Only pin 2,3 and 5 are used on the T100MD+ PLC. T100MX+ PLC uses pin 4 to get power from the host for its opto-isolated serial interface so pin 4 is also required.
For T100md+, all other pins are not connected to power or ground. Pin 1,4,6 are shorted on board and pin 7 and 8 are shorted on board. These are to provide loop backs for the RTS/CTS pair and DTR/DSR if you use a 9 wire conductor. These signals are not used by the PLC but some host computer/modem etc may want to see these signal being looped back in order to allow communication to take place.
