On serial you can write to multiple devices on a 485 network using slave device IDs
On Ether you can do the same, if al the devices have ether and can use your mach comp as the gateway, again you will need to be able to use device numbering.
If you want to use Modbus on a USB physical medium, you would have to get a converter that will convert serial to USB, (NOTE: sometimes it is a crap shoot to find converters that will not screw up the data, often you have to slow down your speed because of the conversion overhead).
You can also Daisy chain, i.e. you can go out to a "Master PLC or device" and then that device controls and talks to all your other devices, thus Mach only sees the Master device. I did this with 2 PLC's, I did Ethernet to my 1st PLC, then from that PLC a 422 to a second PLC the master PLC did all the I/O transactions with the 2cd and then it passes all the I/O to Mach.
Scott