how can determine type connection ?

hi
I have a device that has a communication port but I do not know which type of connection.
I've measured the voltages on pins , that The following values were obtained :

in your opinion , this is which connection??how can determine type connection ?
in The manufacturer's catalog Been picture following :

Also come in the catalog the following descriptions :

Looks like it is an one-way opto-isolated RS232 Serial Input to the PC. It eliminates the need for a common ground between the AquaMaster chassis and the PC chassis. Two of the pins are the input from the AquaMaster; the other two pins are the output to the PC, with galvanic isolation between the two...

thanks dear MikeMl you say :

MikeMl said:

Looks like it is an one-way opto-isolated RS232 Serial Input to the PC.

Click to expand...

why one-way ? i send tab keyboard to Aquamaster and it respond to me for example in putty software . do this is bidirectional ?

MikeMl said:

Two of the pins are the input from the AquaMaster; the other two pins are the output to the PC

Click to expand...

do this means that my GND not true ?

for more detail
I get below voltage in run time on each pin :
top and bottom pin are nearly 3.5 volt and Remain at the same level when pc and device exchange data together .
left pin become nearly -4.92 volt in connect time and change when pc and device exchange data together (for example become -0.8 or -2.3 or -3.2 and then return to -4.92 when pc and device do not exchange data together )
and voltages after Manufacturer Transducer are following :
voltage on rxd pin is -4.9 volt in connect time and Remain at the same level when pc and device exchange data together .
voltage on txd pin is -2.9 volt in connect time and change when pc and device exchange data together (for example become 2.4 or 1.8 or -0.4 and then return to -2.9 when pc and device do not exchange data together )
i do not understand What do Transducer ؟؟

Either is is opto-isolated and only one way, or it is not opto-isolated and it could be two-way. It doesn't have enough pins to be both opto-isolated and two-way...

in your opinion can use below circuit for this work ??
**broken link removed**

Would work, or you can buy ready made (I would).