Null modem
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Null_modem.jpg
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 protocols allow the mapping of a harddisk as a network drive on the other. No networking hardware such as a network interface card or a modem are required for this.
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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 | DB-9 Pin | DB-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:
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Nullmodem.png
Image:Nullmodem.png
See also
External links
- http://ourworld.compuserve.com/homepages/harburg/Link2PCs.htm
- http://www.unh.edu/helpdesk/webdoc/14.227.html
- http://www.angelfire.com/ma/mantasdos/interlnk.html
- http://www.ms-dos.com/dos-i/intexe-i.htm
- http://www.daniel-hertrich.de/rs232/
- http://www.nullmodem.com/NullModem.htm
- http://www.lammertbies.nl/comm/info/RS-232_null_modem.htmlpl:Nullmodem