I don't think modbus is working with emc2.
Here is a bunch of supported hardware
they keep adding hardware as drivers are made. With it being open source - you could write drivers also.
I have the mesa board but have not played with it yet. I also have the pluto-p board but have only done a bit of testing with it.
emc2 has tons of flexablillity - can do servos and stepper. Also has classic ladder - ladder logic integrated into it.
this is our project
Here are some other retrofits