T6615 or similar CO2 sensor

Hi,
does someone has experience with the T6615 or some other member of the 
T6000-family?

I'm not sure how to connect the serial port of this device. It has a pin 
named "Rx" and one "Tx". How to connect with the PC's serial port?

Which one of the two following examples is right?

1

sensor ---- D-Sub9-Pin - PC

2

----------------------------

3

1)  Rx ----      3     - Tx

4

    Tx ----      2     - Rx

5

   GND ----      5     - GND

6

7

2)  Rx ----      2     - Rx

8

    Tx ----      3     - Tx

9

   GND ----      5     - GND

Unfortunately the datasheet says nothing about it.

Thanks for helping

-richard