RS232 null modem pinout and wiring

What is a null modem?

Null modems are used to connect two computers without modems via the RS232 protocol. Originally developed to allow teletype machines to connect to each other through their modems, the RS232 protocol is perfect for use in null modems.

The asymmetric connectivity using the RS232 protocol requires a data source or consumer at one end of the connection and a modem at the other end. There is no standard wiring scheme for an RS232 null modem and you will find that many different variations are used in practice. The reception and transmission lines in a null modem are connected crosswise to enable data transmission in both directions.

The most common use of null modems is to enable older laptops and computers to exchange data. Without a network card or USB port, null modem connections and null modem emulators are the only methods that can be used for data transmission between these legacy machines.

What is a null modem cable?

It might be obvious that you would create null modem connections by using a null modem cable.

What distinguishes a null modem cable from any other cable? Specifically, a null modem cable is constructed using three lines. Two of the lines take care of transmitting data with the third wire acting as a signal ground.

Null modem pinout and wiring

Based on the software you are using when transmitting over a null modem cable, you may need the connection to provide an authenticating handshake.

Here are the most common schemes of null modem cables.

Here is the RS232 cable wiring configuration that is recommended by Microsoft that includes full authentication handshaking.

This is the industry standard for RS232 null modem connections. It uses a cable developed by Microsoft that is manufactured with seven wires.

Virtual Null Modem Emulator

Null modem cables do not provide for high-speed data transmission. Theoretically, the maximum throughput speed of a null modem connection is limited by the speed of the serial port that connects it to the computer. This maximum of 115 kb/s is never reached as the transfer speed is impacted by the length of cable used to make the connection.

Higher transmission speeds can be achieved by using the specialized software of a null modem emulator. This application creates a virtual null modem connection between two computers. It employs a virtual null modem cable to connect to the machine’s serial ports.

Virtual null modem software allows you to create pairs of virtual COM ports. The software enables bi-directional data transmission between serial ports. Data that is transmitted to one COM port in the pair is also simultaneously received by the other port in the pair.

Using virtual null modem software to transmit data is a much more efficient method of communication than that which can be achieved with a physical null modem cable.

A quality null modem emulator such as Virtual Com Port Driver by Eltima allows you to emulate baud rate and enables full HandFlow control. This application supports all standard line signals like RTS/CTS, DRT/DSR, RING, and ERROR. A virtual null modem emulator also enables you to simulate line breaks in serial connections.