[VFDs from Unitronics! One Solution for Control and Automation]

Hi MIke,
After we double checked it seems you are right.

You need to connect a device on the Y1 or Relay output. Use Modbus 200BH, the corresponding signal can be output according to the setting value of Modbus. When it is set to 1, the output is ON, and when it is set to 0, the output is OFF. In this case, it's a way to send a virtual signal to Y1 or Relay output.

[VFDs from Unitronics! One Solution for Control and Automation]

Hi Mike,
the VFDs outputs can't be defined as general purpose outputs hans cannot be set using communication.  

[VFDs from Unitronics! One Solution for Control and Automation]

Hi Mike,
Since our VFDs are communicating using MODBUS RTU the limitation is 31 (If you need more then that you will have to add another modbus port and implement the communication manually) 

I\Os (digital and analog) can be red using communication so it seems that your paln should work.
Please note that since it uses a serial communication it is recommanded using the "read parameter" element instead of reading all values from every VFD and so improve the response time