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Forum Update on Monday, May 27th 2025

read data from serial port

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  • I isan

    @mrjj I should use this :

    for ( const QString& valline : values ) {
        qDebug() << "val =" << valline.toInt();
      }
    

    instead of this :

    x= values[0].toInt();
    

    am I right?

    qDebug() << "val =" << valline.toInt();
    

    always returns zero to me!

    mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by
    #10

    @isan
    hmm
    maybe input is not as you expect then.
    try qDebug() << values;
    to see what it has

    I 1 Reply Last reply
    0
    • mrjjM mrjj

      @isan
      hmm
      maybe input is not as you expect then.
      try qDebug() << values;
      to see what it has

      I Offline
      I Offline
      isan
      wrote on last edited by
      #11

      @mrjj said in read data from serial port:

      qDebug() << values;

      return :
      \n191", "2018/8/3", "23:28:55 \r\n187", "2018/8/3", "23:28:55 \r\n185", "2018/8/3", "23:28:55 \r\n164", "2018/8/3", "23:28:55 \r\n131", "2018/8/3", "23:28:55 \r\n103", "2018/8/3", "23:28:55 \r\n....................................

      1 Reply Last reply
      0
      • mrjjM Offline
        mrjjM Offline
        mrjj
        Lifetime Qt Champion
        wrote on last edited by mrjj
        #12

        Ok so when we only split at ","
        We get \n\r on some/all of the values.
        Using something Like ReadLine might be easier.
        but you seem to use native serial comm and not Qt version?
        but you can try
        for ( const QString& valline : values ) {
        qDebug() << "val =" << valline.trimmed().toInt();
        }
        Trimmed() should remove the \n\r
        http://doc.qt.io/qt-5/qstring.html#trimmed

        ah. wait.
        it looks like some of the values are in same index as timestamp
        "23:28:55 \r\n187"

        1 Reply Last reply
        1
        • mrjjM Offline
          mrjjM Offline
          mrjj
          Lifetime Qt Champion
          wrote on last edited by mrjj
          #13

          Hi
          Im not really sure what is happening.
          Seems to not to be structured as we think/needs more splitting

          QString input{"85,2018/8/3,17:21:3\\n\\r85,2018/8/3,17:21:3\\n\\r95,2018/8/3,17:21:3"};
            QStringList lines = input.split("\\n\\r");
            for ( const QString& line : lines) {
              QStringList values = line.split(",");
              for ( const QString& valline : values ) {
                qDebug() << "val =" << valline.trimmed().toInt();
              }
            }
          

          val = 85
          val = 0
          val = 0
          val = 85
          val = 0
          val = 0
          val = 95
          val = 0
          val = 0

          when do you start processing the input ?
          After it all have been read or how do u know that input is ready ?

          I 1 Reply Last reply
          1
          • mrjjM mrjj

            Hi
            Im not really sure what is happening.
            Seems to not to be structured as we think/needs more splitting

            QString input{"85,2018/8/3,17:21:3\\n\\r85,2018/8/3,17:21:3\\n\\r95,2018/8/3,17:21:3"};
              QStringList lines = input.split("\\n\\r");
              for ( const QString& line : lines) {
                QStringList values = line.split(",");
                for ( const QString& valline : values ) {
                  qDebug() << "val =" << valline.trimmed().toInt();
                }
              }
            

            val = 85
            val = 0
            val = 0
            val = 85
            val = 0
            val = 0
            val = 95
            val = 0
            val = 0

            when do you start processing the input ?
            After it all have been read or how do u know that input is ready ?

            I Offline
            I Offline
            isan
            wrote on last edited by isan
            #14

            @mrjj it's return zero and sometimes 2018

            when do you start processing the input ?
            After it all have been read or how do u know that input is ready ?

