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RS232 cable (wiring for Crossed / Straight)

There are two typical RS232 cables used in industry.

Crossed RS232 cable (or known as a null modem cable)
Straight RS232 cable

RS-232 Cable Wiring: Crossed vs. Straight

When working with RS-232 serial communication, choosing the right cable wiring is critical. Some applications require a straight-through cable, while others need a crossed (null-modem) cable. This guide explains the difference, shows pin-outs, and provides troubleshooting tips.

DTE vs. DCE: Why Cable Type Matters in RS-232

Definitions:

DTE (Data Terminal Equipment): Devices such as PCs, printers, terminals, and data loggers.
DCE (Data Communication Equipment): Devices such as modems, routers, and multiplexers.

Connection Rules:

DTE ↔ DCE: Use a straight-through cable.
DTE ↔ DTE or DCE ↔ DCE: Use a cross (null-modem) cable.

Cross Cable RS232 wiring

Cross-cable wiring for RS-232 usually occurs between two DCE (Data Communication Equipment) devices or two DTE (Data Terminal Equipment) devices.

Typically, for the connection between DTE↔DTE, DCE↔DCE devices,
TXD connects directly to RXD,
RXD connects directly to TXD.

RS232 Cross Cable (Null Cable) Connection — RS232 Cross Cable wiring

*** Click here for RS232 connector pinout (male/female).

Straight Cable RS232 wiring

Typically, for the connection between DTE↔DCE devices,
TXD connects directly to TXD,
RXD connects directly to RXD.

*** Click here for RS232 connector pinout (male/female).

When to Use Each Cable

Straight Cable: Connecting PC ↔ Modem, Router ↔ Terminal.
Cross Cable: Connecting PC ↔ PC, Modem ↔ Modem, Router ↔ Router.
If unsure, start with straight; if no communication occurs, try cross.

Troubleshooting RS-232 Connections

No data or garbage characters → Check Tx/Rx wiring.
Connection drops → Verify RTS/CTS or DTR/DSR control signals.
Cable too long → RS-232 is limited to ~15m (50ft). Use shielded cable for reliability.
Testing → Use a cable tester (like our CCT-01 Cable Connection Tester) to quickly verify wiring connections.

Cable Assembly Tips

Use shielded cable to reduce noise.
Ensure connectors (Dsub-9, Dsub-25) are securely crimped or soldered.
Label cables clearly to avoid mix-ups.
Keep cables under the recommended length for stability.

FAQs

Q: Can I convert a straight cable into a cross cable?
Yes — with an adapter (null-modem adapter) or by re-wiring.

Q: Do I need all pins connected?
Not always. Many simple RS-232 applications only require Tx, Rx, and GND.

Q: What’s the difference between DB-9 and DB-25?
Dsub-25 has more pins, historically used for extra control signals. Most modern RS-232 devices use Dsub-9.

Related Guides

RS-232 Pinout Reference
RS-232 vs RS-485: What’s the Difference?
How to use a Cable Tester?

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RJ45 RS232 Console Cable Pin Out

An RJ45 to RS232 console cable is used to connect a computer to a serial console port on a network device, such as a router, switch, or firewall. One end is connected to the server with RJ45 connector (8P8C, 8 pins), and the other end is to a computer with a D-Sub connector (female 9 pins, often known as d-sub09 connector)

A serial console cable is usually used for conveniently sending serial configuration commands to a server or equipment (usually Cisco equipment, often known as the Cisco Console Cable).

Serial Console Cable Pin Out Diagram (RJ45 to RS232 female cable)

Serial Communication Setup

Typical communication baud rate setup for a serial console cable for Terminal Console.

Baudrate: 9600bps
Data bit: 8 bits
Stop bit: 1 bit
Parity: None
No Hardware Handshaking

For a more detailed pin out of a typical RS232 cable, you can click here for more details.

Detailed RS232 D-Sub 9 Pins Description

In most applications, only pin 02 03 05 on the D-Sub 9 Pins connector is used for the RS232 application. This 3-wire communication mode is non-hardware-handshaking or asynchronous data communication. Meaning that the serial data is sent out based on a pre-set timing clock.

The rest of the pins are meant for controlling the flow of the data. Hardware handshaking (synchronous data communication) is slower. These are hardly used these days because of the faster processing power of IC chips and the timing precision that we have now. For most applications, you don’t need to wire up these wires. But we will still be presenting the purpose of these pins on the D-sub connector for complete documentation of the pin out.

Pin 01 DCD – Data Carrier Detect (IN), indicator signal is detected from the remote terminal (the other end of the terminal).
Pin 02 TXD – Transmitted Data (OUT), data sending out to the remote terminal.
Pin 03 RXD – Received Data (IN), data coming in from the remote terminal.
Pin 04 DTR – Data Terminal Ready (OUT), local terminal side is ready to receive data.
Pin 05 GND – Signal Ground (COMMON), reference pin for all the signal pins.
Pin 06 DSR – Data Set Ready (IN), remote terminal is ready to receive and send data.
Pin 07 CTS – Clear To Send (IN), remote terminal is ready to receive data.
Pin 08 RTS – Request To Send (OUT), the local terminal is requesting the remote terminal to send over data.
Pin 09 RI – Ring Indicator (IN), the local terminal receives the ring from remote terminal.

