A virtual null modem is a PC software driver which emulates two serial COM ports that are connected by a null modem cable. If one PC application sends data on one virtual COM port, a second PC application can receive this data on the second virtual COM port and vice versa.
Virtual COM connections do not give you the same timing as real RS232 connections and usually do not emulate the actual bit-by-bit transmission using a predefined baud rate. Any data packet sent on the first COM port will appear in the second COM port's receive buffer almost immediately. For most debugging and simulation purposes this limitation can be easily tolerated. Some virtual null modem drivers offer an additional baud rate emulation mode, where the data transfer is delayed to emulate a real RS232 connection and its limited transmission rate.
For an Open Source Windows software that has been successfully tested with Docklight, see