I am using an Arduino to send Serial data to Mach3, via serial non plugin, in the Modbus application. I understand how to change what is being sent to mach3 but I do not understand what is being sent back to the Arduino when I set up outputs in serial modbus control. I am using the arduino to send varying discrete inputs to mach3 and set OEM buttons to high or low in mach3 brains. I have a screenshot of my brain file attached as well as how I currently have Modbus setup and how arduino sends data to the slave address and bit selected. I just want to read data back into the arduino and interpret if an output is active or inactive. I just don't know what changes via serial communication. I am using a serial monitor to read the writing and responses on the shared COM port between arduino and mach3 but I do not see a difference when an output is active in my brain setup. I am very new to CNC control as well as Mach3. Any help would be appreciated.