What you see
The port opens but readable text becomes unexpected characters. This is a symptom, not a compiler error or an automatic disconnect.
Likely causes
- The monitor’s baud differs from Serial.begin().
- The sketch transmits binary bytes while the view is being read as text.
- A mismatched board clock or serial interface changes the timing or destination.
Diagnostic steps
Compare both settings
Find Serial.begin(...) in the uploaded sketch. Stop the monitor if its baud selector is disabled, choose the matching value, and reconnect the monitor.
Use a text-only baseline
Upload Serial Hello World at 9600 baud. If this works, inspect how the original sketch formats numbers, line endings or binary output.
Check the target clock
Pro Mini 8 MHz and 16 MHz are separate compile targets. Select the actual voltage/clock variant; do not use the wrong profile just to get a port to open.
Separate readable data from plotting
Serial Plotter accepts supported numeric or labeled series. Text diagnostics such as No echo are not zero-distance measurements and should not be interpreted as numeric samples.
Board-specific notes
Native USB serial implementations may not use the baud setting in the same way as a hardware UART. Always check whether the sketch writes USB Serial, Serial1, or a software UART.
Safe recovery
Restore the known example and original profile. Correct timing/format settings before modifying wiring or replacing hardware.
Evidence and references
Reviewed 2026-10-23 against Circuitera’s actual behavior. An editorial review is separate from a physical board test.
Circuitera implementation evidence
src/App.jsx: Serial baud rate; src/site/ecosystem/tutorials.js: ultrasonic-distance