I use Serial Port Tester, very good COM port testing software. It provide great functionality:
- Collect and record data;
- Monitor multiple ports at a time;
- View data in a convenient way;
- Simulate sending data to serial ports;
- Completely compatible with Modbus RTU and Modbus ASCII protocols;
- Playback and session comparison.
Here you can find detailed description and to try trial version - com port tester.