Here i present my realisation of a PC link interface for Yaesu FT-100. This interface alows you to transmit and receive audio signals from a sound card in order to operate HAM digital modes (FT8, PSK31 etc.).
The PTT command is based on the work of WA2NKF (http://k0lee.com/ft100/pskft100.htm) but with an additionnal protection from OZ6YM (http://www.planker.dk) on audio channels. This protection is based on a 600 ohm 1:1 transformer and avoids the damage of the rig from a DC voltage present on the output of the soundcard.
The full diagram is shown on the fig.1 and the corresponding circuit on the fig.2 :
fig. 1 
fig. 2
The NPN transistor can be a 2n2222 or a similar one, il my case i used a BCW72. The sound level on a soundcard lines is adjusted with a 1 kOhm trimmer. In order to trig the PTT a DTR (pin 4) or a RTS (pin 7) signal can be used. In my case i use a DTR
The circut was designed on KiCAD and milled on a CNC machine.
Some photos of the assembled device:
Configuration of WSJT-X and FLDigi:
FLDigi 
WSJT-X