How do you make the connnection from the Pi to the Classic ?
I'm not David but;
Unless they changed it, newmodbus used ethernet.
You can read the raw data from the serial (USB) port once you tell The Classic to push the data out through it, as well.
Ethernet is much more elegant, however. And you don't need a USB cable to your device reading the registers, just the existing ethernet connection most have already.
Its been awhile but my feeble memory tells me that is how it worked.