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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?

CCT-01 Cable Tester Tool — Click here to check out our **cable tester tool**.

Cable Wire Size and Current Capacity Rating Guide

This is a general cable wire size guide to help you choose the correct wire size for your electrical project.

Choosing a thicker wire provides higher current carrying capacity. It is safer to use thicker wire to conduct the current, but thicker wire is going to cost more. This guides provides you a gauge to select the most appropriate cable size.

The variety of cable brands varies in quality. The cable current conducting material and the insulation jacket material do have an impact of the amount of ampere current that a wire can conduct. The cable length and temperature of the conducting environment also plays a part. A good conducting material has lower resistance or better conductivity, produce less heat, therefore can carry more current. A good insulation jacket that can endure higher heat can handle more current before the jacket gets melted and fails.

*** Important Notes: The following wire size and current is only an estimate for a quick reference guide when selecting your cable size. It is important to check on the datasheet of the cable directly from the manufacturer. It is recommended to provide more buffer to the current capacity for your application.

Wire Size and Current Rating Table Guide

This wire size and current rating table provides a quick lookup table for your cable selection.
It is recommended to use a wire capacity that is 2 times of what your application is expecting to use.
If you want a more detailed computation, check out the next section for the calculation.

Projected Maximum Operating Current	Wire Size (AWG)	Copper Wire Diameter Ø	Cross-sectional Area	Current Capacity (estimated)
0.1A
0.3A
0.5A	30AWG Wire Wrapping wire	0.3mm	0.05mm²	0.8A
1A	28AWG	0.36mm	0.08mm²	1.25A
2A	27AWG
3A	26AWG	0.46mm	0.14mm²	3.5A
5A	24AWG CAT5e Network Cable	0.61mm	0.2mm²	5A
	22AWG	0.78mm	0.33mm²	8.73A
10A	21AWG
	20AWG	0.92mm	0.5mm²	13.87A
25A	17AWG	1.34mm	1.06mm²	29.2A
	16AWG		1.31mm² (closest 1.5mm²)
50A	14AWG Electrical Wire	1.78mm	2.07mm² (closest 2.5mm²)	55.6A
	12AWG Electrical Wire		about 4mm²
	10AWG Electrical Wire		about 6mm²

Table: Estimated wire current carrying capacity.

Calculate current capacity and voltage drop

For application that cannot afford for thicker wire with higher current capacity, it is important to calculate close to your projected situation. We will need to know the amount of current the wire can carry, as well as the expected drop in voltage over the wire distance (to and from).

Wire conducting material and their resistivity.

Wire Material	Resistivity per metre
Copper	0.0174 μΩ.m	1.724 x 10^-8 Ω.m
Aluminum	0.0265 μΩ.m	2.65 x 10^-8 Ω.m
Silver	0.0265 μΩ.m	2.65 x 10^-8 Ω.m
Carbon	0.1 μΩ.m	10 x 10^-8 Ω.m
Iron	0.1 μΩ.m	10 x 10^-8 Ω.m

Table: Wire material resistivity

The formula to compute the resistance of a particular wire material is base on normal room temperature of 20 °C.

Resistance, R = ρ L / A Ω

where
R = resistance of the conductor (ohms, Ω)
ρ = resistivity of the conductor material (ohm metre, Ω m)
L = length of conductor (m)
A = cross-sectional area of conductor (m2)