Controls and reads actual measured distance from LCD bus of a Sndway SW-A40 laser distance meter. Improved version not using Arduino delay that causes random problems with serial interface.

sndway_control2.ino

/**
 * Controls and reads actual measured distance from LCD bus of a Sndway SW-A40 laser distance meter.
 * Improved version not using Arduino delay that causes random problems with serial interface.
 * 
 * Serial control characters:
 * 
 * ?  Return identifier string.
 * c  Clears measurement results and turns off laser. 
 *    Can be used as keep alive command to prevent auto power off after 150s.
 * M  Turns on device when off and triggers a measurement.
 * S  Start continuous measurement mode. Device must be turned on prior to this command.
 *    Command c or M will stop that mode.
 * L  Turns on laser but not triggering measurement.
 * o  Turns off device.
 * u  Cycles distance unit.
 * r  Cycle reference of measurement.
 * R  Get reference of measurement.
 *    f = front
 *    m = middle
 *    b = back (default)
 * 
 * Make sure your Arduino Mini board is 3V compatible! By default it is not!
 * 
 * Developed and tested on Arduino Mini 328P@16MHz.
 * Mictronics 2018 @ www.mictronics.de
 */
#include <avr/wdt.h>

/** 
 * Pin connection to LCD SPI bus. 
 */
#define CLKPIN  2 // LCD clock signal = IO2
#define DATAPIN 4 // LCD data signal = IO4
#define CSPIN   3 // LCD chip select signal = IO3

/**
 * Pin connection for device control.
 * Key signals are low active.
 */
#define UNITKEY     5 // Key changing units = IO5
#define MEASUREKEY  6 // Key switching device ON and taking measurement = IO6
#define CLEARKEY    7 // Key switching device OFF and clearing measurement = IO7
#define REFKEY      8 // Key changing the measurement reference = IO8

const word lutNumber[] = {'8',    '0',    '9',    '6',    '5',    '3',    'E',    '2',    '4',    '7',    '1',    'r',    '-'};
const word lutFrame0[] = {0x0006, 0x0006, 0x0004, 0x0006, 0x0004, 0x0004, 0x0002, 0x0002, 0x0004, 0x0004, 0x0004, 0x0002, 0x0000};
const word lutFrame1[] = {0x0006, 0x0004, 0x0006, 0x0002, 0x0002, 0x0006, 0x0002, 0x0006, 0x0006, 0x0004, 0x0004, 0x0002, 0x0002};
const word lutFrame2[] = {0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000};
const word lutFrame3[] = {0x0006, 0x0006, 0x0006, 0x0006, 0x0006, 0x0004, 0x0006, 0x0004, 0x0002, 0x0004, 0x0000, 0x0000, 0x0000};

#define DIGIT5  0x0002
#define SIGN    0x0001
#define UNIT_ft 0x1000
#define UNIT_in 0x4800
#define UNIT_m  0xC000
#define REF     0x04
#define LASERON 0x08

byte lcdFrameBuffer[70];
byte data = 0;
byte bitCount = 8;
byte byteCount = 0;
char serialOutBuffer[20];
byte serialOutIndex = 0;
bool takeMeasurement = false;
volatile bool laserIsOn = false;
volatile bool deviceIsOn = false;
unsigned long lcdTimeout;
char reference = 'b';
unsigned long keyTimeout = 0;
unsigned long measureKeyDelay = 0;

void setup() {
  pinMode(CLKPIN, INPUT);
  pinMode(DATAPIN, INPUT);
  pinMode(CSPIN, INPUT);
  digitalWrite(UNITKEY, HIGH);
  digitalWrite(MEASUREKEY, HIGH);
  digitalWrite(CLEARKEY, HIGH);
  digitalWrite(REFKEY, HIGH);
  pinMode(UNITKEY, OUTPUT);
  pinMode(MEASUREKEY, OUTPUT);
  pinMode(CLEARKEY, OUTPUT);
  pinMode(REFKEY, OUTPUT);
  Serial.begin(38400);
  Serial.setTimeout(100);
  lcdTimeout = millis();
  wdt_enable(WDTO_4S);
  /* External interrupt 0&1 enable on rising edge. */
  EICRA = _BV(ISC11) | _BV(ISC10) | _BV(ISC01) | _BV(ISC00) ;
  EIMSK = _BV(INT1) | _BV(INT0);
}

static void clickKey(byte pin, unsigned long t){
  digitalWrite(pin, LOW);
  keyTimeout = millis() + t;
}

void loop() {
  word frame0 = 0, frame1 = 0, frame2 = 0, frame3 = 0;
  word mask0 = 0, mask1 = 0, mask2 = 0, mask3 = 0;

  /* Skip LCD init frame and start reception inbetween two frames */
  if((byteCount > 2) && (lcdFrameBuffer[0] != 0x10) && (lcdFrameBuffer[1] != 0x00)){
    delay(50);
    byteCount = bitCount =0;
  }
  
  /* Process a complete LCD frame */
  if(byteCount == 68){
    noInterrupts();
    // Check for correct frame order
    if((lcdFrameBuffer[0] == 0x10) &&(lcdFrameBuffer[1] == 0x00) &&(lcdFrameBuffer[2] == 0xC0)){
      // Save last LCD line data to work variables
      frame0 = lcdFrameBuffer[10];
      frame0 <<= 8;
      frame0 += lcdFrameBuffer[11];
      frame1 = lcdFrameBuffer[27];
      frame1 <<= 8;
      frame1 += lcdFrameBuffer[28];
      frame2 = lcdFrameBuffer[44];
      frame2 <<= 8;
      frame2 += lcdFrameBuffer[45];
      frame3 = lcdFrameBuffer[61];
      frame3 <<= 8;
      frame3 += lcdFrameBuffer[62];
      // Get measurement reference
      if((lcdFrameBuffer[5] & REF)){
        reference = 'm';
      } else if((lcdFrameBuffer[22] & REF)){
        reference = 'b';
      } else if((lcdFrameBuffer[39] & REF)){
        reference = 'f';
      } else {
        reference = '?';
      }
    }
    /* Get ready to receive new frame */
    byteCount = 0;
    interrupts();
    // Process last frame, get distance and unit from LCD data
    // Sign
    if((frame0 & SIGN) == SIGN){ serialOutBuffer[serialOutIndex++] = '-';}
    // Digit 5
    if((frame2 & DIGIT5) == DIGIT5){ serialOutBuffer[serialOutIndex++] = '1';}
    // Remaining 5 digits
    for(byte d = 0; d < 5; d++){      
      for(byte n = 0; n < 13; n++){
        mask0 = lutFrame0[n];
        mask1 = lutFrame1[n];
        mask2 = lutFrame2[n];
        mask3 = lutFrame3[n];
        mask0 = (lutFrame0[n] << (d*2));
        mask1 = (lutFrame1[n] << (d*2));
        mask2 = (lutFrame2[n] << (d*2));
        mask3 = (lutFrame3[n] << (d*2));
        if(((frame0 & mask0) == mask0) && ((frame1 & mask1) == mask1) && ((frame2 & mask2) == mask2) && ((frame3 & mask3) == mask3)){
          serialOutBuffer[serialOutIndex++] = lutNumber[n];
          break;
        }
      }
      // Decimal points
      if((d == 1) && ((frame2 & 0x0020) == 0x0020)){ serialOutBuffer[serialOutIndex++] = '.'; }
      else if((d == 2) && ((frame2 & 0x0080) == 0x0080)){ serialOutBuffer[serialOutIndex++] = '.'; }
      else if((d == 3) && ((frame2 & 0x0200) == 0x0200)){ serialOutBuffer[serialOutIndex++] = '.'; }
    } 
    // Unit
    if((serialOutBuffer[0] != 'E') && (serialOutBuffer[0] != '-')){
      if((frame2 & UNIT_ft) == UNIT_ft){
        serialOutBuffer[serialOutIndex++] = 'f';
        serialOutBuffer[serialOutIndex++] = 't';
      } else if((frame2 & UNIT_in) == UNIT_in){
        serialOutBuffer[serialOutIndex++] = 'i';
        serialOutBuffer[serialOutIndex++] = 'n';
      } else if((frame2 & UNIT_m) == UNIT_m){
        serialOutBuffer[serialOutIndex++] = 'm';
      }
    }
    serialOutBuffer[serialOutIndex++] = '\n';
    if(serialOutIndex > 2){
      Serial.write(serialOutBuffer, serialOutIndex);
    }
    serialOutIndex = 0;
    deviceIsOn = true;
  }

