Null modem

As a null-modem cable is a cable called that connects two computers via the serial interface (RS -232 or EIA- 232) with each other. In contrast to a standard serial cable that connects a computer to a modem, here several lines must be crossed (eg, Receive Data and Transmit Data, pins 2 and 3 of the sockets ). In the field of the Ethernet network technology is called the equivalent cross cable.

The name Null Modem (English for no modem in the sense of modem omitted) stems from the fact that computer ( Data terminal equipment DTE) are often connected to long distance via modems with each other, which are traditionally connected via a serial interface to the computer. With a direct connection, in which the computers are only a few meters away from each other, the modems can be omitted and replaced with a null modem cable.

A null modem cable has both ends female (female) D -sub connector.

Use and history

About a null-modem connection, ie a direct serial connection, the two computers involved can exchange data, which was very popular especially at times of the beginnings of the personal computer in the 1980s and early 1990s. With two personal computers running the DOS operating system is running, is easy to do in this way, a data exchange. Was widespread, for example, the data transfer program Laplink. Also, two Amigas or Atari ST can be connected with a null modem cable. This technique was used in particular of computer games to allow multiplayer games.

With the increasing emergence of ( also in-house ) computer networks ( so-called LANs) as well as newer interface techniques such as USB or Firewire narrowband null modem connection became increasingly unattractive because of this new connectivity data can be much faster and also transmit between more than two computers.

Even today, null modem cables are used mainly in the field of embedded systems, namely computer systems that have enough processing power for the control of high-speed controllers for USB or Ethernet, or where the installation of such a controller would be too expensive, for example, in applications requiring only a few data transfer or if the interface is only rarely used. Typically, the set -top boxes such as satellite television receiver or DVD players that have a serial interface to update their firmware. If these devices are equipped with scaled-down and simplified motherboards of computers and accordingly have a serial port with DTE pin-out is required, how to connect to another computer, a null modem cable.

Construction of a standard null modem cable

Here is a vollbeschaltetes null modem cable, with a hardware handshake can be performed.

Cable with two 9-pin D -sub connectors

The protective earth ( = shield ) is not listed.

For the simplest version of a null-modem cable that supports hardware handshaking, it is sufficient to RxD to TxD and RTS with CTS to cross and establish a ground connection in each case. The remaining lines are usually only required in actual operation with an external device.

The Crossroads of the compounds are symmetrical, ie swapping the ends of the cables does not change the mode of operation. The cable itself is, however, according to the EIA -232 standard, seen electrically unbalanced.

Cable with a 9 - and a 25 -pin D -sub connector

In the 25-pin connector, the shield of the cable is additionally applied to pin 1. Hence, the signal earth is on the functional ground ( cable shield), which is not necessary but usually. It is usually enough to solder the shield to the metal housing of the connector.

With such a cable is, for example, the data transfer between an Amiga and a PC.

Adapter plug

Adapter plug, the implement of a 9-pin male to 25-pin socket, or 9 -pin female to 25 -pin connector, are assigned as follows:

The connection of such an adapter plug with a standard null modem cable with two 9 -pin D- Sub sockets thus corresponds to the shown above 9 - to 25 -pin null modem cable.

Structure of a null-modem cable without hardware flow control

The multiple entries of a pin featuring bridges are connected by normally only the pins of a plug, with no connection to any of the pins on the opposite side. In this case, as the following two bridges arise: pin 6 to pin 9, pin 7 to pin 8

By connecting pin 7 and pin 8, the hardware flow control is undermined, which send both pages without regard to the input buffer of the opposite side. This may be necessary if one of the connected equipment does not support hardware flow control.

In addition to the above variants, there are commercially even more. Since the cables are usually sold without documentation, the variant can often find just using a continuity tester.

Construction of a 3-wire null modem cable with 9- pin D -sub connectors

Here is a minimal null-modem cable. But this is no hardware handshake is possible.

Null-modem adapter

In addition to the cables, there are also null-modem adapter that make a 1:1 associated standard serial cable, a null modem cable. You have both sides female connector and check the above-mentioned lines.

Legend of the pin names ( 9-pin)

Disadvantages

The disadvantage of this serial connection is that all signals only have a common ground and this can lead to transmission problems with very long lines. This can then be solved only by very slow transmission. More modern serial connections ( Ethernet, SATA, USB, HDMI, ... ) so always use a balanced signal transmission with two contrasting data lines plus ground per signal.