Digital (serial) data has a number of attributes including voltage, polarity, data rate. The s300 v2 & v3 have two digital inputs to cope with the two common voltage and polarities.
The S300 normal digital input (pin 'I') accepts 5V idle / 0V active voltages with a variable data rate. This is often referred to as TTL level serial.
A computer serial port uses the RS-232 standard, which uses -12V idle / +12V active voltages. Many serial devices output a non-standard serial which is TTL level voltages but with RS-232 polarity (0V idle / 5V active). The S300 inverted digital input ((pin 'N') accepts these 0V idle / 5V active polarity inverted signals.
Note that only one of the 'I' and 'N' digital input pins can be used at a time.
PLX iMFD devices output 3.3V / 0V (uses normal polarity pin 'I').
Serial output from two PLX devices in daisy chain.
Innovate LC-1 and AEM 30-2310 output 0V / 5V (uses inverse polarity pin 'N').
Serial from the Innovate LC-1.