🧠 DB9 to DB9 Null Modem Serial Cable (Female ↔ Female)
 
(RS‑232 Serial Cable • 9‑Pin D‑Sub Female 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, firmware flashing, and console debugging.
 
🔧 Cable Form
DB9 female connector on both ends
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 (console ports with male DB9)
Industrial controllers (PLCs, CNC machines, sensors)
Embedded systems with male DB9 debug ports
✅ Any two DTE devices requiring direct communication without a modem
 
✅ 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 DB9 ports on both devices
 
🛠️ Quick Tips for Beginners
🧭 Confirm both devices have male 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