Diamond: I/O: An open drain IO drives output high as a 'Z' and drives low as a '0'. This type of IOs is often used when multiple devices are connected to a bus.
On devices which do not have a open drain option you can emulate an open drain circuit using an output enable (OE) on the IO. Here is an example:
assign myoutput = (myenable) ? 1'b0 : 1'bz;