Mach3 will read serial data from ModBus output.
You have to check the box in the first page of; Config/ Ports and Pins/Engine Configuration ...Ports & Pins.
Lower right corner box:
check; ModBus InputOutput Support,
check; ModBus Plugin Supported.
Then you must enable the correct port in:
Config/ General Config; Lower left box "Serial Output, check the serial port#, input BaudRate in the box, select 8-bit 1stop.
Now open "Function Cfg's" on Menu; scroll down to "Setup Serial ModBus Control", and click open the page, "ModBus Configuration".
Fill in the boxes at the top of the page; Port Num:, Baud Rate,
Each ModBus input signal will be listed in one of the Cfg # rows. You can write in a brief description in the "Device" column.
When these are filled in, you can test your I/O with the "Test ModBus" box. In the case of an Analog input, you will see the values change as you turn the potentiometer.
Read the example in the ModBus section of the forum, written by Zafar. He has a complete step by step with pictures of setting up an Analog feedrate including the Brains. You need to write a brain, to input your ModBus analog input into an OEM code input.
After a bit of trial and error, you will get it working. Also have a look at some of the Mach3 videos regarding Modbus.
John