Why RS-232 Still Matters
Embedded systems often rely on serial ports to talk with sensors, modems, and other equipment. RS-232 is a long-standing standard for serial data transfer, dating back to the 1960s.
Today, it remains common in industrial and embedded systems because it’s simple and widely supported.
RS-232 defines not only electrical signals (±voltage levels) but also the connectors and pinouts.
By understanding the pinout, you can reliably connect devices across decades of technology.
RS-232 uses single-ended data lines: think of pins as “lanes” on a road.
There are lanes for transmitting (TX), receiving (RX), ground, and control signals (like RTS/CTS).
Unlike TTL serial (0–5V logic on microcontrollers), RS-232 voltages swing positive/negative (often ±12V).
This means you’ll need a level shifter (such as a MAX232 chip) if you’re connecting to a microcontroller’s UART.
But the payoff is a robust, long-range link that’s resistant to noise.
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DB9 Connector Pinout (9-pin D-Sub)
The 9-pin RS-232 connector (often referred to as DB9 or DE-9) is a standard on PCs and embedded devices. On a PC, the serial port (DTE) is typically male and plugs into a female device port (DCE). Each pin has a defined role. For example, Pin 2 (RXD) is “Receive Data” (incoming to the computer), Pin 3 (TXD) is “Transmit Data” (outgoing), and Pin 5 is Ground.
Other pins handle control signals: Pin 7 is the Request To Send (RTS) signal, and Pin 8 is the Clear To Send (CTS) signal.
Here’s a quick rundown of the DB9 pins:
- Pin 1 (DCD): Data Carrier Detect (IN)
- Pin 2 (RXD): Received Data (IN)
- Pin 3 (TXD): Transmitted Data (OUT)
- Pin 4 (DTR): Data Terminal Ready (OUT)
- Pin 5 (GND): Ground
- Pin 6 (DSR): Data Set Ready (IN)
- Pin 7 (RTS): Request To Send (OUT)
- Pin 8 (CTS): Clear To Send (IN)
- Pin 9 (RI): Ring Indicator (IN)
For most embedded links, you only need the basics: TX, RX, and ground (Pins 2, 3, 5).
The extra lines (CTS/RTS/etc) are hardware flow-control flags.
You can often ignore them if your devices use software handshaking instead.
DB25 Connector Pinout (25-pin D-Sub)
The older RS-232 standard used a 25-pin connector. These days, DB25 ports are rare on PCs, but they can still be found on some industrial equipment. A DB25 offers extra pins for a second channel or more signals, but typically only one channel is used.
Looking into a DB25 male connector (DTE), the essential pins are:
- Pin 1: Protective/Shield Ground
- Pin 2 (TXD): Transmit Data (OUT)
- Pin 3 (RXD): Receive Data (IN)
- Pin 4 (RTS): Request To Send (OUT)
- Pin 5 (CTS): Clear To Send (IN)
- Pin 6 (DSR): Data Set Ready (IN)
- Pin 7 (SGND): Signal Ground
- Pin 8 (DCD): Data Carrier Detect (IN)
- Pin 20 (DTR): Data Terminal Ready (OUT)
- Pin 22 (RI): Ring Indicator (IN)
Pins 12–19 and 21, 23–25 are secondary or reserved signals, and can usually be left unconnected.
Just like DB9, if you only need a one-way link, connecting TX, RX, and GND is enough.
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Straight vs. Null-Modem Cables
When wiring RS-232 cables, a straight-through cable connects like-to-like pins (TX to TX, RX to RX) on a DTE to DCE. This is the standard cable for connecting a computer (DTE) to a modem or device (DCE).
If you ever need to connect two PCs (DTE to DTE), use a null-modem (crossover) cable.
A null-modem swaps TX/RX (and often RTS/CTS) so that each end can transmit to the other.
In embedded projects, you’ll most likely use straight cables.
If both sides use DTE ports, a null cable or adapter will loop the signals.
Remember: PC ports are typically male/DTE, and devices/modems are female/DCE.
Gender changers or null adapters can fix any mismatch.
RS-232 Cable Quick Reference
Here’s a quick table to compare the two connectors:
| Connector Type | Pin Count | Key Signals | Typical Use |
|---|---|---|---|
| DB9 (DE-9) | 9 | TX, RX, GND, RTS, CTS | Modern serial ports (PCs, controllers) |
| DB25 | 25 | TX, RX, GND, DTR, DCD, RTS, CTS, RI | Legacy RS-232C ports |
DB9 is more compact and commonly used on new equipment.
DB25 (RS-232C) was the original standard.
In many cases, a cable will have a DB25 connector on one end and a DB9 connector on the other.
In that case, only the matching signals need wires (the rest go unused).
Frequently Asked Questions
A: No. Microcontrollers use 0–5V TTL logic, whereas RS-232 uses ±12V levels. Use an RS-232 level shifter (e.g., MAX232) to connect.
A: Only when both ends are DTE (like PC-to-PC). If one end is a DCE (modem/device), a straight cable is correct.
A: Not necessarily. For basic one-way links, just TX, RX, and ground are enough. You can leave RTS/CTS disconnected if flow control isn’t needed.
A: Yes! It’s very common in industrial and embedded devices. Even modern PCs often use USB–serial adapters to connect to legacy RS-232 ports.
A: Leave unused pins disconnected.
Make sure the signal ground is connected (Pin 5 on DB9 or Pin 7 on DB25) to complete the circuit.
With these pinout diagrams and tips, you’ll be able to wire RS-232 links confidently in your next embedded project.
Happy serial communicating!
Sam Wu is the Marketing Manager at Romtronic, holding a degree in Mechatronics. With 12 years of experience in sales within the electronic wiring harness industry, he manages marketing efforts across Europe. An expert in cable assembly, wiring harnesses, and advanced connectivity solutions, Sam simplifies complex technologies, offering clear, actionable advice to help you confidently navigate your electrical projects.
