To troubleshoot 3-wire serial connectivity (like RS-232, RS-485, or SPI), you must verify physical connections, match configuration settings, and test signal voltages. 3-wire setups rely on a Transmit line (TX), a Receive line (RX), and a Shared Ground (GND). 1. Check Physical Hardware
Verify Ground: Ensure the GND wire connects firmly to both devices.
Swap TX/RX: Cross the transmit and receive lines between devices.
TX connects to RX: One device’s transmit must go to the other’s receive.
RX connects to TX: Reverse the rule for the opposite data path.
Inspect cable length: Keep unshielded 3-wire cables under 15 meters. 2. Verify Software Configuration Match Baud Rate: Set both devices to the exact same speed. Check Data Bits: Use 8 data bits for most standard systems.
Align Parity: Ensure both sides use None, Even, or Odd parity.
Set Stop Bits: Match the stop bit configuration, usually set to 1.
Disable Flow Control: Turn off hardware flow control (RTS/CTS). 3. Measure Voltages with a Multimeter
Test GND continuity: Verify zero ohms of resistance between device grounds. Measure Idle TX: Check voltage between TX and GND at rest. Identify RS-232 levels: Look for -5V to -15V when idle. Identify TTL levels: Look for 3.3V or 5V when idle. 4. Perform a Loopback Test
Disconnect target device: Unplug the receiving device from the cable. Jumper TX to RX: Connect the TX and RX wires together.
Open terminal software: Use a tool like PuTTY or Serial Studio.
Type test characters: Send text through the terminal configuration.
Verify echo: Confirm that typed characters appear back on screen. 5. Analyze Signals
Connect an oscilloscope: Attach probes to TX/GND and RX/GND.
Look for square waves: Check for clean, sharp voltage transitions.
Spot voltage sagging: Identify weak signals caused by long cables. To isolate your specific issue, please tell me:
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