            I'm not sure what you mean?
            read from serial port in thread and thread start in other class constructor.
            so it's start when program run!
            data send from arduino that connected to raspberry pi with USB port

            mrjjM 1 Reply Last reply
            0
            • I isan

              @mrjj it's return zero and sometimes 2018

              when do you start processing the input ?
              After it all have been read or how do u know that input is ready ?

              I'm not sure what you mean?
              read from serial port in thread and thread start in other class constructor.
              so it's start when program run!
              data send from arduino that connected to raspberry pi with USB port

              mrjjM Offline
              mrjjM Offline
              mrjj
              Lifetime Qt Champion
              wrote on last edited by
              #15

              @isan
              Looking at the code, it seems to me you
              send al data and thread will read it all. and then close serial.
              so

              while (serialDataAvail(fd) > -2) {
                //-------value format is int-------
                value = serialGetchar (fd) ;
                //--------vs format is QString------
                vs.push_back(value);  
               }
               serialClose(fd);
               
               // here u should have complete data and can use 
              to get the values if data is complete
              input would be vs
              
                QStringList lines = input.split("\\n\\r");
                for ( const QString& line : lines) {
                  QStringList values = line.split(",");
                  for ( const QString& valline : values ) {
                    qDebug() << "val =" << valline.trimmed().toInt();
                  }
                }
              }
              
              JonBJ I 2 Replies Last reply
              1
              • mrjjM mrjj

                @isan
                Looking at the code, it seems to me you
                send al data and thread will read it all. and then close serial.
                so

                while (serialDataAvail(fd) > -2) {
                  //-------value format is int-------
                  value = serialGetchar (fd) ;
                  //--------vs format is QString------
                  vs.push_back(value);  
                 }
                 serialClose(fd);
                 
                 // here u should have complete data and can use 
                to get the values if data is complete
                input would be vs
                
                  QStringList lines = input.split("\\n\\r");
                  for ( const QString& line : lines) {
                    QStringList values = line.split(",");
                    for ( const QString& valline : values ) {
                      qDebug() << "val =" << valline.trimmed().toInt();
                    }
                  }
                }
                
                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on last edited by JonB
                #16

                @mrjj
                I have no idea of the implications, but you have pasted code for OP as:

                  QStringList lines = input.split("\\n\\r");
                

                If he is supposed to be copying this, shouldn't he be using

                  QStringList lines = input.split("\r\n");
                

                Or, from his qDebug(), does it mean that the input literally has the 4-character sequence \r\n in it, and not CR-LF? In which case he would want

                QStringList lines = input.split("\\r\\n");
                
                
                mrjjM 1 Reply Last reply
                3
                • JonBJ JonB

                  @mrjj
                  I have no idea of the implications, but you have pasted code for OP as:

                    QStringList lines = input.split("\\n\\r");
                  

                  If he is supposed to be copying this, shouldn't he be using

                    QStringList lines = input.split("\r\n");
                  

                  Or, from his qDebug(), does it mean that the input literally has the 4-character sequence \r\n in it, and not CR-LF? In which case he would want

                  QStringList lines = input.split("\\r\\n");
                  
                  
                  mrjjM Offline
                  mrjjM Offline
                  mrjj
                  Lifetime Qt Champion
                  wrote on last edited by
                  #17

                  @JonB
                  Thank you , yes u are right its \r\n :) (of cause)
                  or \r\n when escaped.

                  1 Reply Last reply
                  2
                  • JonBJ Offline
                    JonBJ Offline
                    JonB
                    wrote on last edited by JonB
                    #18

                    Now, going back to where @isan wrote:

                    return :
                     \n191", "2018/8/3", "23:28:55 \r\n187", "2018/8/3", "23:28:55 \r\n185", "2018/8/3", "23:28:55 \r\n164", "2018/8/3", "23:28:55 \r\n131", "2018/8/3", "23:28:55 \r\n103", "2018/8/3", "23:28:55 \r\n....................................
                    

                    So if that's really right, after first splitting on "\r\n" (not even certain about that if the input really starts as shown with just \n and not \r\n, I'll just assume it's really \r\n), he then needs to split on "\",\"", not just plain ,. Then at the end of that you have 3 clean tokens per line.

