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I have an ESP8266 connected to PMS5003 particulate matter sensor through (hardware) UART. I'm getting many checksum errors while reading from PMS5003.

Here's the library that I'm using to communicate with PMS5003:

PMS5003.cpp

#include "PMS5003.h"
void PMS5003::processDataOn(HardwareSerial &serial) {
  unsigned long timeout = millis();
  int count = 0;
  byte incomeByte[NUM_INCOME_BYTE];
  boolean startcount = false;
  byte data;
  int timeoutHops = 0;
  while (1){
    if (((millis() - timeout) > 1000) && (timeoutHops == 0)) {
      timeoutHops = 1;
      yield();
      ESP.wdtFeed();
    }
    if (((millis() - timeout) > 2000) && (timeoutHops == 1)) {
      timeoutHops = 2;
      yield();
      ESP.wdtFeed();
    }
    if ((millis() - timeout) > 3000){
      Serial.println("SENSOR-ERROR-TIMEOUT");
      break;
    }
    if (serial.available()){
      data = serial.read();
      if (data == CHAR_PRELIM && !startcount) {
        startcount = true;
        count++;
        incomeByte[0] = data;
      } else if (startcount) {
        count++;
        incomeByte[count - 1] = data;
        if (count >= NUM_INCOME_BYTE){
          break;
        }
      }
    }
  }
  unsigned int calcsum = 0;
  unsigned int exptsum;
  for (int i = 0; i < NUM_DATA_BYTE; i++) {
    calcsum += (unsigned int)incomeByte[i];
  }
  exptsum = ((unsigned int)incomeByte[CHECK_BYTE] << 8) + (unsigned int)incomeByte[CHECK_BYTE + 1];
  if (calcsum == exptsum) {
    pm1 = ((unsigned int)incomeByte[PM1_BYTE] << 8) + (unsigned int)incomeByte[PM1_BYTE + 1];
    pm25 = ((unsigned int)incomeByte[PM25_BYTE] << 8) + (unsigned int)incomeByte[PM25_BYTE + 1];
    pm10 = ((unsigned int)incomeByte[PM10_BYTE] << 8) + (unsigned int)incomeByte[PM10_BYTE + 1];
  } else {
    Serial.println("#[exception] PM2.5 Sensor CHECKSUM ERROR!");
    pm1 = -1;
    pm25 = -1;
    pm10 = -1;
  }
  return;
}

int PMS5003::getPM1() {
  return pm1;
}

int PMS5003::getPM25() {
  return pm25;
}

int PMS5003::getPM10() {
  return pm10;
}

PMS5003.h

#ifndef _PMS_5003_H
#define _PMS_5003_H

#include <Wire.h>
#include <Arduino.h>

#define VERSION 0.2
#define Sense_PM        6
#define NUM_INCOME_BYTE 32
#define CHAR_PRELIM     0x42
#define NUM_DATA_BYTE   29
#define CHECK_BYTE      30
#define PM1_BYTE        10
#define PM25_BYTE       12
#define PM10_BYTE       14

class PMS5003 {
public:
    //void processData(int *PM1, int *PM25, int *PM10);
    void processDataOn(HardwareSerial &serial);
    int getPM1();
    int getPM25();
    int getPM10();
private:
    int pm1;
    int pm25;
    int pm10;
};
#endif

Here's how I'm using it:

struct ParticulateMatterMeasurements {
private:
    bool _areValid = false;
    int PM01Value = 0;
    int PM25Value = 0;
    int PM10Value = 0;
public:
    void setAreValid(bool _areValid) {
        ParticulateMatterMeasurements::_areValid = _areValid;
    }

    bool getAreValid() const {
        return _areValid;
    }

    int getPM01Value() const {
        return PM01Value;
    }

    void setPM01Value(int PM01Value) {
        ParticulateMatterMeasurements::PM01Value = PM01Value;
    }

    int getPM25Value() const {
        return PM25Value;
    }

    void setPM25Value(int PM25Value) {
        ParticulateMatterMeasurements::PM25Value = PM25Value;
    }

    int getPM10Value() const {
        return PM10Value;
    }

    void setPM10Value(int PM10Value) {
        ParticulateMatterMeasurements::PM10Value = PM10Value;
    }
};

ParticulateMatterMeasurements getMeasurements() {
    ParticulateMatterMeasurements measurements;
    measurements.setAreValid(false);
    pms5003.processDataOn(Serial);
    measurements.setPM01Value(pms5003.getPM1());
    measurements.setPM25Value(pms5003.getPM25());
    measurements.setPM10Value(pms5003.getPM10());
    if (measurements.getPM01Value() != -1 && measurements.getPM25Value() != -1 && measurements.getPM10Value() != -1) {
        measurements.setAreValid(true);
    }
    return measurements;
}

The problem is that I get many checksum errors. During 60 measurements I get about 100 of: #[exception] PM2.5 Sensor CHECKSUM ERROR!.

What could be the problem here?

