Arduino program to control an 8 relay board from a Serial connection

serial8RelayBoardControl.imo

// Configuration
const int RELAY_CNT = 8;
const int relayPins[RELAY_CNT] = {2, 3, 4, 5, 6, 7, 8, 9};

// Protocol
const char CMD_PREFIX[5] = "rlar"; // 2 chars + null terminator
const int TOTAL_CMD_LEN = 12;    // "rl" (4) + "00000000" (8)

// State Variables
int recvIndex = 0;          // Tracks which character we are waiting for (0 to 9)
char pendingStates[8];      // Temporary buffer to hold "01010101" before applying
char currentPins[RELAY_CNT] = {'0', '0', '0', '0', '0', '0', '0', '0'};

bool prefixFailOnce = 0; // Was the last read a fail?

void setup() {
  Serial.begin(9600);
  for (int i = 0; i < RELAY_CNT; i++) {
    pinMode(relayPins[i], OUTPUT);
    digitalWrite(relayPins[i], HIGH); // Init OFF
  }
  Serial.println("ARDUINO SERIAL CONTROLLER LIVE");
}

void loop() {
  // 1. Process data only if available
  while (Serial.available() > 0) {
    char c = Serial.read();

    // ------------------------------------------------
    // STAGE 1: Check Prefix ("rl")
    // ------------------------------------------------
    if (recvIndex < 4) {
      if (c == CMD_PREFIX[recvIndex]) {
        recvIndex++; // Valid prefix char, move to next
      } else {
        // Character did not match expected prefix.
        // Smart Reset: If this char is actually 'r', start a new sequence immediately.
        // This handles typos like "rrl..." gracefully.
        recvIndex = (c == 'r') ? 1 : 0;
        if (prefixFailOnce == 0) {
          Serial.println("BAD_PREFIX");
          prefixFailOnce = 1;
        } 
      }
    }
    // ------------------------------------------------
    // STAGE 2: Check Data ('0' or '1')
    // ------------------------------------------------
    else if (recvIndex < 12) {
      prefixFailOnce = 0;
      if (c == '0' || c == '1') {
        // Store in temporary buffer. We do NOT write to relays yet.
        // Index 4 corresponds to relay 0, so we subtract 4.
        pendingStates[recvIndex - 4] = c;
        recvIndex++;
      } else {
        // Invalid data char (e.g., '2', 'A', etc).
        Serial.println("ERR_CHAR"); 
        recvIndex = (c == 'r') ? 1 : 0; // Reset and wait for new 'r'
      }
    }

    // ------------------------------------------------
    // STAGE 3: Finalize (Command Complete)
    // ------------------------------------------------
    if (recvIndex == TOTAL_CMD_LEN) {
      // We successfully received "rl" and 8 valid bits.
      // Now apply the temporary buffer to the real hardware.
      applyRelays();
      
      Serial.println("DONE");
      recvIndex = 0; // Reset and wait for next command
    }
  }
}

void applyRelays() {
  for (int i = 0; i < RELAY_CNT; i++) {
    // Only write if state is changing
    if (currentPins[i] != pendingStates[i]) {
      if (pendingStates[i] == '0') {
        digitalWrite(relayPins[i], HIGH); // OFF
      } else {
        digitalWrite(relayPins[i], LOW);  // ON
      }
      currentPins[i] = pendingStates[i];
    }
  }
}