                    I have to say the input looks a bit oddly tokenized/quoted, but that's what corresponds to the input he shows. If you're not careful and leave " characters in, you'll get toInt() returning 0 where you don't expect.

                    mrjjM 1 Reply Last reply
                    2
                    • JonBJ JonB

                      Now, going back to where @isan wrote:

                      return :
                       \n191", "2018/8/3", "23:28:55 \r\n187", "2018/8/3", "23:28:55 \r\n185", "2018/8/3", "23:28:55 \r\n164", "2018/8/3", "23:28:55 \r\n131", "2018/8/3", "23:28:55 \r\n103", "2018/8/3", "23:28:55 \r\n....................................
                      

                      So if that's really right, after first splitting on "\r\n" (not even certain about that if the input really starts as shown with just \n and not \r\n, I'll just assume it's really \r\n), he then needs to split on "\",\"", not just plain ,. Then at the end of that you have 3 clean tokens per line.

                      I have to say the input looks a bit oddly tokenized/quoted, but that's what corresponds to the input he shows. If you're not careful and leave " characters in, you'll get toInt() returning 0 where you don't expect.

                      mrjjM Offline
                      mrjjM Offline
                      mrjj
                      Lifetime Qt Champion
                      wrote on last edited by
                      #19

                      @JonB
                      Hi
                      I think the "s comes from qDebug and is not in the input. (looking at the sending code higher up)
                      It seems he reads the entire load in one go and closes port so that means he should be able to
                      read complete string and then split it. ( i hope )

                      1 Reply Last reply
                      2
                      • mrjjM mrjj

                        @isan
                        Looking at the code, it seems to me you
                        send al data and thread will read it all. and then close serial.
                        so

                        while (serialDataAvail(fd) > -2) {
                          //-------value format is int-------
                          value = serialGetchar (fd) ;
                          //--------vs format is QString------
                          vs.push_back(value);  
                         }
                         serialClose(fd);
                         
                         // here u should have complete data and can use 
                        to get the values if data is complete
                        input would be vs
                        
                          QStringList lines = input.split("\\n\\r");
                          for ( const QString& line : lines) {
                            QStringList values = line.split(",");
                            for ( const QString& valline : values ) {
                              qDebug() << "val =" << valline.trimmed().toInt();
                            }
                          }
                        }
                        
                        I Offline
                        I Offline
                        isan
                        wrote on last edited by isan
                        #20

                        @mrjj said in read data from serial port:

                        @isan
                        Looking at the code, it seems to me you
                        send al data and thread will read it all. and then close serial.
                        so

                        while (serialDataAvail(fd) > -2) {
                          //-------value format is int-------
                          value = serialGetchar (fd) ;
                          //--------vs format is QString------
                          vs.push_back(value);  
                         }
                         serialClose(fd);
                         
                         // here u should have complete data and can use 
                        to get the values if data is complete
                        input would be vs
                        
                          QStringList lines = input.split("\\n\\r");
                          for ( const QString& line : lines) {
                            QStringList values = line.split(",");
                            for ( const QString& valline : values ) {
                              qDebug() << "val =" << valline.trimmed().toInt();
                            }
                          }
                        }
                        

                        Data is always sent and It does not go out of while() and I can not wait for complete data
                        Upon receive, I must use the data in other classes
                        I should not miss any data, if I close the port, the data will be lost

                        mrjjM 1 Reply Last reply
                        0
                        • I isan

                          @mrjj said in read data from serial port:

                          @isan
                          Looking at the code, it seems to me you
                          send al data and thread will read it all. and then close serial.
                          so

                          while (serialDataAvail(fd) > -2) {
                            //-------value format is int-------
                            value = serialGetchar (fd) ;
                            //--------vs format is QString------
                            vs.push_back(value);  
                           }
                           serialClose(fd);
                           