@edit

I ran a test where I print what PMS5003 sends to my ESP8266. It looks like the checksum which is the last byte is sometimes not sent. Instead, I get 66 usually but I can see sometimes 66 77 instead of the last 2 bytes as well.

66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 201 2 216 0 86 0 8 0 3 0 1 145 0 3 172 
16
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 201 2 216 0 86 0 8 0 3 0 1 145 0 3 172 
16
66 77 0 28 0 14 0 18 0 22 0 14 0 18 0 22 10 32 2 239 0 93 0 9 0 3 0 1 145 0 3 45 
18
66 77 0 28 0 14 0 18 0 22 0 14 0 18 0 22 10 32 2 239 0 93 0 9 0 3 0 1 145 0 3 66 
18
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 10 50 2 231 0 88 0 9 0 3 0 1 145 0 3 42 
17
66 77 0 28 0 12 0 16 0 20 0 12 0 16 0 20 9 225 2 208 0 90 0 8 0 3 0 1 145 0 3 190 
16
66 77 0 28 0 13 0 17 0 21 0 13 0 17 0 21 9 225 2 208 0 90 0 8 0 3 0 1 145 0 3 196 
17
66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 249 2 211 0 76 0 5 0 0 0 0 145 0 3 188 
15
66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 249 2 211 0 76 0 5 0 0 0 0 145 0 3 188 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 210 2 188 0 70 0 5 0 0 0 0 145 0 3 118 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 210 2 188 0 70 0 5 0 0 0 0 145 0 3 118 
15
66 77 0 28 0 13 0 16 0 17 0 13 0 16 0 17 9 198 2 183 0 78 0 5 0 0 0 0 145 0 3 115 
16
66 77 0 28 0 13 0 16 0 17 0 13 0 16 0 17 9 198 2 183 0 78 0 5 0 0 0 0 145 0 3 66 
16
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 16 0 18 0 12 0 16 0 18 9 234 2 195 0 87 0 8 0 1 0 0 145 0 3 176 
16
66 77 0 28 0 12 0 16 0 18 0 12 0 16 0 18 9 234 2 195 0 87 0 8 0 1 0 0 145 0 3 176 
16
66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 186 2 184 0 77 0 6 0 1 0 0 145 0 3 101 
15
66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 186 2 184 0 77 0 6 0 1 0 0 145 0 3 107 
16
66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 186 2 184 0 77 0 6 0 1 0 0 145 0 3 107 
16
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 165 2 180 0 76 0 6 0 1 0 0 145 0 3 83 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 165 2 180 0 76 0 6 0 1 0 0 145 0 3 83 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 156 2 186 0 76 0 6 0 1 0 0 145 0 3 80 
17
66 77 0 28 0 12 0 16 0 17 0 12 0 16 0 17 9 156 2 186 0 76 0 6 0 1 0 0 145 0 3 66 
16
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 16 0 17 0 12 0 16 0 17 9 249 2 200 0 67 0 6 0 1 0 0 145 0 3 172 
16
66 77 0 28 0 12 0 16 0 17 0 12 0 16 0 17 9 249 2 200 0 67 0 6 0 1 0 0 145 0 3 172 
16
66 77 0 28 0 12 0 16 0 17 0 12 0 16 0 17 9 231 2 197 0 73 0 7 0 1 0 0 145 0 3 158 
16
66 77 0 28 0 12 0 16 0 17 0 12 0 16 0 17 9 231 2 197 0 73 0 7 0 1 0 0 145 0 3 158 
16
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 243 2 194 0 73 0 7 0 1 0 0 145 0 3 173 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 243 2 194 0 73 0 7 0 1 0 0 145 0 3 173 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 231 2 190 0 70 0 8 0 1 0 0 145 0 3 155 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 231 2 190 0 70 0 8 0 1 0 0 145 0 3 155 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 65 2 219 0 68 0 7 0 1 0 0 145 0 3 66 
17
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 65 2 219 0 68 0 7 0 1 0 0 145 0 3 18 
17
66 77 0 28 0 13 0 18 0 20 0 13 0 18 0 20 10 86 2 218 0 79 0 11 0 2 0 0 145 0 3 58 
18
66 77 0 28 0 13 0 18 0 20 0 13 0 18 0 20 10 86 2 218 0 79 0 11 0 2 0 0 145 0 3 58 
