Documentation updated as to serial ports

CLAUDE.md

@@ -16,6 +16,40 @@ arduino-cli upload -p /dev/ttyACM0 --fqbn arduino:avr:mega Gaugecontroller.ino
 
 Serial monitor: 115200 baud (`Serial` is both CMD_PORT and DEBUG_PORT).
 
+## Switching serial ports (debug → production)
+
+Two `#define`s at the top of `Gaugecontroller.ino` control where commands and debug output go:
+
+```cpp
+#define CMD_PORT   Serial   // command channel (host sends SET, HOME, etc.)
+#define DEBUG_PORT Serial   // diagnostic prints (homing, boot messages)
+```
+
+**Debug / USB-only (default):** both point to `Serial` (the USB-CDC port). Connect via `minicom` or the Arduino IDE serial monitor at 115200 baud.
+
+**Production (hardware UART):** change `CMD_PORT` to a hardware serial port so a host MCU or Raspberry Pi can drive it without occupying the USB port:
+
+```cpp
+#define CMD_PORT   Serial1  // TX1=pin18, RX1=pin19
+#define DEBUG_PORT Serial   // keep USB for monitoring, or silence it (see below)
+```
+
+Arduino Mega hardware UARTs:
+
+| Port    | TX pin | RX pin |
+|---------|--------|--------|
+| Serial1 | 18     | 19     |
+| Serial2 | 16     | 17     |
+| Serial3 | 14     | 15     |
+
+`setup()` calls `DEBUG_PORT.begin(115200)` only. If `CMD_PORT` differs from `DEBUG_PORT` you must also begin it — add a second `begin` call in `setup()`:
+
+```cpp
+CMD_PORT.begin(115200);
+```
+
+**Silencing debug output entirely:** point `DEBUG_PORT` at a null stream, or wrap all `DEBUG_PORT` calls in an `#ifdef DEBUG` guard. The simplest option is to replace the define with a no-op object, but the easiest production approach is just to leave `DEBUG_PORT Serial` and ignore the USB output.
+
 ## Architecture
 
 The sketch controls `GAUGE_COUNT` stepper-motor gauges using a trapezoidal velocity profile and a simple text serial protocol.