tri-state buffer with Xilinx ECS

[Mai]

i am using Xilinx ECS to implement an i2c interface.
i ve got an input sda_i and an output sda_o .
it s working well but i now would like to use an input/output SDA. I tried
to use the symbol "iobuf" with:

IO=SDA with a pull-up
T=sda_o with an inverser
I=pull down
O=sda_i

Simulation shows that '0' is ok, '1' goes like 'X' ...

thanks for help