18
66 77 0 28 0 13 0 18 0 19 0 13 0 18 0 19 10 86 2 216 0 76 0 8 0 1 0 0 145 0 3 47 
18
66 77 0 28 0 13 0 18 0 19 0 13 0 18 0 19 10 86 2 216 0 76 0 8 0 1 0 0 145 0 3 47 
18
66 77 0 28 0 14 0 18 0 20 0 14 0 18 0 20 10 212 2 250 0 75 0 8 0 1 0 0 145 0 3 210 
18
66 77 0 28 0 14 0 18 0 20 0 14 0 18 0 20 10 212 2 250 0 75 0 8 0 1 0 0 145 0 3 210 
18
66 77 0 28 0 12 0 17 0 20 0 12 0 17 0 20 10 137 2 234 0 86 0 10 0 1 0 0 145 0 3 126 
17
66 77 0 28 0 12 0 17 0 20 0 12 0 17 0 20 10 137 2 234 0 86 0 10 0 1 0 0 145 0 3 126 
17
66 77 0 28 0 12 0 17 0 20 0 12 0 17 0 20 10 137 2 234 0 86 0 10 0 1 0 0 145 0 3 66 
17
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 164 2 240 0 95 0 10 0 1 0 0 145 0 3 178 
19
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 164 2 240 0 95 0 10 0 1 0 0 145 0 3 178 
19
66 77 0 28 0 13 0 19 0 21 0 13 0 19 0 21 10 110 2 223 0 95 0 10 0 1 0 0 145 0 3 105 
19
66 77 0 28 0 13 0 19 0 21 0 13 0 19 0 21 10 110 2 223 0 95 0 10 0 1 0 0 145 0 3 105 
19
66 77 0 28 0 13 0 19 0 21 0 13 0 19 0 21 10 128 2 227 0 101 0 10 0 1 0 0 145 0 3 133 
19
66 77 0 28 0 13 0 19 0 21 0 13 0 19 0 21 10 128 2 227 0 101 0 10 0 1 0 0 145 0 3 133 
19
66 77 0 28 0 14 0 20 0 24 0 14 0 20 0 24 10 158 2 254 0 106 0 13 0 4 0 0 145 0 3 211 
20
66 77 0 28 0 13 0 19 0 23 0 13 0 19 0 23 10 158 2 254 0 106 0 13 0 4 0 0 145 0 3 205 
19
66 77 0 28 0 13 0 19 0 23 0 13 0 19 0 23 10 212 3 10 0 107 0 12 0 4 0 0 145 0 3 66 
19
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 14 0 20 0 24 0 14 0 20 0 24 10 212 3 10 0 107 0 12 0 4 0 0 145 0 3 22 
20
66 77 0 28 0 14 0 20 0 24 0 14 0 20 0 24 10 236 3 11 0 109 0 13 0 4 0 0 145 0 3 50 
20
66 77 0 28 0 14 0 20 0 24 0 14 0 20 0 24 10 236 3 11 0 109 0 13 0 4 0 0 145 0 3 50 
20
66 77 0 28 0 15 0 20 0 23 0 15 0 20 0 23 10 254 3 29 0 105 0 9 0 3 0 0 145 0 3 77 
20
66 77 0 28 0 15 0 20 0 23 0 15 0 20 0 23 10 254 3 29 0 105 0 9 0 3 0 0 145 0 3 77 
20
66 77 0 28 0 14 0 19 0 22 0 14 0 19 0 22 11 22 3 40 0 99 0 9 0 3 0 0 145 0 2 101 
19
66 77 0 28 0 14 0 19 0 22 0 14 0 19 0 22 11 22 3 40 0 99 0 9 0 3 0 0 145 0 2 101 
19
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 149 3 6 0 93 0 8 0 3 0 0 145 0 2 184 
19
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 149 3 6 0 93 0 8 0 3 0 0 145 0 2 66 
19
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 14 0 18 0 20 0 14 0 18 0 20 10 140 3 2 0 77 0 5 0 3 0 0 145 0 2 148 
18
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 140 3 2 0 77 0 5 0 3 0 0 145 0 2 142 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 104 2 251 0 77 0 5 0 3 0 0 145 0 3 98 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 104 2 251 0 77 0 5 0 3 0 0 145 0 3 98 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 10 104 2 251 0 77 0 5 0 3 0 0 145 0 3 98 
17
66 77 0 28 0 13 0 18 0 20 0 13 0 18 0 20 10 116 3 11 0 77 0 8 0 3 0 0 145 0 2 134 
18
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 116 3 11 0 77 0 8 0 3 0 0 145 0 2 140 
19
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 137 3 18 0 76 0 7 0 3 0 0 145 0 2 166 
19
[update] This is the newest version.
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 10 137 3 18 0 76 0 7 0 3 0 0 145 0 2 166 
19
66 77 0 28 0 12 0 17 0 18 0 12 0 17 0 18 10 77 2 241 0 77 0 5 0 0 0 0 145 0 3 66 
17
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 17 0 18 0 12 0 17 0 18 10 77 2 241 0 77 0 5 0 0 0 0 145 0 3 54 
