On the Mach3 side, you basically have to do two things. First, you have to go into Function Configs and go into the setup Modbus tcp. What I did was set one config for each address. If you have more than one address that are next to each other, you can combine them, but I didn't, it made it easier to remember what I was using in my brain. I then set up a brain to do just what you are trying to do. It was to start and stop the motors, change direction, and set the speed. Here is the code in my brain. Hope this helps.