Press and hold the DCOMM button until the DCOMM LED on the controller lights up steadily.
This will place the Channel 0 serial port (the round one) into the default settings: 19200 baud, 8 data bits, no parity, 1 stop bit, CRC error checking.
These are not the default settings for a new DF1 Full Duplex driver in RSLinx Classic. You must change the Device Type to "SLC Ch.0" and change the Error Checking to CRC, or use the Autoconfigure feature.
The DCOMM feature does not affect the Channel 1 serial port on 1764-LRP controllers.