Modbus tips and troubleshooting

Most of the difficulties with Modbus communications arise from deviations from the standards, which are not enforced within Modbus. Whether you are connecting via Modbus to a solar inverter, power meter, or flow meter, the information provided here can help you overcome the challenges, and successfully gather data into a Campbell data logger. Further information on Modbus can be found at:

• www.simplyModbus.ca/FAQ.htm
• www.Modbus.org/tech.php
• www.lammertbies.nl/comm/info/modbus.html

Error codes

Modbus defines several error codes, which are reported back to a client from a server. ModbusClient() displays these codes as a negative number. A positive result code indicates no response was received.

Result code -01: illegal function

The illegal function error is reported back by a Modbus server when either it does not support the function at all, or does not support that function code on the requested registers. Different devices support different functions (consult the device documentation). If the function code is supported, make sure you are not trying to write to a register labeled as read-only. It is common for devices to have holding registers where read-only and read/write registers are mapped next to each other.

An uncommon cause for the -01 result is a device with an incomplete implementation of Modbus. Some devices do not fully implement parsing Modbus commands. Instead, they are hardcoded to respond to certain Modbus messages. The result is that the device will report an error when you try selectively polling registers. Try requesting all of the registers together.

Result code -02: illegal data address

The illegal data address error occurs if the server rejects the combination of starting register and length used. One possibility, is a mistake in your program on the starting register number. Refer to the earlier section about register number and consult the device documentation for support information. Also, too long of a length can trigger this error. The ModbusClient() instruction uses length as the number of values to poll. With 32-bit data types, it requests twice as many registers as the length.

An uncommon cause for the -02 result is a device with an incomplete implementation of Modbus. Some devices do not fully implement parsing Modbus commands. Instead, they are hard coded to respond to certain Modbus messages. The result is that the device will report an error when you try selectively polling registers. Try requesting all of the registers together.

Result code -11: COM port error

Result code -11 occurs when the data logger is unable to open the COM port specified. For serial connections, this error may indicate an invalid COM port number. For Modbus TCP, it indicates a failed socket connection.

If you have a failed socket connection for Modbus TCP, check your TCPOpen() instruction. The socket returned from TCPOpen() should be a number less than 99. Provided the data logger has a working network connection, further troubleshooting can be done with a computer running Modbus software. Connect the computer to the same network and attempt to open a Modbus TCP connection to the problem server device. Once you resolve the connection between the computer and the server device, the connection from the data logger should work.