Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using a RS-232 serial cable. The original RS-232 standard only defined the connection of DTEs with DCEs i.e. modems. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by a standard.
Null modems are commonly used for file transfer between computers, or remote operation. Under the Microsoft Windows operating system, the direct cable connection can be used over a null modem connection. The later versions of MS-DOS were shipped with the InterLnk program. Both pieces of software allow the mapping of a hard disk on one computer as a network drive on the other computer. No Ethernet hardware (such as a network interface card or a modem) is required for this.
The popularity and availability of faster information exchange systems such as Ethernet made the use of null-modem cables less common. Nowadays, such a cable can still be useful to kernel hackers though, since it allows the user to remotely debug a kernel with a minimum of device drivers and code (a serial driver mainly consists of two FIFO buffers and an interrupt service routine). ddb or KGDB can be used to remotely debug BSD systems, for instance. This can also provide a serial console through which the in-kernel debugger can be dropped to in case of kernel panics, in which case the local monitor and keyboard may not be usable anymore (the X11 server reserves those resources if a GUI is used, and dropping to the debugger in the case of a panic won't free them).
A null modem cable is an 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.
This is one very common mapping which will work with software that relies on proper assertion of the CD signal.
|Signal Name||DB-25 Pin||DE-9 Pin||DE-9 Pin||DB-25 Pin|
|FG (Frame Ground)||1||-||X||-||1||FG|
|TD (Transmit Data)||2||3||-||2||3||RD|
|RD (Receive Data)||3||2||-||3||2||TD|
|RTS (Request To Send)||4||7||-||8||5||CTS|
|CTS (Clear To Send)||5||8||-||7||4||RTS|
|SG (Signal Ground)||7||5||-||5||7||SG|
|DSR (Data Set Ready)||6||6||-||4||20||DTR|
|CD (Carrier Detect)||8||1||-||4||20||DTR|
|DTR (Data Terminal Ready)||20||4||-||1||8||CD|
|DTR (Data Terminal Ready)||20||4||-||6||6||DSR|
Here is another pin connection diagram for a null modem:
Connecting two DTE devices together requires a null modem that acts as a DCE between the devices by swapping the corresponding signals (TD-RD, DTR-DSR, and RTS-CTS). This can be done with a separate device and two cables, or using a cable wired to do this. If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the Yost standard is that a null modem cable is a "rollover cable" that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.
This cable could be used with more software but it had no actual enhancements over its predecessor. The software would work thinking it had hardware flow control but could suddenly stop when higher speeds were reached and with no identifiable reason.
Virtual null modem is also communication method to connect two computer applications directly using virtual serial port. Unlike null modem cable, virtual null modem is a software solution which emulates hardware null modem in computer. All features of hardware null modem are available in virtual null modem as well. There are some advantages to this:
Another common example consist 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.