                           // here u should have complete data and can use 
                          to get the values if data is complete
                          input would be vs
                          
                            QStringList lines = input.split("\\n\\r");
                            for ( const QString& line : lines) {
                              QStringList values = line.split(",");
                              for ( const QString& valline : values ) {
                                qDebug() << "val =" << valline.trimmed().toInt();
                              }
                            }
                          }
                          

                          Data is always sent and It does not go out of while() and I can not wait for complete data
                          Upon receive, I must use the data in other classes
                          I should not miss any data, if I close the port, the data will be lost

                          mrjjM Offline
                          mrjjM Offline
                          mrjj
                          Lifetime Qt Champion
                          wrote on last edited by
                          #21

                          @isan
                          ok so it must be on the fly.

                          Then you need to use 2 buffers as not to parse already received data over and over.
                          so in pseudo code

                          Read char from serial
                          tempbuffer += char
                          if char is \n then
                          split tempbuffer
                          emit value
                          clear tempbuffer

                          //i assume u want to store all ?
                          Mainbuffer +=char

                          I 1 Reply Last reply
                          1
                          • mrjjM mrjj

                            @isan
                            ok so it must be on the fly.

                            Then you need to use 2 buffers as not to parse already received data over and over.
                            so in pseudo code

                            Read char from serial
                            tempbuffer += char
                            if char is \n then
                            split tempbuffer
                            emit value
                            clear tempbuffer

                            //i assume u want to store all ?
                            Mainbuffer +=char

                            I Offline
                            I Offline
                            isan
                            wrote on last edited by isan
                            #22

                            @mrjj so the code is like:

                            void MyThread::run()
                            {
                            
                                qDebug("Thread id inside run %d",(int)QThread::currentThreadId());
                            
                                int fd ,x;
                                
                                  
                                if ((fd = serialOpen ("/dev/ttyACM0",230400)) < 0)
                                {
                                    fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
                                }
                            
                                while (serialDataAvail(fd)>-2)
                                {
                                     //-------value format is int -------//
                                    value=serialGetchar (fd) ;
                                 //--------vs format is QString------//
                                     vs.push_back(value);
                                //--------tempbuffer format is QString------//
                               tempbuffer .append( vs);
                               if (vs=="\n")
                              //-------- values format is QStringList------//
                                 values = tempbuffer.split(",");
                                  x= values[0].toInt();
                             msleep(1); 
                                   emit signalValueUpdated(x);
                                   tempbuffer.clear();
                              
                                }
                                serialClose(fd); 
                            }
                            

                            is it true?

                            mrjjM 1 Reply Last reply
                            0
                            • I isan

                              @mrjj so the code is like:

                              void MyThread::run()
                              {
                              
                                  qDebug("Thread id inside run %d",(int)QThread::currentThreadId());
                              
                                  int fd ,x;
                                  
                                    
                                  if ((fd = serialOpen ("/dev/ttyACM0",230400)) < 0)
                                  {
                                      fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
                                  }
                              
                                  while (serialDataAvail(fd)>-2)
                                  {
                                       //-------value format is int -------//
                                      value=serialGetchar (fd) ;
                                   //--------vs format is QString------//
                                       vs.push_back(value);
                                  //--------tempbuffer format is QString------//
                                 tempbuffer .append( vs);
                                 if (vs=="\n")
                                //-------- values format is QStringList------//
                                   values = tempbuffer.split(",");
                                    x= values[0].toInt();
                               msleep(1); 
                                     emit signalValueUpdated(x);
                                     tempbuffer.clear();
                                
                                  }
                                  serialClose(fd); 
                              }
                              

                              is it true?

