Trying to pull down 1.2V Dataline to GROUND, Does not fully drop to 0V
If both are outputs the answer is: you can't. 

If there are more than one devices, which can control the same line it has to be open collector or open drain. 

If you connect two active outputs it is very likely to damage the pin. You drive the line low, the device high. Your resistor is 8 Ohm. The input and output resistance of the pins are usually 1-5 Ohm. Let take 3 Ohm. The current will be 1.2V / 14 Ohm = 85mA (much too much for the pin).