why bytesAvailable() returns '0' when there are answer data from serial port monitor

[to_guliang]

As far as I remember bytesAvailable() doesn't work with QExtSerialPort.

supposed that bytesAvailable() doesn't work ,i amend my code as:

RS232->writeData(Data,13); //write data 
      usleep(120000);                  //wait 
 
      int i= RS232->QIODevice::bytesAvailable();//if there are bytes availble to read
 
      QString str;
      str.setNum(i);
      Com->setText(str);//show the bytes availble to read on the 'Com' Button
 
      char *pStr=NULL;
      RS232->read(pStr,13);    //read data from serial port to pStr
      Com1->setText(pStr);     //show the data on 'Com1' Button
      RS232->close();

RS232->writeData(Data,13); //write data 
      usleep(120000);                  //wait 
      
      int i= RS232->QIODevice::bytesAvailable();//if there are bytes availble to read
      
      QString str;
      str.setNum(i);
      Com->setText(str);//show the bytes availble to read on the 'Com' Button
    
      char *pStr=NULL;
      RS232->read(pStr,13);    //read data from serial port to pStr
      Com1->setText(pStr);     //show the data on 'Com1' Button
      RS232->close();

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i still can't get any data! why?

to zeratul: i don't use the serial monitor.

[wysota]

I just checked - bytesAvailable() works fine with the serial port class. If it doesn't work for you, there has to be some other reason for that. What did open() return and what did you pass to it?

[to_guliang]

I just checked - bytesAvailable() works fine with the serial port class. If it doesn't work for you, there has to be some other reason for that. What did open() return and what did you pass to it?

thanks for your reply
i have develop a application that could write and read program to the MELSEC FX-3U PLC.
open() return true. i write command that read program from PLC to the serial port.
i can observe the answer data from PLC through serial port, that means i have pass my data succeedly to the PLC and the PLC has answer me.
but the bytesAvailable() always returns '0' and i can't get any data by using readData(char * data, qint64 maxSize).
i don't know why

sorry, my expression is poor,Could you understand me? if don't, please tell me, i will try my best to express myself

[wysota]

How exactly do you "observe" the data passing through the port? If you use another application for that, it could be that it steals the data, as it was already suggested.

[to_guliang]

How exactly do you "observe" the data passing through the port? If you use another application for that, it could be that it steals the data, as it was already suggested.

i make sure i have write the data succeedly. the PLC can run with my program that i write to it. According to the protocol of the MELSEC-FX PLC, the PLC will answer me some information.
i can't get any data even if i close the serial port monitor as i have state it after i try as the suggestion.

[to_guliang]

the problem has been solved. QIODevice::bytesAvailable()
i can read data the PLC answer, and the bytesAvailable() returns the right value.
i should use "RS232->bytesAvailable();", not "RS232->QIODevice::bytesAvailable();"