NULL MODEM DEFINED…
Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using an RS-232 serial cable. The name stems from the historical use of the RS-232 cable to connect two teleprinter devices to modems in order to communicate with one another; null modem communication was possible by instead using RS-232 to connect the teleprinters directly to one another.
NULL MODEM CABLE DEFINED…
A null modem cable is a RS-232 serial cable where the transmit and receive lines are crosslinked. In some cables there are also handshake lines crosslinked. In many situations a straight through serial cable is used, together with a null modem adapter. The adapter contains the necessary crosslinks between the signals.
VIRTUAL NULL MODEM DEFINED…
A virtual null modem is a communication method to connect two computer applications directly using a virtual serial port. Unlike a null modem cable, a virtual null modem is a software solution which emulates a hardware null modem within the computer. All features of a hardware null modem are available in a virtual null modem as well. There are some advantages to this:
Higher transmission speed of serial data (limited by computer performance only).
Virtual connection over network or Internet is possible, mitigating cable length restrictions.
An unlimited number of virtual connections is possible.
No serial cable is needed.
The computer’s physical serial ports remain free.
For instance, DOSBox has allowed older DOS games to use virtual null modems.
Another common example consists of unix pseudo terminals (pty) which present a standard tty interface to user applications, including virtual serial controls. Two such ptys may easily be linked together by an application to form a virtual null modem communication path.
You fill find that there are still rs232 printers alive and active in the world and sometimes you will need to
1. network an rs232 printer
2. share an rs232 printer
3. create a virtual connection with an rs232 printer across a network
4. create a virtual connection with an rs232 printer and an application
5. converting a parallel printer to rs232 to gain virtual null modem functionality (bidirectional rs232/parallel cable link)