Record the complete serial configuration
| Baud rate | Bit timing; a wrong rate produces corruption rather than a clean protocol error. |
|---|---|
| Data bits, parity, stop bits | Framing must match both endpoints; parity errors can be discarded by drivers. |
| Flow control | None, RTS/CTS, DTR/DSR or software XON/XOFF affects whether bytes move. |
| Direction | Requests and responses must remain distinguishable. |
| Timing | Inter-byte and inter-message gaps may define boundaries or device readiness. |
Capture methods and tradeoffs
A passive dual-channel tap can observe TX in both directions while sharing signal ground appropriately. A hardware protocol analyzer may preserve direction and timestamps more reliably. Software interception on the host can be convenient but may miss traffic below the API, alter timing or be impossible with the original application.
Confirm voltage levels before connecting logic analyzers or TTL adapters. RS232 signaling is not TTL UART, and an unsuitable input can be damaged.
Make the capture analyzable
- Save raw binary, not only a formatted screen log.
- Annotate operator action, device state and clock time.
- Capture initialization from power-on as well as the visible command.
- Repeat identical actions, then vary one parameter.
- Retain failed exchanges; timeouts and malformed responses reveal framing and state.
- Check whether a textual-looking protocol also contains binary lengths or checksums.
If no bytes appear, investigate pinout and handshake lines. Use captured corpora in device reverse engineering or the wider unknown protocol workflow. The serial pillar keeps electrical, transport and protocol faults separated.
Provide
Raw capture files, tool/setup diagram, serial settings, cable pinout, direction labels and a log of actions performed during capture.