USB to Serial Port Cable: Getting the Wiring Right
The adapter works, the driver is installed, and nothing talks. On serial links the reason is almost always the wiring between the adapter and the equipment, not the adapter itself.
Quick Answer
Match three things: connector gender, straight through versus null modem wiring, and the equipment manual wiring. A USB to serial port cable ends in a DB9 plug, usually male. For console access to network equipment you need a rollover cable wired for that vendor, not a standard RS-232 cable. Keep RS-232 runs within about 15 m at typical rates.
Cable Variants
| Variant | Wiring | Used for |
|---|---|---|
| Straight through DB9 | Pin to pin | DTE to DCE links, most instruments |
| Null modem | RX and TX crossed plus handshake | DTE to DTE, two computers or controllers |
| Rollover | Pin order reversed | Network equipment console ports, usually RJ45 |
| DB9 to DB25 | Converted pinout | Older modems, plotters, legacy printers |
DB9 Pinout Reference
| Pin | Signal | Direction on a DTE |
|---|---|---|
| 1 | DCD | In |
| 2 | RXD | In |
| 3 | TXD | Out |
| 4 | DTR | Out |
| 5 | GND | Ground |
| 6 | DSR | In |
| 7 | RTS | Out |
| 8 | CTS | In |
| 9 | RI | In |
Only pins 2, 3 and 5 carry the actual data and ground. The rest are handshake and status lines that some equipment requires and others ignore.
Choosing the Right Assembly
Gender
Adapters normally end in a male DB9. Equipment often presents a female socket, so a male to female cable is the usual pairing.
Screw locking
Industrial equipment vibrates. Thumb screw locking prevents the intermittent faults that a friction fit causes.
Length and rate
RS-232 is specified for about 15 m at 19.2 kbps, longer at lower rates. Beyond that, use an RS-232 to RS-485 converter or Ethernet serial device server.
Real-World Applications
- Console cable in an engineer toolkit for switch and firewall access.
- Fixed installation cable for a PLC programming point.
- Instrument connection in a calibration lab.
- Legacy printer or plotter kept in service on a new PC.
Selection Checklist
- Read the equipment manual for the required wiring and gender.
- Check whether the link is DTE to DCE or DTE to DTE.
- Match the adapter chipset drivers to the operating system.
- Choose screw locked connectors for fixed installations.
- Test the loopback before travelling to site.
Related Guides
For the adapter itself, read USB 2.0 serial adapter. For the connector family, see USB to USB 3 cable. For portable kits, start with 2 port USB 3.0 hub.
FAQ
Do I need a null modem cable or a straight cable?
Straight for connecting a computer to a modem or instrument, null modem for computer to computer or controller to controller.
Can I use a rollover cable with a USB serial adapter?
Yes, if the adapter ends in RJ45 or you use an RJ45 to DB9 adapter wired for the vendor. Wiring must match the equipment.
How long can a serial cable be?
About 15 m at common rates. Lower baud rates tolerate longer runs, but beyond that use a converter or device server.
Why does the equipment only respond intermittently?
Usually a loose connector or missing handshake lines. Check the screw locks and the flow control settings.