pySerial setDTR not working well

Question

At first I'm using PYthon 2.7.5, pySerial in last version for Python 2.7.x, Windows 7 (64) and FTDIFT232RL (so not real RS232 Serial port). Just for the tests I created this code:

class cProduction:

    g_comPort_name = "COM1"
    g_comPort = 0

    # Instance of object -------------------------------------------------------
    def __init__(self, portName):
        self.data = []
        self.g_comPort_name = portName

    # Main functions -----------------------------------------------------------
    def start(self):
        """ Open port, configure port, set pins of serial line to default state
        DTR true, RTS false """
        try:
            self.g_comPort = serial.Serial(
                port = self.g_comPort_name,
                baudrate = 9600,
                parity = serial.PARITY_NONE,
                stopbits = serial.STOPBITS_ONE,
                bytesize = serial.EIGHTBITS,
                timeout = 10,
                dsrdtr = False,
                xonxoff = False,
                rtscts = False)

            if self.g_comPort.isOpen():
                return True
            else:
                return False
        except:
            return False
        return False

Main:

prod = cProduction("COM33")
if prod.start():
    print Style.BRIGHT + Fore.YELLOW + "Open" + Style.RESET_ALL
    while True:
        key = raw_input('key: ')
        if key == "R":
            prod.g_comPort.setRTS(True)
        if key == "r":
            prod.g_comPort.setRTS(False)
        if key == "D":
            prod.g_comPort.setDTR(True)
        if key == "d":
            prod.g_comPort.setDTR(False)

else:
    print Style.BRIGHT + Fore.RED + "Error while starting production module" + Style.RESET_ALL

RTS is working well but I have a problem with DTR. When I set DTR to False I can see on osciloscope that DTR was really set to low level, but after ~700ms it is return back to high level (without any interrupt from my side). In this time setDTR(False) is not working anymore. I have to first call setDTR(True) and after it setDTR(False) is working again (but again after 700ms it goes back to high level). The time 700ms is allwas the same. I tried to change configuration of 'dsrdtr' or 'rtscts' but it was not helpful.

I have Qt application based on C++ which is working with my device correctly and now I'm just trying to rewrite it to Python so I'm on 100% sure that my device is correct.

Edit: I tried it with real RS232 COM port on my laptop and it is working corectly. So it seems that pySerial has a problem with Virtual COM port with FTDI chip...

shodanex shodanex · Accepted Answer · 2013-10-29T11:08:53

It looks more like a hardware or driver problem than a python issue. Is there something connected to dtr ? It could be an hardware protection triggering because to much current is drawn.

You could try to use logger or apimonitor on your application and on the Qt one to see if there is any difference.

pySerial setDTR not working well

1 Answers