17
66 77 0 28 0 13 0 18 0 19 0 13 0 18 0 19 10 32 2 224 0 80 0 6 0 0 0 0 145 0 3 2 
18
66 77 0 28 0 13 0 18 0 19 0 13 0 18 0 19 10 32 2 224 0 80 0 6 0 0 0 0 145 0 3 2 
18
66 77 0 28 0 13 0 19 0 19 0 13 0 19 0 19 10 47 2 240 0 81 0 5 0 0 0 0 145 0 3 35 
19
66 77 0 28 0 12 0 18 0 18 0 12 0 18 0 18 10 47 2 240 0 81 0 5 0 0 0 0 145 0 3 29 
18
66 77 0 28 0 14 0 20 0 20 0 14 0 20 0 20 10 173 3 6 0 90 0 6 0 1 0 1 145 0 2 202 
20
66 77 0 28 0 14 0 20 0 20 0 14 0 20 0 20 10 173 3 6 0 90 0 6 0 1 0 1 145 0 2 202 
20
66 77 0 28 0 14 0 21 0 21 0 14 0 21 0 21 10 233 3 24 0 90 0 9 0 1 0 1 145 0 3 31 
21
66 77 0 28 0 13 0 19 0 22 0 13 0 19 0 22 10 242 3 25 0 84 0 12 0 4 0 2 145 0 3 38 
19
66 77 0 28 0 13 0 19 0 22 0 13 0 19 0 22 10 242 3 25 0 84 0 12 0 4 0 2 145 0 3 38 
19
66 77 0 28 0 13 0 19 0 22 0 13 0 19 0 22 10 242 3 25 0 84 0 12 0 4 0 2 145 0 3 66 
19
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 15 0 20 0 23 0 15 0 20 0 23 11 79 3 40 0 88 0 9 0 4 0 2 145 0 2 156 
20
66 77 0 28 0 15 0 20 0 23 0 15 0 20 0 23 11 79 3 40 0 88 0 9 0 4 0 2 145 0 2 156 
20
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 11 67 3 38 0 83 0 9 0 4 0 2 145 0 2 129 
19
66 77 0 28 0 14 0 19 0 21 0 14 0 19 0 21 11 67 3 38 0 83 0 9 0 4 0 2 145 0 2 129 
19
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 11 58 3 43 0 82 0 8 0 4 0 2 145 0 2 121 
18
66 77 0 28 0 15 0 19 0 22 0 15 0 19 0 22 11 58 3 43 0 82 0 8 0 4 0 2 145 0 2 127 
19
66 77 0 28 0 15 0 19 0 21 0 15 0 19 0 21 11 31 3 37 0 79 0 7 0 4 0 2 145 0 2 88 
19
66 77 0 28 0 15 0 19 0 21 0 15 0 19 0 21 11 31 3 37 0 79 0 7 0 4 0 2 145 0 2 88 
19
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 203 3 11 0 84 0 8 0 4 0 2 145 0 2 66 
18
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 203 3 11 0 84 0 8 0 4 0 2 145 0 2 235 
18
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 197 3 5 0 84 0 9 0 4 0 2 145 0 2 224 
18
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 197 3 5 0 84 0 9 0 4 0 2 145 0 2 224 
18
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 179 3 8 0 81 0 8 0 3 0 1 145 0 2 203 
18
66 77 0 28 0 14 0 18 0 21 0 14 0 18 0 21 10 179 3 8 0 81 0 8 0 3 0 1 145 0 2 203 
18
66 77 0 28 0 14 0 17 0 20 0 14 0 17 0 20 10 98 2 243 0 75 0 8 0 3 0 1 145 0 3 90 
17
66 77 0 28 0 14 0 17 0 20 0 14 0 17 0 20 10 98 2 243 0 75 0 8 0 3 0 1 145 0 3 90 
17
66 77 0 28 0 14 0 17 0 19 0 14 0 17 0 19 10 116 2 246 0 77 0 5 0 3 0 1 145 0 3 108 
17
66 77 0 28 0 14 0 17 0 19 0 14 0 17 0 19 10 116 2 246 0 77 0 5 0 3 0 1 145 0 3 66 
17
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 14 0 17 0 19 0 14 0 17 0 19 10 74 2 232 0 80 0 3 0 1 0 1 145 0 3 51 
17
66 77 0 28 0 14 0 17 0 19 0 14 0 17 0 19 10 74 2 232 0 80 0 3 0 1 0 1 145 0 3 51 
17
66 77 0 28 0 14 0 17 0 19 0 14 0 17 0 19 10 74 2 232 0 80 0 3 0 1 0 1 145 0 3 51 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 246 2 220 0 79 0 3 0 1 0 1 145 0 3 205 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 246 2 220 0 79 0 3 0 1 0 1 145 0 3 205 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 9 255 2 222 0 82 0 6 0 1 0 1 145 0 3 224 
17
66 77 0 28 0 13 0 17 0 19 0 13 0 17 0 19 9 255 2 222 0 82 0 6 0 1 0 1 145 0 3 224 
17
66 77 0 28 0 14 0 18 0 20 0 14 0 18 0 20 10 89 2 242 0 85 0 6 0 1 0 1 145 0 3 88 
18
66 77 0 28 0 14 0 18 0 20 0 14 0 18 0 20 10 89 2 242 0 85 0 6 0 1 0 1 145 0 3 66 
18
#[exception] PM2.5 Sensor CHECKSUM ERROR!

