In the last lesson we did some really awesome stuff. We simulated a simple Modbus network and you got to see a live Query / Response cycle. I hope that seeing it has helped to further solidify the Modbus concepts that you have been learning about all along.
At the end of the lesson, we saw a live example of a Modbus error code. In this lesson, we will dig deeper into Modbus error codes to give you even more tools to troubleshoot Modbus networks.
Modbus error codes (often called Modbus exception Codes) are codes returned by a Modbus slave in a response to a Query byte stream from the Modbus master. When we get an error code coming back in the response, then it means that even though the Modbus communication is working, something other issue is causing the incomplete exchange of data.
You don’t have to know all of the Modbus codes by heart but you have to know how to decipher and interpret them to solve Modbus network issues. All of the Modbus error codes can be found in the Modicon Modbus Protocol Reference Guide. It can be downloaded for free here.
For purposes of this lesson, I will list the first five error codes as they appear in the reference documentation. They are as follows: