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What is a Null Modem ?

Posted on 2005-04-23
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Hi...everybody......can someone please explain to me in detail what  exactly a null modem is ?.......thanks........
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Question by:robin_alexander
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by:rburello
ID: 13851494
A null modem is a connection to the serial port that crosses the transmit(TX) and receive(RX) lines.  It creates a loopback for ther serial line.
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rburns50 earned 375 total points
ID: 13852189
Actually, it's a little more complex than what rburello stated above. A null modem cable was used in the past (and I suppose still used) to connect together two DTE (Date Terminal Equipment) devices. Normally, you connect DTE's to DCE (Data Communications Equipment) devices- an example would be your PC's serial port (DTE) connecting to a modem (DCE). The wiring standards for DTE and DCE connectors make sure that transmit on one is connected ot receive on the other and vice versa, so that communications can happen. There are also other matches, such as RTS/CTS (request to send, clear to send), DTR/DSR (data terminal ready, data set ready).

What a null modem adapter or cable does is switch some of the wires in the middle, so that a DTE device can talk to another DTE device. If it was a straight through cable, you'd have a problem, as both ends would be transmitting on the same wire, trying to receive on the same wire, try to signal readiness on the same wire etc. Null modem cables swap around the pinouts, so that there are no conflicts.

A good example of what a null modem used to be used for was for connecting PC's back to back for file transfers etc. Before network cards and crossover CAT5 cables, this was the easiest way to conect two PC's together- serila port to serial port. I also used to use them (albeit a different cable, same concept) to connect Cisco routers back to back via their serial ports, to simulate them connecting across a WAN- one woudl act as DTE (normal), the other was DCE (even though it was DTE in nature, the null cable made it appear as DCE).

If you want the actual pinouts and description, here's a good link:
http://www.nullmodem.com/NullModem.htm


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