After some time I get more errors:

66 77 0 28 0 13 0 17 0 66 77 0 28 0 13 0 17 0 17 0 13 0 17 0 17 9 129 2 177 0 65 0 
7168
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 17 0 17 0 66 77 0 28 0 13 0 17 0 17 0 13 0 17 0 17 9 129 2 177 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 17 0 17 0 66 77 0 28 0 12 0 16 0 16 0 12 0 16 0 16 9 186 2 186 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 16 0 16 0 12 0 16 0 16 9 186 2 186 0 67 0 6 0 0 0 0 145 0 3 92 
16
66 77 0 28 0 12 0 16 0 16 0 12 0 16 0 16 9 174 2 193 0 62 0 5 0 0 0 0 145 0 3 81 
16
66 77 0 28 0 13 0 17 0 17 0 13 0 17 0 17 9 174 2 193 0 62 0 5 0 0 0 0 145 0 3 87 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 180 2 197 0 62 0 6 0 1 0 1 145 0 3 102 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 180 2 197 0 62 0 6 0 1 0 1 145 0 3 102 
17
66 77 0 28 0 13 0 17 0 18 0 13 0 17 0 18 9 165 2 190 0 62 0 6 0 1 0 1 145 0 3 80 
17
66 77 0 28 0 13 0 17 0 66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 135 2 182 0 65 0 
7168
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 16 0 18 0 66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 135 2 182 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 117 2 180 0 64 0 3 0 2 0 1 145 0 3 20 
16
66 77 0 28 0 13 0 16 0 18 0 66 77 0 28 0 13 0 16 0 18 0 13 0 16 0 18 9 117 2 180 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 153 2 196 0 64 0 3 0 2 0 1 145 0 3 66 
15
66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 153 2 196 0 64 0 3 0 2 0 1 145 0 3 66 
15
[update] This is the newest version.
66 77 0 28 0 12 0 16 0 19 0 12 0 16 0 19 9 162 2 206 0 71 0 6 0 5 0 2 145 0 3 105 
16
66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 9 162 2 206 0 71 0 6 0 5 0 2 145 0 3 111 
17
66 77 0 28 0 13 0 16 0 19 0 13 0 16 0 19 9 123 2 189 0 74 0 6 0 5 0 2 145 0 3 54 
16
66 77 0 28 0 13 0 16 0 66 77 0 28 0 12 0 16 0 20 0 12 0 16 0 20 9 54 2 166 0 76 0 
7168
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 12 0 16 0 20 0 12 0 16 0 20 9 54 2 166 0 76 0 7 0 6 0 2 145 0 2 222 
16
66 77 0 28 0 11 0 15 0 19 0 66 77 0 28 0 11 0 15 0 19 0 11 0 15 0 19 9 21 2 166 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 11 0 15 0 19 0 66 77 0 28 0 12 0 16 0 20 0 12 0 16 0 20 9 21 2 166 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 120 2 189 0 83 0 9 0 6 0 1 145 0 3 65 
16
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 120 2 189 0 83 0 9 0 6 0 1 145 0 3 65 
16
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 186 2 198 0 74 0 9 0 5 0 1 145 0 3 130 
16
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 186 2 198 0 74 0 9 0 5 0 1 145 0 3 130 
16
66 77 0 28 0 13 0 15 0 19 0 13 0 15 0 19 9 171 2 184 0 74 0 9 0 5 0 1 145 0 3 97 
15
66 77 0 28 0 13 0 15 0 66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 198 2 190 0 77 0 
7168
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 16 0 20 0 13 0 16 0 20 9 198 2 190 0 77 0 8 0 5 0 1 145 0 3 136 
16
66 77 0 28 0 13 0 16 0 19 0 66 77 0 28 0 13 0 16 0 19 0 13 0 16 0 19 9 225 2 190 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 10 23 2 214 0 81 0 9 0 3 0 0 145 0 2 246 
17
66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 10 23 2 214 0 81 0 9 0 3 0 0 145 0 2 246 
17
66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 10 23 2 214 0 81 0 9 0 3 0 0 145 0 2 246 
17
66 77 0 28 0 13 0 17 0 20 0 66 77 0 28 0 13 0 17 0 20 0 13 0 17 0 20 10 98 2 234 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!

The lonely number in the new line after 32 bytes is the value of PM2.5 which shouldn't get high. However, it gets sometimes high and sometimes not when the checksum is incorrect.

I wonder why the situation changes over time... Maybe I could keep resetting the UART somehow?

