Modbus can be very frustrating but if you keep going you tend to get there in the end. From a S7-1200 I have managed Modbus TCP talking to a Linux embedded computer that converts it to Modbus RTU to read data from an ABB ACS355 VSD and a Mains Supply multimeter. Be prepared for some diagnostics, especially over TCP. Wireshark means you can look at the data going to and fro and see what the S7-1200 is sending out and if the remote device is responding. You probably won't get it working in an afternoon, so give yourself some extra fiddling time. Make sure you have an up to date firmware on the PLC and at least V14 TIA Portal.