RS232 Cable Guide: Two Wiring Types, One Connector
Quick answer / TL;DR: Short answer: a straight-through cable connects a computer to a modem or device, while a null modem cable crosses the transmit and receive pairs to connect two computers or two like devices. They look identical, so check the wiring or the part number before use. Most industrial equipment uses straight-through; connecting two controllers usually needs null modem.
Three data families: USB carries power too, Ethernet spans buildings, fiber spans kilometers.An RS232 cable connects serial equipment, and the connector is the same whether the cable is straight through or null modem.
Choosing the wrong one produces a connection that is electrically correct and communicates nothing.
This article covers the two wirings, the equipment types that need each, and the pinout check that identifies what a cable actually is.
RS232 uses separate transmit and receive lines, so each device's transmit must reach the other's receive.
A straight-through cable assumes the equipment does that crossing internally.
A null modem cable does it in the cable.
Wiring Types
| Type | Use | Wiring |
|---|
| Straight through | Computer to modem | Pin 2 to pin 2 |
| Null modem | Computer to computer | Pin 2 to pin 3 |
| Null modem with handshake | Full control | Crossed plus loops |
| Adapter | Convert one to other | Gender changer |
The handshake row matters where the software expects hardware flow control, since a minimal null modem cable may omit those lines.
The adapter row is how a straight-through cable is converted without buying a second cable.
Identifying a Cable
A continuity test between pin 2 at one end and pin 2 or 3 at the other identifies the type in seconds.
A cable marked only by length and connector cannot be identified from the outside.
Recording the type when a cable is installed saves that test later.
The test is worth doing before blaming the equipment.
Practical Use
- Identify the equipment pairing first
- Choose straight-through or null modem accordingly
- Check handshake lines if flow control is used
- Test continuity before use
- Label the cable with its type
Real-World Applications
Two controllers do not communicate on a straight-through cable. They need a null modem, and the connection works once changed.
A device communicates but software flow control fails. The cable omits the handshake lines, and a full null modem resolves it.
A cable of unknown type is tested for continuity, and pin 2 to pin 3 identifies it as null modem.
A gender changer converts a straight-through cable for a device with the opposite connector.
Standards and Verification
RS232 is defined in the TIA/EIA-232 standard, and device pinouts are published by each equipment manufacturer.
Verification is a continuity check on pins 2 and 3 plus a communication test at the intended settings.
Check the wiring. A cable can be intact and still be the wrong type.
Next Step
Send us the two devices and their connector types, and we will confirm whether you need straight-through or null modem wiring and which handshake lines.
Use the contact form on this site with both model numbers and we will reply with the pinout that suits the pair.
Modern Substitutes
Many computers no longer have a serial port, so a USB to serial adapter replaces it.
The adapter must present a real serial port to the software, which most do.
The cable type beyond the adapter still follows the straight-through or null modem rule.
Changing the adapter does not change the wiring requirement.
That is the point most often missed when a serial link fails on a modern machine.
Length and Rate
RS232 is specified for modest distances at low rates, and the practical limit depends on the cable's capacitance.
A long cable at a high rate produces errors that look like a protocol fault.
Reducing the rate or shortening the cable is the usual remedy.
That is worth trying before replacing equipment.
The standard's limits are conservative and real.
Where the equipment is old enough to need a serial port, it may also expect specific handshake behaviour that a minimal cable does not provide.
The equipment's manual states the requirement, and it is worth reading before buying a cable.
A full null modem cable covers most cases.
Where the equipment uses a different connector, such as a terminal block rather than a DB9, the manufacturer's pinout is the only reliable reference.
Assuming a standard pinout on non-standard equipment produces a cable that does not work.
The manual is the authority.
Reading it takes minutes and prevents a wasted build.
Where the equipment requires a specific handshake, the cable's wiring must include those lines rather than only the data pair.
A cable that works for one device may fail for another with the same connector.
The manual's pinout is the only reliable guide.
Where the equipment is a controller with a proprietary connector, an adapter to DB9 is usually available from the manufacturer.
Using that adapter preserves the standard pinout beyond it.
A hand-built cable from the controller is the last resort.
Related reading: USB to serial adapter — blogs/news/usb-to-serial-adapter
Related reading: Data cable types — blogs/news/data-cable-types
Related reading: USB cable for file transfer — blogs/news/usb-cable-for-file-transfer
Frequently Asked Questions
What is the difference between straight and null modem?
Null modem crosses transmit and receive; straight-through does not.
How do I tell which cable I have?
Continuity test between pin 2 and pin 3 at the far end.
Do I need handshake lines?
Only if the software uses hardware flow control.
Can I convert one to the other?
Yes, with a null modem adapter.
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