🧠 DB9 to DB9 Null Modem Serial Cable (Male ↔ Female)
 
(RS‑232 Serial Cable • 9‑Pin D‑Sub Male to Female • TX/RX Crossed Pinout)
 
This cable enables peer‑to‑peer RS‑232 communication by swapping the transmit and receive pins. It is commonly used for PC‑to‑PC links, console debugging, and firmware flashing when devices have opposite gender DB9 ports.
 
🔧 Cable Form
DB9 male connector on one end
DB9 female connector on the other
Null modem wiring: TX ↔ RX crossover
Shielded cable with molded strain relief
Lengths range from 6ft to 10ft
RS‑232 compliant
 
🖥️ Compatible Devices
PCs and laptops with male DB9 serial ports
Routers and switches with female DB9 console ports
Industrial controllers (PLCs, CNC machines, sensors)
Embedded systems with DB9 debug ports
✅ Any two DTE devices requiring direct communication without a modem, but with opposite gender DB9 ports
 
✅ Common Uses
🔄 PC‑to‑PC communication (file transfer, terminal emulation)
🔌 Router/switch console access for configuration and debugging
🧩 Firmware flashing of embedded systems via serial console
🧪 Industrial automation links between controllers and diagnostic tools
🧵 Legacy device interfacing (GPS, lab instruments, barcode scanners)
 
🧠 Signal Format
⚡ RS‑232 standard
🔌 Null modem pinout (typical):
Pin 2 (TX) ↔ Pin 3 (RX)
Pin 3 (RX) ↔ Pin 2 (TX)
Pin 5 (GND) ↔ Pin 5 (GND)
Optional handshaking lines crossed depending on cable variant (RTS/CTS, DTR/DSR)
🔁 Allows two DTE devices to communicate directly
 
⚠️ Things to Watch Out For
📉 Not interchangeable with straight‑through cables (which do not cross TX/RX)
🔌 Both devices must expect null modem signaling
🧯 Using this cable in a straight‑through scenario will result in communication failure
📏 Gender matters — only works with male ↔ female DB9 port pairing
 
🛠️ Quick Tips for Beginners
🧭 Confirm both devices have opposite gender DB9 ports and require null modem wiring
🔌 Use for peer‑to‑peer communication, not host‑to‑peripheral setups
🧪 For PC‑to‑modem or PC‑to‑sensor links, use a straight‑through cable instead
🧯 Label clearly — DB9 cables look identical externally but differ internally