This port is a green port located externally on all Solis inverters on the bottom of the inverter.  
Pin 1 = +5 volts DC
pin 2 = -5 volts DC 
Pin 3= +RS485
Pin 4 = -RS485
Checking the voltage on pins 1 and 2 with a multimeter will ensure power is provided to the communication device (ex: wifi datalogger. The voltage reading should be 5V on DC.
 
Checking the voltage on pins 3 and 4 with a multimeter will ensure RS485 signal between the inverter and the communication device. RS-485 voltage should read ~1.7Volts on DC