Null Modem

Information about Null Modem

A null modem adapter

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).

Null modem cables and adapters

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.

Null modem cable pin mapping

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:

Virtual null modem

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:

more transmission speed of serial data (limited by computer performance only)
unlimited distance because virtual connection over network or Internet is possible
unlimited number of virtual connections is possible
no serial cable is needed.
serial port of computer is free

DOSBox has allowed older DOS games to use virtual null modems.

External links

Data terminal equipment (DTE) is an end instrument that converts user information into signals for transmission or reconverts received signals into user information. A DTE device communicates with the data circuit-terminating equipment (DCE).
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computer is a machine which manipulates data according to a list of instructions.

Computers take numerous physical forms. The first devices that resemble modern computers date to the mid-20th century (around 1940 - 1941), although the computer concept and various machines
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A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system. A computer terminal is an instance of a human-machine interface(HMI).
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A computer printer, or more commonly a printer, produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper transparencies.
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In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports.
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serial cable is a cable that can be used to transfer information between two devices. Serial cables use the RS-232 standards for their connectors. The serial cable can be any combination of male or female in a DE-9 (often mistakenly called DB-9) or DB25 connector type.
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A Data circuit-terminating equipment (DCE) is a device that sits between the data terminal equipment (DTE) and a transmission circuit. It is also called data communications equipment and data carrier equipment.
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Modem (from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information.
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In telecommunications, transmission is the forwarding of signal traffic over distances that are too great to be simply connected by a twisted pair wires. Techniques available now may be microwave link, satellite link, coaxial cable or fibre optic cable.
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handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins.
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standardization or standardisation can have several meanings depending on its context. Common use of the word standard implies that it is a universally agreed-upon set of guidelines for interoperability.
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Microsoft Windows

Screenshot of Windows Vista Ultimate, the latest version of Microsoft Windows.
Company/developer: Microsoft Corporation
OS family: MS-DOS/9x-based, Windows CE, Windows NT
Source model: Closed source

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An operating system (OS) is the software that manages the sharing of the resources of a computer. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the
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Direct Cable Connection (DCC), is a feature of Microsoft Windows that allows a computer to transfer and share files (or connected printers) with another computer, via a connection using either the serial port, parallel port or the infrared port of each computer.
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MS-DOS (short for Microsoft Disk Operating System) is an operating system commercialized by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the dominant operating system for the PC compatible
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Hard disk drive

An IBM hard disk drive with the metal cover removed. The platters are highly reflective.
Date Invented: September 13 1956
Invented By: An IBM team led by Reynold Johnson
Connects to:
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Hardware is a general term that refers to the physical artifacts of a technology.It may also mean the physical components of a computer system.

Hardware historically meant the metal parts and fittings that were used to make wooden products stronger, more functional, longer
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Network Card

A 1990s Ethernet network interface controller card which connects to the motherboard via the now-obsolete ISA bus.

Connects to:

Motherboard via one of

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Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media
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kernel is the central component of most computer operating systems (OS). Its responsibilities include managing the system's resources (the communication between hardware and software components).
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FIFO is an acronym for First In, First Out. This expression describes the principle of a queue or first-come, first-served (FCFS) behaviour: what comes in first is handled first, what comes in next waits until the first is finished, etc.
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An interrupt handler, also known as an interrupt service routine (ISR), is a callback subroutine in an operating system or device driver whose execution is triggered by the reception of an interrupt.
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DDB can mean:

DDB Worldwide, an advertising agency
ddb (software), a BSD interactive kernel debugger.
DDB Design Development & Building, an Australian domestic building company
Dortmund Data Bank, a factual data bank for thermophysical and thermodynamic data

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KGDB is a debugger for the Linux kernel. It requires two machines that are connected via a serial connection. The serial connection may either be an RS-232 interface using a null modem cable, or via the UDP/IP networking protocol (KGDB over Ethernet, KGDBoE).
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Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the UNIX derivative distributed by the University of California, Berkeley, starting in the 1970s.
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X Window System (commonly X11 or X) is a display protocol which provides windowing on bitmap displays. It provides the standard toolkit and protocol to build graphical user interfaces (GUIs) on Unix-like operating systems and OpenVMS, and has been ported to all other
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Adapters in computing are hardware or software devices that convert transited data from one presentation to another. The data can be, for example, a message between objects in an application process, or a transition sent through a network.
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Application software is a subclass of computer software that employs the capabilities of a computer directly and thoroughly to a task that the user wishes to perform. This should be contrasted with system software which is involved in integrating a computer's various capabilities,
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