How can I simulate open drain outputs using IBIS models?

The general purpose I/O IBIS models provided by Altera can be modified to simulate open drain behavior. Before making modifications, you should save a copy of the IBIS file for the device or design that you are using. Follow the instructions below:

1. Locate the model or models you wish to modify in the .ibs file.
2. Change the Model_type from "I/O" to "I/O_open_drain" under the [Model] keyword section.
3. Delete the [Pullup] keyword and all of the data in the [Pullup] section.
4. Remove the rising falling waveform data for V_fixture = 0
5. Save the file.