Start with endpoint roles
DTE and DCE conventions explain why a PC serial port normally transmits on the pin where a modem receives. Two DTE endpoints commonly need a null-modem crossover; DTE-to-DCE commonly uses straight-through wiring. Equipment vendors do not always follow the convention, especially on non-DB connectors.
| Signal | Diagnostic role |
|---|---|
| TX | Output from an endpoint; identify by documentation or safe measurement. |
| RX | Input receiving the other endpoint’s TX. |
| Signal ground | Reference required for meaningful voltage measurements. |
| RTS/CTS | Hardware flow control; a device may remain silent if expected state is absent. |
| DTR/DSR/DCD | Readiness/presence lines sometimes repurposed by instruments. |
Safe troubleshooting order
- Identify connector numbering from the correct viewing side.
- Confirm RS232 voltage levels; do not assume a UART header is RS232.
- Determine grounds and likely outputs without tying two outputs together.
- Establish DTE/DCE behavior and whether straight-through or null-modem wiring is expected.
- Check handshake requirements before reducing the cable to TX/RX/GND.
- Verify continuity and isolation with equipment powered down where appropriate.
- Only then capture traffic with known serial settings.
Why a three-wire cable may still fail
The original application may wait for CTS, DSR or DCD; a device may require DTR to enter communication mode. Looping handshake lines locally can help diagnose expectations but may defeat real readiness signaling and should not become an undocumented permanent fix.
After wiring is known, continue with traffic capture. Unknown behavior belongs in device reverse engineering; replacement software is covered by RS232 development. See the serial pillar for the full layer model.
Provide
Photos and connector labels, manuals, continuity results, measured idle voltages, cable construction and the original working equipment pairing.