If a different manufacturer’s hardware is placed on a Modbus Link and a problem occurs, it is probably in the Modbus master interface software, and therefore reading the device manual is a must. There might be problems with a memory offset the original addressing in the Modbus message started at zero, while the memory location used in the PLC was started at one, e.g. coil 1 has a message memory location of 0 and holding register address of zero in the Modbus message has a memory location of 40001. This offset is retained in many of the modern Modbus interfaces and devices. 

       Also, Modbus uses big-endian data flow for addresses and data (the most significant data byte sent first, lower significant byte second) but some Modbus interfaces for vendor-specific equipment may use little-endian addresses (least significant data byte sent first, most significant byte second). With the help of the programs which allows to see and send messages to a slave allows addressing, function codes, data sent, and what error codes are being returned from what slave/server. Modbus 07 and 08 function codes and sub-codes can be used to get additional diagnostic information on serial links.