How to Build a Null Modem Cable

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How to Build a Null Modem Cable

Null modem cable is used to directly connect two systems, and establish an Internet connection between the computers. Read on to learn how to build a null modem cable.

A null modem cable (RS-232 serial cable) is used when one needs to connect a DTE (Data Terminal Equipment) to another via their communication ports. 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. Hence the name.

Two DTEs (computers) can be interconnected with this specially designed cable without the use of DCEs (Data Circuit-terminating Equipment) i.e modem. A DTE is a device, which communicates with DCE, and provides data communication control functions, which is to be executed according to the link protocols, by converting the user information to signals.

Our Computer (PC), is a DTE. DCE is a device that sits between two DTEs, and communicates with it using the standards which is primarily RS-232.

Now, in our experiment we are not going to use any DCE to make the connection between the DTEs, we would be using a RS-232 serial cable. This communication method is temporarily used to make a connection between a laptop computer and a desktop device. It is mainly used in simple data-transfer network connections.

A serial data communication is directly made between two computers using the RS-232 serial cable. The basic applications of this cable are:

  • Switching networks and embedded systems
  • Remote Operation
  • File transfer between two computers
  • Providing serial administration consoles


  • No Hardware Handshaking: The connection is made only for data and ground pins.
  • Loop back Handshaking: In this type of connection, the cable pin loops back, and the software is made to perform, thinking it had hardware flow control but, the disadvantage is that, the functioning could stop at any time.
  • Partial handshaking: Specially designed software utilizes this type of cable, because, of the functionality of the flow control signals.
  • Full Handshaking: The cable supports the software control flow, due to its RTS/CTS crossover pins. It is better than its predecessors.
  • Virtual Null modem: Virtual serial port is used in this cable connection. A software is employed to emulate a hardware null modem, between the two systems.

Making a Null Modem Cable Connection

The hardware flow control between the two DTEs is done using RS-232 pin connections. First you need to identify, which type of RS-232 connector, your PC has. It could be 9-pin ports or 25 pin ports. One of the significant signals used in the connection is TD (Transmit Data) and RD (Receive Data).

25 Pin Out Connection

25 Pin-Out Connection

9 Pin Out Connection

9 Pin-Out Connection

TD is used by one system to indicate that it is going to transmit some data, and RD is the signal which is sent by the other system, after it receives the data. Ring Indicate (RI) port is not used in this cable.

Embedded Signals and Its Function

The signals are sent from one DTE to another, in order to establish a serial communication. Take a look at the direction of the signal mentioned as from DTE and to DTE, considering only one DTE.

For example, the direction of the TD signal is mentioned as to DTE, which means the TD signal is transmitted by one DTE to another. The RD signal is from DTE, which means that the signal is received from another computer.

FG – Frame Ground (Protective Ground) – From DTE
TD – Transmit Data – To DTE
RD – Receive Data – From DTE
RTS – Request to Send (DTE asserts this signal when it is ready to receive data) – To DTE

CTS – Clear to Send (DTE asserts this signal when it is ready to accept data) – From DTE
DSR – 
Data Set Ready – To DTE
DTR – Data Transmit Ready (DTE asserts this signal after its powered on and ready to operate) – To DTE

SG – Signal Ground (This signal establishes as reference point for all RS-232interface voltages) – From DTE
CD – Data Carrier Detect (This signal indicates that the device can detect the carrier signal) – To DTE
RI – Ring Indicator (This signal indicates that phone line is ringing) – To DTE

Not all the pins that are present on the RS-232 connector are utilized to make the null modem cable. Once the port connections are made using this cable, the connection parameters should be set between the two computers.

If your computer is running with Windows operating system, you need a Hyperterminal program to establish serial communication between the two devices. This program first needs to be installed in order for you to run it, after which it can be set in the following way:

In your computer, go to Start → Accessories → Communications → Hyperterminal. Double click on HYPERTRM.EXE, and enter a new name for the connection and click OK. A dialog box will appear, on which you need to select COM1 or COM2 and click OK. With the advent of Ethernet, this communication method is not used that often.

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