Virtual Serial Ports Driver (VSPD) is a Windows device driver. Using it you can create two virtual serial COM ports in your system which will be virtually connected to each other. For example, after installing VSPD, you will have Virtual Serial Port COM5 and Virtual Serial Port COM6 added to your system. These ports will behave like real common serial ports to other Windows applications. These ports will not interact with any hardware. The idea is: all data sent by other application to COM5 will arrive at COM6, and all data sent by other applications to COM6 will arrive at COM5. So, the application, that listening COM5 will get all data, sent by other application to COM6 and vice versa.