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RS232 Pin Out

Reference pinout diagrams for DB9 connector (sometimes known as DE9 connector or D-sub connector) for RS232 serial communication ports. RS232 is a serial data communication signaling protocol. D-sub 9pins or 25pins connector is commonly used for RS232. These days RJ45 (or 8P8C) modular connector is also commonly used for RS232 purposes.

Note:
For 3 wires RS232 serial communication (non-hardware-handshaking or asynchronous data communication) wiring, you only need pin 2, 3 (RX, TX), and pin 5 (Gnd)

Typically the TX pin of an RS232 D-sub connector represents data flowing out of the connector, flowing into the RX pin of its mating connector. This is a communication channel between two devices.

Male (D-Sub 9 pins)

The pin out of an RS232 male connector/socket, typically use on a computer terminal (DTE, Data Terminal Equipment)

Note: Most systems use only 3 wires for asynchronous data communication. In those cases, only pin 02 RXD, pin 03 TXD, and pin 05 GND are important. The rest are not so important. Refer to crossed RS232 cable for more information.

Female (D-Sub 9 pins)

The pin out of an RS232 female connector/socket typically used on an electronic device (DCE, Data Communication Equipment)

Note: Most systems use only 3 wires for asynchronous data communication. In those cases, only pin 02 TXD, pin 03 RXD, and pin 05 GND are important. The rest are not so important. Refer to crossed RS232 cable for more information.

RS232 D-Sub Connector Information

D-Sub 9 pins male connector for RS232 is typically used on a computer terminal device (master). D-Sub 9 pins female connector for RS232 is typically used on an electronic device (slave).

RS232 standard was also implemented using a D-Sub 20pins connector. Nowadays is quite rare to use such a big connector.

The modern computer system uses RJ45 modular plug for its RS232 data communication. A common IT terminal console cable is known as “RS232 DB9 to RJ45”. This is typically for accessing the terminal console of IT equipment. Click here for more information about this cable.

Another common type of cable where RS232 is found is the “USB to RS232 converter cable“. Modern computing hardware uses USB connection more, due to its size and plug-and-play feature. It is commonly used between modern computer and industrial devices that still uses the RS232 communication method.

Typical RS232 Wire Connection (D-sub 9 pins)

RS232 cable typically has two types.

Crossed RS232 cable (or known as null modem cable)
Straight RS232 cable

Click here for more wiring information on the Crossed or Straight RS232 cable.

RS232 Max Length

The RS232 often uses a data baud-rate of 9600bps. This baud-rate is low and the cable length distance can be up to about 150 meters (about 500ft).

As the speed of data transmission increases on the cable, the physical signal of the data can easily be coupled to the adjacent wire. This induces noise onto the cable. The noise can be in the form of garbage data on the adjacent data wire. To reduce the effect of induced noise from the adjacent wire, the length is kept shorter. The insulation between wires is capacitance which high-frequency signals can easily pass over. The shorter length will keep the capacitance lower which makes it harder for the higher-frequency signal to be coupled over to the adjacent cable.

The following table is a general guide to the cable length base on the baud-rate. Please note that this is only a general guide. The material and the construction of the cable also play a part in the length of the cable. To be safe, it is better to have a margin factor of 2. If you really want to stretch the cable length further, ensure that you get yourself a high-quality RS232 cable and also use a better serial data protocol to detect data corruption and do the necessary data re-transmission.

	Cable Length	Cable Length
Baud Rate	(in Metres)	(in Feets)
2400 bps	900m	3000ft
4800 bps	300m	1000ft
9600 bps	*150m*	*500ft*
19200 bps	15m	50ft
115200 bps	5m	16ft

RS232 Cable Length to Data Baud-rate table reference guide

You can easily do an experiment to see the effect of noise induced on the adjacent wire. Set up 3 wires (Tx, Rx, Gnd) of 30m to 50m distance between two RS232 devices (or computer). Have the wire twisted or taped up so that the gap between the wires is as small as possible. The best is no gap between the wires. Set up the baud-rate to 19200bps (with hardware handshaking).

Transfer a string of bytes into the Tx wire and observed the garbage bytes returning from the Rx wire. These garbage data are signals from the Tx wire coupled to the Rx wire. The disturbance noise triggered the receiver circuit thinking that a valid physical RS232 signal is received. These mis-triggered bytes received on the Rx channel are the noise induced from the Tx line. Change the baudrate lower to 9600bps. You will notice that the probability of this garbage generated is lesser or none. Increasing the baud-rate will also increase the probability of noise-induced.