Null Modem Adapter: One Job, Done Properly
Quick answer / TL;DR: Short answer: a null modem adapter swaps TX and RX and normally crosses RTS and CTS as well. Choose DB9 or DB25 to match the ports, and check whether the device needs handshake lines looped back. If data flows one way only, the crossover is incomplete for that device.
A patch cord is a stranded assembly: moulded boot, eight contacts, T568B.A null modem adapter crosses transmit and receive so two devices that both expect to be the host can talk to each other.
It is a small part that solves a specific wiring problem, and getting the handshake pins wrong makes it look faulty.
What Is Crossed
| Signal | Straight cable | Null modem |
|---|
| TX to RX | Pin 3 to pin 3 | Pin 3 to pin 2 |
| RX to TX | Pin 2 to pin 2 | Pin 2 to pin 3 |
| RTS to CTS | Straight | Crossed |
| DTR to DSR | Straight | Crossed or looped |
Some equipment only needs TX and RX crossed and uses no handshake, which is why a full adapter sometimes fails where a three-wire cable works.
Diagnosis
- Check with a breakout box or meter
- Confirm the device is DTE or DCE
- Try loopback plugs for handshake pins
- Test at a low baud rate first
- Document the pinout used
Knowing whether a port is DTE or DCE decides whether a null modem is needed at all: two DTEs need one, a DTE and a DCE do not.
Real-World Applications
A switch console port connected to a laptop needs a null modem or a purpose-built console cable, depending on the vendor. Industrial controllers with DB9 ports are the other common case, where the adapter stays attached permanently.
Standards and Verification
| Reference | Relevance |
|---|
| TIA-232-F | Serial interface |
| ITU-T V.24 | Circuit definitions |
| Vendor console specifications | Pinout per device |
Pinout for a specific device is not provided in the knowledge base. Check the equipment manual before building a cable.
Practical Notes
Label the adapter with its pinout. Null modem, straight and crossover adapters look identical.
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Frequently Asked Questions
Does it work for all serial devices?
Not all. Handshake wiring varies.
When is one needed?
When both ends are DTE.
Why one-way data?
Crossover or handshake incomplete.
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