Defozo
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    in your question you say you are using "virtual UART", but in your code you are using `Serial` for reading sensor data at the same time as printing debug info. Have you tried using a dedicated `SoftwareSerial` for the sensor? I am guessing that serial communications are being interrupted/garbled when the Arduino tries to talk to the serial moniter while simultaneously talking to the sensor, thus causing a checksum error. – ContinuousLoad Sep 26 '17 at 16:34
  • Oops, you're right, I'm using the hardware UART. My mistake. However, I'm not using this `Serial` often. On my serial monitor, I can see only this checksum error and one line of information about update availability every 60 measurements. My devices aren't even connected to the PC when a measurement is being done (I mean, the ESP8266 sends the measurements to my server and I noticed that there are delays, so I connected the device to my PC to investigate but in normal conditions it's not connected). So, I'm not sure if this is the case and I can't change the hardware design now. – Defozo Sep 26 '17 at 19:19
  • If you want to determine if checksum errors are occurring even when the PC is disconnected, you could program the ESP8266 to flash an LED for moment whenever an error occurs. Or, you could log errors into a file using the ESP8266 built-in filesystem library called [SPIFFS](http://www.instructables.com/id/Using-ESP8266-SPIFFS/). – ContinuousLoad Sep 26 '17 at 21:33
  • I know, but I'm pretty sure they do. The delays are the evidence. My ESP8266 should send the measurements every minute but it does every ~3 minutes. All indicate that checksums don't match. Though, I appreciate your help. – Defozo Sep 26 '17 at 21:47
  • Thanks for the update. Hmm.. I noticed that the PMS5003 is a 5V sensor (and of course ESP8266 is 3.3V). You probably already thought of this, but is each device getting the voltage it expects? Also, sometimes a logic-level converter is required to make 3v3 device play nicely with 5v one. In a pinch, you can just connect a ~4.7k to the RX of the 3v3 device. Hope this helps! – ContinuousLoad Sep 26 '17 at 22:37
  • That's not it. PMS5003 is powered 5V (from USB) while ESP8266 gets 3.3V (through stabilizer) but the PMS5003's UART operates on 3.3V. So, the UART lines are connected without anything. Any other ideas? – Defozo Sep 26 '17 at 23:21
  • I noticed that the number of checksum errors increases over time. There are usually 7 errors at the beginning and then after some time it increases to ~15, then to ~30... – Defozo Sep 27 '17 at 11:55
  • Reasons why UARTs and RS-232 has been around for so long in spite of its drawbacks are that it is cheap & easy to implement and reliable. IOW unless you're operating in an EMI storm with long cables, checksum errors should not be occurring (assuming you have the same checksum calculation on each end). You fail to mention (and verify) the fundamental stuff, such as the port configurations of both sides. Also consider the possibility that your code is reading too slow. The data view with "errors" has numerous truncated records as indicated by `66 77` byte pairs. – sawdust Sep 28 '17 at 01:25
  • Your code has a potential bug. You have hardcoded the message length, instead of using the message length embedded in the message itself. You are also only checking for the first start char, when there are two chars that should be scanned (to avoid false positives) . – sawdust Sep 28 '17 at 01:33

2 Answers2

2

For each of the packets that fail the checksum test, you find a CHAR_PRELIM (66) either in the middle or at the end. This means the sensor is occasionally dropping packets and causing misalignment.

One solution is to restart packet reading each time a 66 is read. This code should do it:

UPDATE: as per @sawdust's comment, the presence of both 66 and 77 should be used as a start condition because it may be possible for 66 to appear by itself in the data. The other consideration is to use the packet length provided by the 3rd and 4th bytes instead of assuming the length to be 32. Hopefully these improvements make the code more durable.

size_t length;
incomingByte[0] = 66; // the first two bytes are always known
incomingByte[1] = 77;

... 

if (serial.available()) {
  if (serial.read() == 66 && serial.read() == 77)
    incomingByte[2] = serial.read(); // length high byte
    incomingByte[3] = serial.read(); // length low byte
    int length = (incomingByte[2] << 8) + incomingByte[3];

    // starting at index 4, read `length` bytes
    serial.readBytes(incomingByte + 4, length);
    break;
  }
}

// when the code breaks out of the while(1) loop, you still need to evaluate the checksum.

According to the protocol defined by this source, the packet length is fixed at 32 bytes, so the encoded frame length (bytes 3 and 4) should always equal 0 28 (32 bytes - 2 start bytes - 2 frame length bytes = 28).

However, this code should work even for variable length packets (thanks @sawdust).

Fair warning: I do not have one of these sensors, so obviously I didn't test this, but the concept remains.

I recognize that this code won't solve the issue of characters being dropped, since it just ignores incomplete packets and you still rely on the validity of the checksum.

Finally, I find it interesting that the reason that the checksum is failing is because the checksum bytes are not even being received in those cases!

Hope this helps!

UPDATE #2: This is more or less a revised answer in it of itself.

Using this code to read packets, the following criteria (which are defined by the protocol) are guaranteed:

  1. The packets begins with [66 77]
  2. The packet contains 32 bytes
  3. The start condition [66 77] will never occur in the body of the packet.

