Make A Null Modem Cable

Null modem with partial handshaking.

Make a null modem cable.

As we already know a null modem connection is based on a null modem cable which offers the easiest path to connect two machines together. Transmit data to receive data in both directions that s two of the pins and ground to ground. This means lugging around a null modem cable in addition to the non standard rj 45 to db 9 console cable required by most cisco equipment. Despite these warnings this type of null modem cable has been used successfully between windows 95 98 me computers with a direct cable connection.

Null modem cable with handshake. Along with full fledged cables there are also null modem adapters that can be used to change a straight thru cable into null modem. But after putting together four simple custom adapters i now have an ethernet cable a crossover ethernet cable a modem cable a null modem cable and a cisco console cable all by carrying around just one. The startech mxt100ff is an example of a straight through cable.

A null modem or crossover cable is used to connect to dte devices together. The cable is comprised of three lines. When using an adapter make sure to only put a null modem adapter on one end. Here are the magical three pins for my 3 wire null modem cable.

Rs232 null modem pinout and wiring. Simple null modem cable. Null modem cables are still somewhat common today having been used on many machines for a few decades. If you need to make your own null modem cable follow this wiring diagram for the 3 wire null modem cable method.

As we have seen null modem connections are made possible through the use of a null mode cable. Therefore although this null modem cable is cheap and easy to make use it with care. It s much easier than most others. Null modem is a communication method to directly connect two dtes computer terminal printer etc using an rs 232 serial cable the name stems from the historical use of rs 232 cables to connect two teleprinter devices or two modems in order to communicate with one another.

When such a cable is employed to establish a connection between two dtes both the dtes are made to believe that they are connected through modems. One wire is the signal ground the second one is the receiving line and the third one is the transmitting wire. For example to make a 9 to 25 straight cable wire the 9 pin plug according to the leftmost column and the 25 pin plug according to the second column from the left. But what is a null modem cable.

Null modem communication refers to using a crossed over rs 232 cable to connect the teleprinters directly to one. If you are making a straight extension cable not a null modem cable just stay on the same side of the table. For this to work the transmit txd pin of one device needs to be connected to the receive rxd pin of the other device and vice versa. The adapters change the pin.

This is the simplest solution to connecting the two computers.