There is an in-depth article covering several Samsung devices in the SamsungSerial page
|Device||I/O voltage||ID<->GND||SOC TX||SOC RX||switch||Comments|
|I9300||1.8V1||619 kOhms3||D-||D+||It gives a shell once Replicant has booted|
|Nexus S||150 kOhm||FSA9480|
|Galaxy Nexus||619 kOhms4|
1 Measured between USB GND and USB VCC
2 Code is required to use the switch as you need to configure both the MUIC and DP3T switch
There is a page on Serial debugging on the postmarketOS wiki that contains information for using the serial port for many smartphones models.