🛠 Passerelle série sur les 4 UART matériels

Objectives

Build the signature project for the Mega 2560: communicate with four serial devices simultaneously using its 4 hardware UARTs — something an Uno cannot do (only one UART). Official pin assignments: Serial (0 RX / 1 TX, shared with USB), Serial1 (RX 19 / TX 18), Serial2 (RX 17 / TX 16), Serial3 (RX 15 / TX 14). You will write a non-blocking byte-by-byte router.

Hardware

• 1 × Arduino Mega 2560 + USB cable
• 1 × GPS module (typically 9600 bauds) on Serial1
• 1 × modem module or second device (typically 115200) on Serial2
• 1 × RS-485 transceiver (or any 9600 device) on Serial3
• Breadboard and wires

Respect the TX→RX/RX→TX crossover between the Mega and each module, and ensure a common ground. Warning: the USB-CP link is managed by the ATmega16U2; if the PC writes a lot without anyone reading, the flow can block the board (documented CDC behavior).

Complete Code

// Passerelle 4 UART — PC + GPS + modem + RS-485
void setup {
  Serial.begin(115200);    // console USB (broches 0/1)
  Serial1.begin(9600);     // GPS typique
  Serial2.begin(115200);   // modem typique
  Serial3.begin(9600);     // transceiver RS-485
  while (!Serial && millis < 3000) {}  // attend l'USB CDC
}

void loop {
  // Routage non bloquant, octet par octet :
  if (Serial1.available) Serial.write(Serial1.read);    // GPS -> PC
  if (Serial.available)  Serial2.write(Serial.read);    // PC -> modem
  if (Serial2.available) Serial3.write(Serial2.read);   // modem -> RS-485
  if (Serial3.available) Serial.write(Serial3.read);    // RS-485 -> PC
}

Expected Verification

The serial monitor at 115200 bauds continuously displays the GPS NMEA frames. What you type in the monitor goes to Serial2; the modem's response is relayed to Serial3. No bytes are lost at usual rates because each loop empties the four buffers.

Common Errors

• Confusing the pairs: Serial1 = 19/18, Serial2 = 17/16, Serial3 = 15/14 (RX/TX in this order) — not pins 10/11 of the SPI.
• Connecting TX to TX instead of TX to RX crossed: nothing passes, or worse, line conflict.
• Forgetting the common ground between separately powered modules: corrupted data.
• Flooding the USB port without reading it: the board may stop responding over USB (specificity of the 16U2's CDC flow control); limit the rate or read on the PC side.