Cables
5. In-the-middle connection
With this setup your PC will be in between the A (DTE) and B (DCE).
The PC will repeat whatever it receives on one serial port to the other serial port. PC A will be connected to A (DTE) and will require a crossed serial cable, since your PC is also DTE.
Cable with two serial cables, one to A (DTE) and another to B (DCE).