If you've ever set up a network switch, programmed a PLC, configured a UPS, or worked with industrial equipment, you've probably encountered a DB9 serial port. Despite being introduced in the 1960s, RS-232 serial communication is still everywhere — and learning how to use it properly is a useful skill that pays off in any technical career.
What is DB9 / RS-232?
RS-232 is the electrical specification for serial communication. DB9 (technically DE-9) is the physical 9-pin connector that's most commonly associated with RS-232. The terms are often used interchangeably, though technically you can have RS-232 over other connector types and DB9 connectors used for other protocols.
The DB9 connector has 9 pins arranged in two rows (5 pins on top, 4 on bottom) inside a D-shaped metal shell. Like VGA connectors, DB9 has thumbscrews on either side that lock the cable in place.
The "DB" prefix originally indicated the shell size — a DB-25 connector has the same shell as a DB9. To be technically accurate, the 9-pin version should be called DE-9, but DB9 has become the universal name through decades of usage.
Male vs female: which do you need?
DB9 connectors come in both male (with 9 pins sticking out) and female (with 9 holes) versions. Equipment varies widely in which gender is used:
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Computers and laptops typically have male DB9 ports (for the COM1/COM2 serial ports)
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Network equipment (routers, switches, firewalls) usually has female DB9 console ports
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PLCs and industrial controllers can have either, depending on manufacturer
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Modems historically had female DB9 ports
Standard DB9 cables come in three configurations: male-to-male, male-to-female, and female-to-female. Buying the wrong gender is a common frustration. When in doubt, look at the equipment specifications or photos before ordering.
Straight-through vs null modem cables
This is the most important distinction in serial cable shopping, and the source of countless hours of frustration when ignored.
Straight-through cable — Pin 1 connects to pin 1, pin 2 to pin 2, and so on. Used for connecting a computer to a modem or other DCE (Data Communications Equipment).
Null modem cable — The transmit (TX) and receive (RX) pins are crossed, plus several other handshaking pins are swapped. This allows two computers (both DTE — Data Terminal Equipment) to communicate directly without a modem in between. You need a null modem cable when connecting two PCs together, connecting a PC to a console port on a network switch or router, programming a PLC, configuring a UPS, or any direct DTE-to-DTE communication.
The most common mistake in serial troubleshooting is using a straight-through cable when a null modem is required, or vice versa. Symptoms include the connection appearing to work but no data flowing, or only one direction working.
USB to serial adapters
Most modern computers no longer have built-in DB9 serial ports. To use serial equipment with a modern laptop, you need a USB-to-serial adapter.
These adapters present a virtual COM port to your operating system. Once installed, software like PuTTY, HyperTerminal, or Tera Term sees the adapter as a regular serial port.
Quality matters significantly with USB-to-serial adapters. Cheap adapters often use counterfeit FTDI chips or unreliable Prolific chipsets that can cause Windows compatibility issues, dropped connections, or driver conflicts. Look for adapters using genuine FTDI FT232 chips for the most reliable performance with industrial and networking equipment.
Common serial cable applications in 2026
Despite being decades old, serial communication remains the standard interface for many critical systems:
Network equipment console ports — Cisco, Juniper, HP, and most enterprise network gear uses RJ45-to-DB9 console cables for out-of-band management. When the network is down, you connect to the console port to diagnose and fix the device.
Industrial PLCs and HMIs — Allen-Bradley, Siemens, Schneider Electric, and other industrial control vendors use serial communication extensively. Modbus, the most common industrial protocol, runs over RS-232 (and the related RS-485).
UPS systems — APC, Eaton, and other UPS manufacturers use serial ports for status monitoring and shutdown signaling.
Test and measurement equipment — Oscilloscopes, multimeters, signal generators, and power supplies often have serial ports for remote control and data logging.
Embedded systems and development boards — Microcontroller boards (Arduino, ESP32, etc.) often expose serial ports for debugging and programming.
Marine electronics — GPS receivers, AIS transponders, and chartplotters frequently use NMEA 0183 over RS-232.
Common serial cable types
RJ45 to DB9 console cable — One end is RJ45 (used to plug into the console port of a network switch), the other end is female DB9 (to connect to a USB-to-serial adapter or a PC's serial port). The classic Cisco "rollover cable."
DB9 male to DB9 female extension cable — Used to extend an existing serial connection.
DB9 to DB25 adapter — Used for older equipment that uses the larger DB25 connector (common on serial printers and some legacy modems).
Null modem DB9 male to DB9 female — For connecting two DTE devices together.
At Kentek, we carry serial cables in every common configuration — straight-through, null modem, RJ45-to-DB9 console cables, gender changers, and USB-to-serial adapters with FTDI chipsets.