Modem and null modem

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They required a modem or similar equipment as a go-between. Null modem cables changed that old standard, allowing devices to be linked up directly with no middleman equipment. This allows older computers and other machines with serial ports to transfer data between each other directly, similar to more modern ethernet crossover cables. Null modem cables work by switching around wire pairs when going from one end of the cable to the other.

Returning to the DB9 example, a null modem cable would have Pin 2 on one side connected to Pin 3 on the other side.

Then Pin 4 would be connected to Pin 6 and Pin 7 to Pin 8. Pins 1 and 9 are unused while Pin 5 still connects to itself, acting as the ground wire. This type of null modem cable is called a Handshake cable.

The way these cables are wired allows two connected devices to talk and listen to each other at the same time. Standard AT cables attempt to do the same thing cannot due to their straight-thru pinout. While there are a few different types of handshake cables , the chart below shows the most common version and is the only type available as a stock item today. With serial cables now considered obsolete, the odds of running into one of the other types is practically non-existent unless you happen to be using very old, very specific equipment.

A standard pinout featuring the two most common null modem cables, DB9 and DB Null modem cables are still somewhat common today, having been used on many machines for a few decades.

Along with full-fledged cables, there are also null modem adapters that can be used to change a straight-thru cable into null modem. When using an adapter, make sure to only put a null modem adapter on one end. The adapters change the pin configuration from straight-thru to null modem.

So if null modem adapters are installed on both sides, the pins will line up like a straight-thru cable again. This solution is not perfect, as it does not provide high-speed data transfer. Employing the dedicated software of a Null Modem Emulator , you can construct a virtual null modem connection. Using a virtual RS null modem cable, you can transfer your data much more quickly and are not constrained by the distance between the connected machines. The RS protocol was originally used to connect a telephone modern to a teletype machine.

The teletypes communicated to each other through the modems. This asymmetric connection using the RS required a modem on one side and a data source or consumer at the other end. Null-modem connection are not standardized, allowing for multiple possible wiring configurations. In this type of connection, the transmission and reception lines are connected crosswise, enabling two-way data transmission.

Today, null modems are used primarily to exchange data between older computers or laptops. These machines may not have a USB port or network card.

For these types of machines, a null modem is the only option when data transmission is required. As we have seen, null-modem connections are made possible through the use of a null mode cable.

This is the simplest solution to connecting the two computers. But what is a null modem cable? The cable is comprised of three lines. One wire serves as the signal ground with the other two lines fostering the transmitting and receiving of your data. Based on the software used in this connection, some kind of authenticating handshake may be necessary.

RS cable wiring scheme by Microsoft has become the industry standard, using a cable made up of seven wires.