                              mrjjM Offline
                              mrjjM Offline
                              mrjj
                              Lifetime Qt Champion
                              wrote on last edited by mrjj
                              #23

                              @isan
                              Hi
                              Almost, you need to clear tempbuffer as soon as you have used it.
                              And u need to check last read char (value) being the \n

                              void MyThread::run() {
                              
                                qDebug("Thread id inside run %d", (int)QThread::currentThreadId());
                              
                                int fd, x=0;
                              
                              
                                if ((fd = serialOpen ("/dev/ttyACM0", 230400)) < 0) {
                                  fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
                                }
                              
                                while (serialDataAvail(fd) > -2) {
                                  value = serialGetchar (fd) ;
                                  vs.push_back(value);
                                  tempbuffer += value;
                                  if (value == '\n') {
                                    values = tempbuffer.split(",");
                                    x = values[0].toInt();
                                    tempbuffer.clear();
                                  }
                                  msleep(1);
                                  emit signalValueUpdated(x);
                                  
                              
                                }
                                serialClose(fd);
                              }
                              
                              JonBJ 1 Reply Last reply
                              1
                              • mrjjM mrjj

                                @isan
                                Hi
                                Almost, you need to clear tempbuffer as soon as you have used it.
                                And u need to check last read char (value) being the \n

                                void MyThread::run() {
                                
                                  qDebug("Thread id inside run %d", (int)QThread::currentThreadId());
                                
                                  int fd, x=0;
                                
                                
                                  if ((fd = serialOpen ("/dev/ttyACM0", 230400)) < 0) {
                                    fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
                                  }
                                
                                  while (serialDataAvail(fd) > -2) {
                                    value = serialGetchar (fd) ;
                                    vs.push_back(value);
                                    tempbuffer += value;
                                    if (value == '\n') {
                                      values = tempbuffer.split(",");
                                      x = values[0].toInt();
                                      tempbuffer.clear();
                                    }
                                    msleep(1);
                                    emit signalValueUpdated(x);
                                    
                                
                                  }
                                  serialClose(fd);
                                }
                                
                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #24

                                @mrjj , @isan
                                This code looks wrong. It doesn't help that we don't see the declarations of vs or tempbuffer. Comment:

                                //--------tempbuffer format is QStringList------//

                                No, it isn't, because later you go tempbuffer.split(",");. So it's probably a QString.

                                Then: you read 1 char, you append it to vs. Then you append vs to tempbuffer. If value is not \n, you pick up the next char, append that to vs (now 2 chars long), append that to tempBuffer (now 3 chars long)...

                                Hmm, vs must be a single char, not a QString like the comment says it is? Who knows....

                                Separate issue:

                                while (serialDataAvail(fd)>-2)
                                

                                Nope. serialDataAvail(fd) returns -1 on error. The code accepts that as a legal. It also should therefore never exit the while loop. Further, if 0 bytes are available code goes into the value=serialGetchar (fd) call. After 10 seconds of no data that will return -1. At which point accepts that as the legal char received. Finally, if error opening device in the first place, the code writes a message and then continues into the loop, which makes no sense.

                                All this code really needs correcting....

                                Finally, have a think about the fact that sometimes you are using Qt & C++ functions, sometimes you are doing lowest-level C library calls. Do you really need to mix them?

                                mrjjM 1 Reply Last reply
                                2
                                • JonBJ JonB

                                  @mrjj , @isan
                                  This code looks wrong. It doesn't help that we don't see the declarations of vs or tempbuffer. Comment:

                                  //--------tempbuffer format is QStringList------//

                                  No, it isn't, because later you go tempbuffer.split(",");. So it's probably a QString.

                                  Then: you read 1 char, you append it to vs. Then you append vs to tempbuffer. If value is not \n, you pick up the next char, append that to vs (now 2 chars long), append that to tempBuffer (now 3 chars long)...

                                  Hmm, vs must be a single char, not a QString like the comment says it is? Who knows....

                                  Separate issue:

                                  while (serialDataAvail(fd)>-2)
                                  

                                  Nope. serialDataAvail(fd) returns -1 on error. The code accepts that as a legal. It also should therefore never exit the while loop. Further, if 0 bytes are available code goes into the value=serialGetchar (fd) call. After 10 seconds of no data that will return -1. At which point accepts that as the legal char received. Finally, if error opening device in the first place, the code writes a message and then continues into the loop, which makes no sense.

