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Guest Slick504

Null modem

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Can someone please explain in plain English, what is a null modem, and where would one be used. Thanks in advance

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A null modem is used for an RS-232 bale to swap the Tx and Rx so that you can communicate with certain devices. It is like a telephone of sorts, your transmit line (TX) has to go to the recieve line (RX) on the other device, the null modem effectively swaps the pins through its connection. I hope that helps.

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You asked for "simple" so I'm leaving out the finer points but I think this is what you're looking for. First things first. What is a modem? Suppose you and I are located several miles apart and that we each have a computer. We decide to have these two computers communicate with each other over the telephone lines. The problem is that the serial communication ports on our computers use digital signals but the telephone signals need to be analog signals. So we each buy a MODEM. The name comes from MOD-ulate/DEM-odulate. In other words, change digital-to-analog and then change analog-back-to-digital. We each hook our  modems up to the telephone lines with a little snap-in type telephone cable. Now how about the cable from the computer to the modem? Suppose (to keep it simple) that the serial comm port on the computer is a 9-pin and that the port on the modem is a 9-pin also. The cables that you and I will each need are usually called "straight-through" cables. In other words, pin 2 at the computer end is wired "straight-through" to pin 2 of the modem end. And, pin 3 at the computer end is wired "straight-through" to pin 3 of the modem end. And, pin 5 at the computer end is wired "straight-through" to pin 5 of the modem end. These three wires are (referenced from the computer's viewpoint) used for RECEIVE, TRANSMIT, and SIGNAL COMMON. There are other signal wires in these cables but we'll skip them in this simple discussion. Another less common (but still perfectly valid) name for this particular type of cable is a "MODEM" cable. Now we're ready to talk over the phone lines ... and (more importantly) we know what a MODEM CABLE is. Next suppose that we relocate our two computers and place them both in the same room. Now we don't need the telephone lines anymore and so we don't need the modems either. So ... let's just take one of the "straight-through" or "modem" cables and hook our two serial communication ports together. OOPS. That won't work - because when my computer TRANSMITS on pin 3, the signal is being wired "straight through" to your computer's TRANSMIT pin 3. We need to cross the wires as follows: When I transmit on my pin 3 - the signal needs to go over to your receive on pin 2. When you transmit on your pin 3 - the signal needs to go over to the receive on my pin 2. The SIGNAL COMMONS on both pins number 5 are ok. Now what do you think we should call the cable we need now? How about a "NO MODEM" cable? Or (if we're trying to confuse and/or impress people) how about calling it a "NULL MODEM" cable? And, yes, some people do indeed call it a "cross-over" cable. That's as simple as I can make it but others may certainly do a better job. To sum up. A "NULL MODEM" cable (think "NOT MODEM" cable) is a serial communication cable to be used between two computers if we're NOT using modems. It can also be a little adapter that looks like a gender changer but has the pins cross-wired 2-to-3 and 3-to-2 as described above. Another application for this "NULL MODEM" type of cable would be to have your programming computer communicate with the Channel 0 port on the front of an SLC-5/04. Hope this helps. Finally, the pin assignments are different with 25-pin connectors.

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Since all of you are so knowledgable about this stuff, how about registering on this fine forum and quit lurking in the background.

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I agree. Excellent post Ron! Come join us! Also my two cents. If you guys want to know how to make a Null Modem cable, I frequent http://www.hardwarebook.net This is a DIRECT LINK to the serial section. Bookmark these links, they are LIFE savers. http://www.hardwarebook.net/cable/index.html#Serial

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Thanks to everyone who replied, plain English is mutch easier to understand than Geek speak. Cheers Graeme

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you can buy a null modem adapter at radio shack for $3.00 ? it comes in very handy if you need to talk to tons of different devices through an RS-232 port. sometimes you need 2-3 cross and sometimes you don't

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