Here's the code. I manage to reduce it down to a few if statements

void PMS5003::processDataOn(HardwareSerial &serial) {
    bool possibleStart = false;
    incomeByte[0] = 66;
    incomeByte[1] = 77;
    uint8_t count = 0;

    ...

    while (1) {
        ...

        if (serial.available()) {    
            uint8_t c = serial.read();

            if (possibleStart) {
                possibleStart = false;
                if (c == 77) count = 2;
            }
            if (c == 66) possibleStart = true;

            if (count >= 2) incomeByte[count++] = c;
            if (count == NUM_DATA_BYTE) break;      
        }
    }

    // at this point, incomeByte must:\
    // > begin with [66 77]
    // > contain 32 bytes
    // > not contain [66 77] anywhere after the first two bytes
    // > therefore, it is guaranteed to contain a checksum

    // now is the right time to evaluate the checksum.
    // I expect all of the checksums to match, but you might as well check
}

At the time of posting, the OP has already coded a solution which fulfills the requirements. I am posting this because I believe this code improves upon the OP's by being more concise, more readable/declarative, and hopefully more easily manageable.

This code can also serve as a general solution for any case in which two characters define a start condition, provided the packet length is known or can be determined.

ContinuousLoad
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    *"One solution is to restart packet reading each time a 66 is read."* -- That could generate false positives. Since two byes are defined as start codes, 0x42 and 0x4D, both bytes should be used, rather than just the first. – sawdust Sep 28 '17 at 01:19
  • Thank you for the answer. I was working on the solution in the same time your answer was posted. In comparison to yours, in my code, I added checking if there's another `66 77` between the found `66 77` and the data I'm looking for. I hope it'll make the code even more durable. I'm not sure if I should edit your answer or provide a new one or edit my own question? – Defozo Sep 28 '17 at 09:56
  • I'm not sure if I understand your last comment correctly. I can't evaluate the checksum if there's no checksum at all, right? I just need to hope that the data is correct. – Defozo Sep 28 '17 at 10:00
  • @Defozo: Sorry, I did not mean to introduce confusion :) You are right, I was just commenting that it was ironic that in most cases the missing checksum bytes *were the reason for the failed checksum check*. The ideal purpose of a checksum is to verify that all the *data bytes* were received correctly, but in this case, it is the *checksum* itself that got dropped. This occurs, for example, in the first error you provided in your question: all 28 *data bytes* are present, but no checksum! – ContinuousLoad Sep 28 '17 at 14:05
  • I think there is a bug in your UPDATE #2 code, should be `count = 1` and `if (count >= 1)` to avoid writing `77` to third position (`incomeByte[2]`). – Igor Jerosimić Mar 24 '21 at 19:56
0

While the question why the data comes corrupted still remains, here is a workaround I managed to achieve:

  1. I'm checking the checksum, if it's incorrect, then:
  2. I'm looking for 66 77 in the whole data. When I find it:
  3. I'm checking if in the next 16 bytes there's another 66 77. If it's not found:
  4. I'm presuming the values that are distanced by 10-15 bytes from 66 77 are the ones I'm looking for (PM1, PM2.5, PM10).

Here's the code:

void PMS5003::processDataOn(HardwareSerial &serial) {
  unsigned long timeout = millis();
  int count = 0;
  byte incomeByte[NUM_INCOME_BYTE];
  boolean startcount = false;
  byte data;
  int timeoutHops = 0;
  while (1){
    if (((millis() - timeout) > 1000) && (timeoutHops == 0)) {
      timeoutHops = 1;
      yield();
      ESP.wdtFeed();
    }
    if (((millis() - timeout) > 2000) && (timeoutHops == 1)) {
      timeoutHops = 2;
      yield();
      ESP.wdtFeed();
    }
    if ((millis() - timeout) > 3000) {
      yield();
      ESP.wdtFeed();
      Serial.println("SENSOR-ERROR-TIMEOUT");
      break;
    }
    if (serial.available()) {
      data = serial.read();
      if (data == CHAR_PRELIM && !startcount) {
        startcount = true;
        count++;
        incomeByte[0] = data;
      } else if (startcount) {
        count++;
        incomeByte[count - 1] = data;
        if (count >= NUM_INCOME_BYTE){
          break;
        }
      }
    }
  }
  unsigned int calcsum = 0;
  unsigned int exptsum;
  for (int a = 0; a < NUM_INCOME_BYTE; a++) {
    Serial.print((unsigned int)incomeByte[a]);
    Serial.print(" ");
  }
  Serial.println();
  Serial.println(((unsigned int)incomeByte[PM25_BYTE] << 8) + (unsigned int)incomeByte[PM25_BYTE + 1]);
  for (int i = 0; i < NUM_DATA_BYTE; i++) {
    calcsum += (unsigned int)incomeByte[i];
  }
  exptsum = ((unsigned int)incomeByte[CHECK_BYTE] << 8) + (unsigned int)incomeByte[CHECK_BYTE + 1];
  if (calcsum == exptsum) {
    pm1 = ((unsigned int)incomeByte[PM1_BYTE] << 8) + (unsigned int)incomeByte[PM1_BYTE + 1];
    pm25 = ((unsigned int)incomeByte[PM25_BYTE] << 8) + (unsigned int)incomeByte[PM25_BYTE + 1];
    pm10 = ((unsigned int)incomeByte[PM10_BYTE] << 8) + (unsigned int)incomeByte[PM10_BYTE + 1];
  } else {
    Serial.println("#[exception] PM2.5 Sensor CHECKSUM ERROR!");
    pm1 = -1;
    pm25 = -1;
    pm10 = -1;
    for (int a = 0; a < NUM_INCOME_BYTE; a++) {
      bool valid = true;
      if (((unsigned int)incomeByte[a] == 66) && ((unsigned int)incomeByte[a+1] == 77)) {
        if (a+16 < NUM_INCOME_BYTE) {
          for (int b = a+1; b < a+15; b++) {
            if (((unsigned int)incomeByte[b] == 66) && ((unsigned int)incomeByte[b+1] == 77)) {
              valid = false;
              break;
            }
          }
          if (valid) {
            pm1 = ((unsigned int)incomeByte[a+10] << 8) + (unsigned int)incomeByte[a+11];
            pm25 = ((unsigned int)incomeByte[a+12] << 8) + (unsigned int)incomeByte[a+13];
            pm10 = ((unsigned int)incomeByte[a+14] << 8) + (unsigned int)incomeByte[a+15];
            Serial.println("valid: ");
            Serial.print(pm1);
            Serial.print(" ");
            Serial.print(pm25);
            Serial.print(" ");
            Serial.print(pm10);
            Serial.println();
            break;
          }
        }
      }
    }
  }
  return;
}

