Simply, it’s an Arduino library for communicating with Modbus slaves over RS232/485 via RTU (remote terminal unit) protocol. In conclusion, the Modbus 2-channel 5V relay module is a very useful device ...

This Arduino Uno setup can be powered by a 6F22 9V battery or ... A few more significant details about this Chinese “2 Relay Module” are worth noting, especially if you are using it for the first time ...