Okay seeing this cable is no longer supplied, lets take a look at what it does.

The FTDI chip uses a driver on the PC/Laptop that makes it show as a virtual 
Serial Com Port but it plugs into the USB port.
You can either use it by sending serial data down the Virtual Com Port or send 
commands directly to it.

Most programs these days are sending data directly to it such as ECUFLASH, 
EVOSCAN and Mitsulogger which will soon be called UniLogger.

My test program does not do this and manages to get the same result sending 
Data straight down the virtual Com port.

Okay lets get down to it.

Most of the circuit is easy enough, bung a few caps and resistors in place 
as per the spec sheets to allow the FTDI chip to run.

The bits we are interested in are the Control lines and looking at the 
diagram they are:

TXD
RXD
RTS
DTR
DCD
Hven
VppEN

Okay so lets look closer.

For the basics the TXD data line simply connects to the K-Line of the 
ECU and sends request for information. This goes via the 74HC257D switch.

The RXD data line receives answers to the requests on the K-Line of the 
ECU and this goes via the 74HC257D switch.

The RTS line gets either taken Hi or Low to switch the 74HC257D switch, 
this puts the data out on either the K-Line or the L-line.

The DTR line sends information to the L-Line again going through the 74HC257D switch.

The DSR lines receives data back on the L-line going through the 74HC257D switch.

The Hven line switches 16Volts onto the Flash Read/Write line when using ECUFLASH.

The VppEN line simply turns on power.

Running out of time tonight, but will list the exact (Or as close as I can get to 
explain what I see and how the circuit works.) over the next couple of days.