Null Transactions

How can a driver be disconnected (i.e., not influencing the output at all)?
- Use the null waveform element

Example:

                       bus_out <= null after 17 ns;

What happens if all drivers of a resolved signal are disconnected?
- Use register kind in signal declaration to keep most recently determined value
- Use bus kind in signal declaration if resolution function must determine the value
- Example:
  - signal t: wired_bus bus;
  - signal u : bit register;