Null modem cable
A null modem cable is a cable that connects two computers via the serial interface ( RS-232 or EIA-232). In contrast to a standard serial cable that connects a computer with a modem , different lines have to be crossed here (e.g. Receive Data and Transmit Data , pins 2 and 3 of the sockets). In the field of Ethernet - network technology is called the equivalent cross cable .
The name null modem (Engl. For a modem within the meaning of modem omitted) comes from the fact that computer ( data terminal equipment , DTE) often on long distance via modems are connected to each other, which are traditionally each connected via a serial interface to the computer. With a direct connection where the computers are only a few meters apart, the modems can be omitted and replaced by a null modem cable.
A null modem cable has both sides female (female) D-Sub - Connector .
Usage and history
Via a null modem connection, i. H. a direct serial connection, the two computers involved can exchange data , which was particularly popular at the time of the early days of personal computers in the 1980s and early 1990s. With two personal computers running the DOS operating system , data exchange is easy to accomplish in this way. The data transmission program Laplink , for example, was widespread . Two Amigas or Atari STs can also be connected with a null modem cable . This technique was used in particular by computer games to enable multiplayer games.
With the increasing emergence of (also in-house) computer networks (so-called LANs ) and newer interface technologies such as USB or Firewire , the narrow-band null-modem connection has become increasingly unattractive, as these new connection options allow data to be transferred much faster and between more than just two computers.
Null modem cables are still used today, primarily in the area of embedded systems , i.e. H. Computer systems that do not have enough computing power to control high-speed controllers for USB or Ethernet , or where installing such a controller would be too expensive, e.g. B. in applications that only transfer a small amount of data or when the interface is rarely used. Typically these are set-top boxes such as satellite TV receivers or DVD players that have a serial interface for updating their firmware . If these devices are equipped with reduced or simplified main boards of computers and accordingly have a serial connection with DEE pin assignment, a null modem cable is required, as is the case for connecting to another computer.
Structure of a standard null modem cable
This is a fully wired null modem cable with which a hardware handshake can also be carried out.
Cable with two 9-pin D-Sub sockets
| Designation connector 1 | DCD, DSR | RxD | TxD | DTR | GND | RTS | CTS |
| Pin connector 1 | 1 and 6 | 2 | 3 | 4th | 5 | 7th | 8th |
| Pin connector 2 | 4th | 3 | 2 | 1 and 6 | 5 | 8th | 7th |
| Designation connector 2 | DTR | TxD | RxD | DCD, DSR | GND | CTS | RTS |
The protective ground (= shielding) is not listed.
For the simplest variant of a null modem cable that supports hardware handshake, it is sufficient to cross RxD with TxD and RTS with CTS and establish a ground connection. The other lines are usually only required in real operation with an external device.
The intersections of the connections are symmetrical, i. H. swapping the two cable ends does not change the functionality. The cable itself, however, is electrically unbalanced in accordance with the EIA-232 standard .
Cable with a 9-pin and a 25-pin D-Sub socket
| designation | DCD, DSR | RxD | TxD | DTR | GND | RTS | CTS | RI |
| 9 pin | 1 and 6 | 2 | 3 | 4th | 5 | 7th | 8th | 9 |
| 25-pin | 20th | 2 | 3 | 6 and 8 | 7th | 5 | 4th | 22nd |
| designation | DTR | TxD | RxD | DSR, DCD | GND | CTS | RTS | RI |
With the 25-pin connector, the cable shield is also applied to pin 1. This means that the signal ground is on the system ground (cable shield), but this is usually not necessary. It is usually sufficient to solder the shield to the metal housing of the connector.
With such a cable z. B. the data transfer between an Amiga and a PC is possible.
Adapter plug
Adapter plugs that convert from 9-pin plug to 25-pin socket or 9-pin socket to 25-pin plug have the following assignment:
| 9-pin side | Shield | 3 | 2 | 7th | 8th | 6th | 5 | 1 | 4th | 9 |
| designation | Shield | TxD | RxD | RTS | CTS | DSR | GND | DCD | DTR | RI |
| 25-pin side | 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th | 20th | 22nd |
The connection of such an adapter plug with a standard null model cable with two 9-pin D-Sub sockets corresponds to the 9 to 25-pin null modem cable shown above.
Structure of a null modem cable without hardware flow control
| Designation connector 1 | DCD | RxD | TxD | DTR | GND | DSR, RI | RTS, CTS |
| Pin connector 1 | 1 | 2 | 3 | 4th | 5 | 6 and 9 | 7 and 8 |
| Pin connector 2 | 1 | 3 | 2 | 4th | 5 | 6 and 9 | 7 and 8 |
| Designation connector 2 | DCD | TxD | RxD | DTR | GND | DSR, RI | RTS, CTS |
The multiple entries of a pin indicate bridges through which normally only the pins of a plug are connected to one another without a connection to one of the pins on the opposite side. In this case, the following two bridges result: pin 6 to pin 9, pin 7 to pin 8.
The connection of pin 7 and pin 8 overrides the hardware flow control, which means that both sides send regardless of the input buffer of the opposite side. This may be necessary if one of the two connected devices does not support hardware flow control.
In addition to the above variants, there are others in stores. Since the cables are usually sold without documentation, the variant can often only be determined with the help of a continuity tester.
Structure of a 3-wire null modem cable with 9-pin D-Sub sockets
This is a minimal null modem cable. However, no hardware handshake is possible with this.
| Designation connector 1 | RxD | TxD | GND |
| Pin connector 1 | 2 | 3 | 5 |
| Pin connector 2 | 3 | 2 | 5 |
| Designation connector 2 | TxD | RxD | GND |
Null modem adapter
In addition to the cables, there are also null modem adapters that turn a 1: 1 connected serial standard cable into a null modem cable. They have female connectors on both sides and cross the cables mentioned above.
Legend of the pin designations (9-pin)
| Pin code | Short name | designation | Signal direction | description |
|---|---|---|---|---|
| 1 | DCD | Data carrier detect | Transmission device → end device | The data carrier signal was detected by the transmission device (e.g. modem) |
| 2 | RxD | Receive (x) data | Transmission device → end device | Line that receives a data bit from the transmission device (e.g. modem) on the end device (e.g. PC) |
| 3 | TxD | Transmit (x) data | End device → transmission device | Line that sends a data bit from the end device (e.g. PC) to the transmission device (e.g. modem) |
| 4th | DTR | Data Terminal Ready | End device → transmission device | The end device (e.g. PC) is ready for use |
| 5 | GND | Ground ("mass") | no | Reference potential to 0 V |
| 6th | DSR | Data set ready | Transmission device → end device | Transmission device (e.g. modem) is ready for use |
| 7th | RTS | Request To Send | End device → transmission device | End device (e.g. PC) indicates that the remote station should send (send request) |
| 8th | CTS | Clear To Send | Transmission device → end device | Transmission device (e.g. modem) indicates readiness to receive (send permission) |
| 9 | RI | Ring indicator | Transmission device → end device | Transmission device (e.g. modem) receives a ringing or calling signal on the telephone line |
disadvantage
The disadvantage of this serial connection is that all signals only have one common ground, which can lead to transmission interference with very long lines. This can only be circumvented by very slow transmission. More modern serial connections (RS422, RS485, Ethernet, SATA, USB, HDMI ...) therefore always use symmetrical signal transmission with two opposing data lines plus ground per signal.
See also
- IEEE 1284 (parallel port)
Web links
Individual evidence
- ↑ Null-Modem Serial Cables . Website of the Amiga Forever project. Retrieved January 8, 2014.