                                  All this code really needs correcting....

                                  Finally, have a think about the fact that sometimes you are using Qt & C++ functions, sometimes you are doing lowest-level C library calls. Do you really need to mix them?

                                  mrjjM Offline
                                  mrjjM Offline
                                  mrjj
                                  Lifetime Qt Champion
                                  wrote on last edited by mrjj
                                  #25

                                  +@JonB
                                  Thx, yes value should be added to tempbuffer.
                                  Then i can learn not to edit code in forum directly :)
                                  So something like this + all than @JonB said

                                  while (serialDataAvail(fd) > -2) { // fix condition! 
                                      value = serialGetchar (fd) ;// read char
                                      vs.push_back(value); // store in ful buffer
                                      tempbuffer += value; // add to tempbuffer
                                      if (value == '\n') { // if we just read \n
                                        values = tempbuffer.split(","); // split on , to QStringList
                                        x = values[0].toInt(); // take first index and convert to int
                                        tempbuffer.clear(); // clear it for next 
                                      }
                                  ......
                                  
                                  I 1 Reply Last reply
                                  1
                                  • J.HilkJ Offline
                                    J.HilkJ Offline
                                    J.Hilk
                                    Moderators
                                    wrote on last edited by
                                    #26

                                    I‘ve been following this thread for a while now, and you guy‘s doing a great job :-)

                                    Just one thing that‘s been buging me from the beginning.

                                    The call off split on the string. It‘s used for nothing but temporary storage to than turn the first entry into an INT.

                                    I would highly recommand to at least use splitref.


                                    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                                    Q: What's that?
                                    A: It's blue light.
                                    Q: What does it do?
                                    A: It turns blue.

                                    mrjjM 1 Reply Last reply
                                    3
                                    • mrjjM mrjj

                                      +@JonB
                                      Thx, yes value should be added to tempbuffer.
                                      Then i can learn not to edit code in forum directly :)
                                      So something like this + all than @JonB said

                                      while (serialDataAvail(fd) > -2) { // fix condition! 
                                          value = serialGetchar (fd) ;// read char
                                          vs.push_back(value); // store in ful buffer
                                          tempbuffer += value; // add to tempbuffer
                                          if (value == '\n') { // if we just read \n
                                            values = tempbuffer.split(","); // split on , to QStringList
                                            x = values[0].toInt(); // take first index and convert to int
                                            tempbuffer.clear(); // clear it for next 
                                          }
                                      ......
                                      
                                      I Offline
                                      I Offline
                                      isan
                                      wrote on last edited by isan
                                      #27

                                      @mrjj Thanks for keep helping to solve my problem
                                      It's work!

                                      mrjjM 1 Reply Last reply
                                      2
                                      • I isan

                                        @mrjj Thanks for keep helping to solve my problem
                                        It's work!

                                        mrjjM Offline
                                        mrjjM Offline
                                        mrjj
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #28

                                        @isan
                                        Super :) \O/

                                        1 Reply Last reply
                                        0
                                        • J.HilkJ J.Hilk

                                          I‘ve been following this thread for a while now, and you guy‘s doing a great job :-)

                                          Just one thing that‘s been buging me from the beginning.

                                          The call off split on the string. It‘s used for nothing but temporary storage to than turn the first entry into an INT.

                                          I would highly recommand to at least use splitref.

                                          mrjjM Offline
                                          mrjjM Offline
                                          mrjj
                                          Lifetime Qt Champion
                                          wrote on last edited by
                                          #29

                                          @J.Hilk
                                          Yes i agree. but in this case we wanted to split very small sample so
                                          i decided not to introduce new classes. (QVector/QStringRef)
                                          as to focus on getting it running. But yes, much better to use normally as the speed gain is huge for large dataset. (more than i did imagine)

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