A null serial cable, also known as a null modem cable, is a type of RS-232 serial cable used to directly connect two devices (like two PCs) without a modem or intermediary hardware. Unlike a standard serial cable, a null modem cable swaps specific transmission and reception lines so that the devices can “talk” to each other directly.
null serial cable
In this guide, we’ll explain what a null serial cable is, how it differs from standard serial cables, typical wiring (pinout), and where it’s still used today.
What Is a Null Serial Cable?
A null serial cable is a special type of DB9 or DB25 serial cable that crosses the transmit (TX) and receive (RX) signals between two devices. This allows both devices to communicate in a peer-to-peer fashion over the RS-232 standard, which is typically designed for device-to-host connections.
It is called 'null modem' because it eliminates the need for a modem, simulating modem functionality through internal wiring.
Null Serial Cable vs Standard Serial Cable
| Feature | Standard Serial Cable | Null Serial Cable |
|---|---|---|
| Purpose | Connect host to device | Connect two hosts (e.g., PC-PC) |
| Wiring | Pin-to-pin (1:1, 2:2...) | Crossover (TX-RX, RTS-CTS...) |
| Use Case | Modems, printers, serial devices | Direct device-to-device communication |
| Common Connectors | DB9, DB25 | DB9, DB25 |
Typical Null Serial Cable Pinout (DB9)
For null modem wiring, the key is to cross the data and control signals:
| Pin (Side A) | Pin (Side B) | Signal |
|---|---|---|
| 2 | 3 | RX <-> TX |
| 3 | 2 | TX <-> RX |
| 5 | 5 | GND <-> GND |
| 7 | 8 | RTS <-> CTS |
| 8 | 7 | CTS <-> RTS |
| 6 | 4 | DSR <-> DTR |
| 4 | 6 | DTR <-> DSR |
Some simplified versions only cross TX, RX, and GND, which is enough for basic serial communication.
Applications of Null Modem Cables
Even in the USB and Ethernet era, null serial cables are still used in:
Legacy PC-to-PC communication
Router and switch configuration (console ports)
Data transfer between older hardware
Serial debugging and development
Industrial equipment communication
DB9 Null Modem Cable vs Null Modem Adapter
DB9 Null Modem Cable – A complete cable with crossover wiring.
Null Modem Adapter – A small in-line device that converts a standard serial cable into a null modem setup.
If you already have a straight-through serial cable, a null modem adapter can be a cost-effective solution.
How to Choose a Null Serial Cable
Connector Type: DB9 Male to Female is the most common. DB25 is used in some legacy systems.
Wiring: Confirm it’s wired for null modem (not straight-through).
Cable Length: Common lengths are 3ft, 6ft, 10ft, up to 50ft. Longer cables may require shielding.
Shielding: For industrial or EMI-prone environments.
Color-Coding/Labeling: Helps identify TX/RX lines during troubleshooting.
USB to Null Modem Option
Modern computers lack RS-232 ports. You can use a USB to DB9 null modem cable, which includes:
USB plug on one end
Built-in USB-to-RS232 chip
DB9 null modem wiring on the other end
This makes it easy to interface legacy serial equipment with modern laptops or PCs.
Final Thoughts
A null serial cable remains a reliable and low-cost solution for serial communication between two DTE (Data Terminal Equipment) devices. Whether you're working on industrial automation, retro computing, or serial debugging, a properly wired null modem cable can save you hours of setup time.
Need a Quality Null Modem Cable?
At TOT Wire & Cable, we offer:
DB9 null modem cables (male-female, male-male)
Custom lengths and shielding options
USB to null modem solutions
Bulk supply for OEMs and system integrators
👉 Contact us to get a quote or discuss your serial communication needs.