  // Device auto turns off after 150s
  if(deviceIsOn && ((millis()-lcdTimeout) > 150000)){
    laserIsOn = false;
    deviceIsOn = false;
    Serial.println("off");
  }

  if(keyTimeout && (millis() > keyTimeout)){
    keyTimeout = 0;
    digitalWrite(UNITKEY, HIGH);
    digitalWrite(MEASUREKEY, HIGH);
    digitalWrite(CLEARKEY, HIGH);
    digitalWrite(REFKEY, HIGH);   
  }

  if(measureKeyDelay && (millis() > measureKeyDelay)){
    measureKeyDelay = 0;
    clickKey(MEASUREKEY, 30);
  }
 
  wdt_reset();    
}

void serialEvent(){
  /* Get and process serial input */
    char c = (char)Serial.read();
    switch(c){
      // Return device id
      case '?':
        Serial.println("Sndway Rangefinder v1.1");  
        break;
      // Clear measurement result and switch off laser
      case 'c':
        clickKey(CLEARKEY, 30);
        laserIsOn = false;
        lcdTimeout = millis();        
        break;
      // Switch on device and take measurement
      case 'M':
        // Switch on device
        if(deviceIsOn == false){
          laserIsOn = false;
          clickKey(MEASUREKEY, 30);
          measureKeyDelay = millis() + 2000;
          break;
        }
        // Trigger laser on
        if(laserIsOn == false){
          clickKey(MEASUREKEY, 30);
          measureKeyDelay = millis() + 800;
          break;
        }
        clickKey(MEASUREKEY, 30);
        lcdTimeout = millis();
        laserIsOn = false;
        break;
      case 'S':
        clickKey(MEASUREKEY, 1500);        
        laserIsOn = true;
        break;
      // Switch on laser
      case 'L':
        if(laserIsOn == false){
          clickKey(MEASUREKEY, 30);
          laserIsOn = true;
          lcdTimeout = millis();
        }
        break;
      // Switch off device
      case 'o':
        Serial.println("off");
        clickKey(CLEARKEY, 2000);  
        laserIsOn = false;
        deviceIsOn = false;
        break;
      // Cycle unit
      case 'u':
        clickKey(UNITKEY, 30);
        lcdTimeout = millis();        
        break;
      case 'r':
        clickKey(REFKEY, 30);
        lcdTimeout = millis();        
        break;
      case 'R':
        Serial.println(reference);
        break;
      default:
        break;
    }
}

/**
 * Reads the data pin on CLK rising edge.
 */
ISR(INT0_vect) {
  if(digitalRead(CSPIN) == LOW) {
    if(bitCount == 8) {
      // Skip data/command identifier bit
      --bitCount;
      return;
    }
    asm volatile("nop;");
    data |= (digitalRead(DATAPIN) << bitCount);
    --bitCount;
  }
}

/**
 * Save a single data byte to LCD frame buffer on CS rising edge.
 */
ISR(INT1_vect) {
  lcdFrameBuffer[byteCount] = data;
  ++byteCount;
  data = 0;
  bitCount = 8;
}