// uart-slave-red.ino — Node 1 (slave A), Arduino UNO R3
// Week 11 — receive-only node on the shared UART bus at 9600 baud.
// SoftwareSerial on D8 keeps the hardware UART (D0/D1) free for the USB monitor.
// Reads every frame, verifies checksum, and acts only when DEST_NID matches.
 
#include <SoftwareSerial.h>
 
#define MY_NID      1
#define NID_MASTER  0
 
#define FRAME_START 0xFF
#define CMD_LED     0x03
 
#define RX_PIN      8     // bus RX (master TX -> here)
#define TX_PIN      9     // unused, required by SoftwareSerial constructor
#define LED_PIN     7     // red LED via 220 ohm to GND
 
SoftwareSerial busSerial(RX_PIN, TX_PIN);
 
uint8_t buf[6];
uint8_t idx = 0;
 
void handleFrame() {
  uint8_t dest = buf[1], src = buf[2], cmd = buf[3], payload = buf[4], chk = buf[5];
 
  if ((uint8_t)(dest ^ src ^ cmd ^ payload) != chk) {
    Serial.println(F("RX  bad checksum, frame dropped"));
    return;
  }
  if (dest != MY_NID) {
    Serial.print(F("RX  frame for NID "));
    Serial.print(dest);
    Serial.println(F(" - not me, ignored"));
    return;
  }
  if (cmd == CMD_LED) {
    digitalWrite(LED_PIN, payload ? HIGH : LOW);
    Serial.print(F("RX  DEST_NID=1 (me)  LED="));
    Serial.println(payload ? "ON" : "OFF");
  }
}
 
void setup() {
  Serial.begin(9600);       // USB monitor
  busSerial.begin(9600);    // UART bus on D8
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
  Serial.println(F("Slave A (NID 1, red LED) ready - Arduino UNO."));
}
 
void loop() {
  while (busSerial.available()) {
    uint8_t b = busSerial.read();
    if (idx == 0 && b != FRAME_START) continue;   // resync: wait for START
    buf[idx++] = b;
    if (idx == 6) {
      idx = 0;
      handleFrame();
    }
  }
}