Theoretically, it may produce false positives or negatives but in practice, it just works.

66 77 0 28 0 12 0 15 0 17 0 12 0 15 0 17 9 102 2 176 66 77 0 28 0 12 0 15 0 16 0 12 
15
#[exception] PM2.5 Sensor CHECKSUM ERROR!
valid: 
12 15 17
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 114 2 175 0 73 0 4 0 1 0 0 145 0 3 12 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 114 2 175 0 73 0 4 0 1 0 0 145 0 3 12 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 141 2 190 0 72 0 3 0 1 0 0 145 0 3 52 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 141 2 190 0 72 0 3 0 1 0 0 145 0 3 52 
15
66 77 0 28 0 12 0 16 0 16 0 12 0 16 0 16 9 198 2 202 0 75 0 3 0 0 0 0 145 0 3 125 
16
66 77 0 28 0 12 0 16 0 16 0 66 77 0 28 0 12 0 16 0 16 0 12 0 16 0 16 9 198 2 202 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
valid: 
12 16 16
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 174 2 199 0 71 0 3 0 0 0 0 145 0 3 92 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 174 2 199 0 71 0 3 0 0 0 0 145 0 3 92 
15
66 77 0 28 0 12 0 15 0 16 0 12 0 15 0 16 9 174 2 199 66 77 0 28 0 13 0 16 0 16 0 13 
15
#[exception] PM2.5 Sensor CHECKSUM ERROR!
valid: 
12 15 16
66 77 0 28 0 13 0 16 0 16 0 13 0 16 0 16 9 213 2 205 0 72 0 3 0 0 0 0 145 0 3 142 
16
66 77 0 28 0 13 0 16 0 16 0 13 0 16 0 16 9 213 2 205 0 72 0 3 0 0 0 0 145 0 3 142 
16
66 77 0 28 0 13 0 16 0 17 0 13 0 16 0 17 9 207 2 208 0 83 0 6 0 1 0 0 145 0 3 156 
16
66 77 0 28 0 13 0 16 0 17 0 13 0 16 0 17 9 207 2 208 0 83 0 6 0 1 0 0 145 0 3 156 
16
66 77 0 28 0 13 0 17 0 17 0 13 0 17 0 17 9 159 2 202 0 87 0 5 0 1 0 0 145 0 3 107 
17
66 77 0 28 0 13 0 17 0 17 0 66 77 0 28 0 13 0 17 0 17 0 13 0 17 0 17 9 159 2 202 0 
19712
#[exception] PM2.5 Sensor CHECKSUM ERROR!
valid: 
13 17 17
Defozo
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    You could technically save just a bit of memory by using `uint8_t` instead of `unsigned int` for bytes and `uint16_t` for words (2-byte values, like checksum) since bytes, by definition, consist of 8 bits. Since `int` types on ESP8266 use 4 bytes, you would save 2 or 3 bytes per instance. As an added bonus, `uintXX_t` is a few characters shorter! From a viewers perspective, it would clearly show how many bytes are expected to be stored in a given variable. Anyway, glad to here you are getting satisfying results! – ContinuousLoad Sep 28 '17 at 14:25
  • *"4. I'm presuming the values that are distanced by..."* -- **Using data from a message that fails validation is a very bad idea.** For a basic parsing algorithm (which you partially already figured out) see https://stackoverflow.com/questions/16177947/identification-of-packets-in-a-byte-stream/16180135#16180135 Since there's a two-byte start code (but your code still only tests one byte), you need to account for byte alignment i.e. see the 2-byte test & parsing code in https://stackoverflow.com/questions/43280740/parsing-complete-messages-from-serial-port/43287464#43287464. – sawdust Sep 28 '17 at 23:21