为什么print（）影响com1的读取方式？

Question

I'm writing an interface for an old piece of electronic equipment that uses RS232 serial. I've run into a problem that I can't seem to solve. Here is the original code

def readMyPort():
    global readPort
    while True:   #always loop
        if readPort: # only do something of the port is active
            dataString = b''
            while (mySerialPort.inWaiting()>0):
                data = mySerialPort.read(1)
                if (data == b'') or (data == b'\r') or (data == b'\n'):
                    if (dataString != b''):
                        myOutput.insert('1.0', (dataString + b'\n').decode())
                        dataString = b''
                else:
                    dataString += data

The problem I face is that the instrument sends a string of 12 characters in response to a command, and I only seem to catch the last 4, and no, there are no '', '\\r', or '\\n' in the string. In an effort to troubleshoot I added a print(), as shown below. Magically I started to get all the data.

def readMyPort():
    global readPort
    while True:   #always loop
        if readPort: # only do something of the port is active
            dataString = b''
            while (mySerialPort.inWaiting()>0):
                data = mySerialPort.read(1)
                print(data)   #<-------- the added line
                if (data == b'') or (data == b'\r') or (data == b'\n'):
                    if (dataString != b''):
                        myOutput.insert('1.0', (dataString + b'\n').decode())
                        dataString = b''
                else:
                    dataString += data

Now I don't want to have all that printing going on normally. I tried just adding a print('') and that works as well, but I still have all those \\n getting printed. I tried print('', end = '\\r') but that didn't work. Does anyone have an idea why? I don't think it is a speed issue. I'm only running 9600 baud. FYI: python 3.2 on a Win32 machine. This routine is launched in it's own thread.

Answer 1

The operation print(data) requires some time (relatively large) to print something to the console. So by adding the line print(data) you just add some delay inside your loop. You may verify this theory by substituting print(data) for time.sleep(0.1 or whatever small value) and checking that the problem is hopefully gone.

I think that the delay helps because without it mySerialPort.inWaiting() may becomes 0 sometimes (receive buffer is empty) before the transaction is actually completed. Your instrument can't simply output data that fast. In your final version of the code you may add time.sleep() instead of print(data) .

Answer 2

Following the answer of Konstantin, it appears that the serial port needs some time to adjust the buffer and counters, so adding a small delay in the loop solved my problem. 1 ms is not enough but 10 ms is. The final code is:

def readMyPort():
    global readPort
    while True:   #always loop
        if readPort: # only do something if the port is active
            dataString = b''
            while (mySerialPort.inWaiting()>0):  #if something is in the buffer
                data = mySerialPort.read(1)
                if (data == b'') or (data == b'\r') or (data == b'\n'):
                    if (dataString != b''):  #don't output empty strings
                        myOutput.insert('1.0', (dataString + b'\n').decode())
                        dataString = b''
                else:
                    dataString += data
                    time.sleep(0.01)   #slow the loop down a bit 

I do continue to wonder if there is a more elegant solution.