DB9 Serial Cable vs. Null Modem

When working with old computers, industrial machinery, or some communication ports, it is crucial to distinguish between DB9 serial cables and null modem cables. 

Serial connection has remained one of the most used methods of connecting devices, and the DB9 connectors are part of it.

Going by the above description, it might be easy to think that DB9 serial cables and null modem cables are the same, but they are not, and they are used for different purposes.

DB9 serial cables are mainly used to connect such devices as computers and peripherals to ensure the right transfer of information. 

Conversely, null modem cables are designed to connect two or more computers or devices directly without using the modem. 

It is also important to know when to utilize each cable to avoid wasting time and possible communication breakdowns. 

The following blog will briefly explain the two cable types and in what circumstances they can be used.

What is a DB9 serial cable?

DB9 serial cable is a type of connector used in serial communication, and it is generally found in old computer models and numerous other devices. 

 It features a D-shaped connector with nine pins arranged in two rows. On the first layer and on top, there will be five beds, while on the second layer, there will be four beds. This allows for the sharing of information from one or more units, and this can be done using standards like RS-232.

A straight-through connection of the DB9 serial cable entails the following pin configuration: Pin 2 for transmit data (TX), Pin 3 for receive data (RX), and Pin 5 for ground (GND). 

These contacts enable the cable to transmit and receive signals and can be useful for tasks such as using a computer with a modem or a serial printer. 

 Although DB9 cables are relatively old, they are still used today wherever there is a need for a stable serial connection.

DB9 serial cable is used in the following applications:

  • Extension of cable networks for older computer peripherals like mice and keyboards to PCs.

  • Interacting with computer controlled systems like computer-numerically controlled (CNC) machines and programmable logic controllers (PLCs).

  • Interconnection between older routers and switches in the network.

  • Direct interaction with embedded systems and microcontrollers in programming or transferring data.

  • Connection of GPS devices to computers for data download.

  • Setting up serial connections for point-of-sale (POS) terminals.

  • USB-to-serial adapters transfer data from outdated PCs or terminals to current equipment.

  • Scanning of automotive systems using OBD interfaces for diagnosis.

computer cables

What is a null modem cable?

A null modem cable is a serial cable that is used to connect two systems or other equipment without using a modem.

 Another unique feature of this cable is the crossing over of the transmitting and receiving lines, where the TX of the first end is the RX of the second, and vice versa. 

This crossed configuration enables data to be transferred directly through the devices. A null modem cable has pin 2 connected to pin 3 and pin 3 to pin 2, while a DB9 serial cable normally has all the pins connected in a straight manner, that is, pin 2 is connected to pin 2, and so on. 

 That said, depending on the particular type of null modem cable, other control leads such as RTS and CTS could be crossed as well. 

A null modem cable is in fact a regular crossover cable for direct, point-to-point serial interfacing.

Some of the uses of the null modem cable include:

  • Sharing files between two computers or creating a link between them for the purpose of data exchange or interaction.

  • Facilitating serial communication from one device to another without using a modem.

  • Enabling serial debugging by plugging development boards or microcontrollers into a computer.

  • Direct connection of older serial peripherals, like printers, to computers.

  • Setting up a link for remote terminal connections in embedded systems.

  • Serving to convey signals from one industrial device or control system to another.

    • DB9 Serial Cable vs. Null Modem Cable

    Feature DB9 Serial Cable Null Modem Cable
    Purpose Standard serial communication between DTE (Data Terminal Equipment) and DCE (Data Communication Equipment) Direct communication between two DTEs without the need for DCE
    Wiring Configuration Straight-through wiring (Pin 1 to Pin 1, Pin 2 to Pin 2, etc.) Crossed wiring (Tx and Rx lines are crossed, e.g., Pin 2 to Pin 3, Pin 3 to Pin 2)
    Typical Use Case Connecting a computer to a modem or other DCE device Direct connection between two computers or other DTE devices
    Handshaking Lines Handshaking lines are not crossed; RTS/CTS and DTR/DSR lines are connected directly Handshaking lines (RTS/CTS, DTR/DSR) are crossed to simulate communication with a DCE device
    Common Pins Used Pin 2 (Tx), Pin 3 (Rx), Pin 5 (Ground) Pin 2 (Rx), Pin 3 (Tx), Pin 5 (Ground)
    Applications Data transmission between a computer and peripheral devices like modems, printers Data transfer between two PCs or other devices that act as DTEs
    Signal Flow Unidirectional (DTE to DCE) Bidirectional (DTE to DTE, cross-communication)
    Connector Type Usually DB9 Male to DB9 Female Usually DB9 Female to DB9 Female
    Standard Example RS-232 Standard Custom implementation also referred to as null modem RS-232

    Conclusion

    In conclusion, distinguishing between the DB9 serial and null modem cables is important to ensure correct connectivity between the devices. 

    DB9 serial cables are most useful when connecting two devices that transmit data directly, for instance, between a computer and a modem. 

     On the other hand, null modem cables are used when two similar devices, for instance, two computers, require a connection devoid of a central device. 

    Selecting the right cable type achieves a normal flow of data while minimizing or eliminating any cable connectivity problems. 

    The current setup must be assessed to identify the most suitable cable to use when connecting the devices. 

    It is inconceivable to think that selecting the cable is merely a technical decision; it is critical to the performance. 

    So, it is high time to evaluate your requirements to determine whether the cable you are using is the best one.