Issue with python serial: ValueError: Attempting to use a port that is not open

Question

I am trying to get the data from a blood pressure monitor using the python in raspberry pi3. I googled and found a few examples to get the data using python.

My code:

#!/usr/bin/python
import serial
neo = serial.Serial(
    port='/dev/ttyUSB0',
    baudrate=9600,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS,
    timeout=0
)
print("connected to: " + neo.portstr)

#neo.open()      #opens port
print "Is port open ? ",neo.isOpen()    #returns true?

#ser.write("help\n");
while True:
    dataline = neo.readline();
    if dataline:
        print(dataline), neo.close()

When I ran above code using "sudo python pyusb.py" command, it is returning the following error:

connected to: /dev/ttyUSB0
Is port open ?  True
None
Traceback (most recent call last):
  File "pyusb.py", line 18, in <module>
    dataline = neo.readline();
  File "/usr/lib/python2.7/dist-packages/serial/serialposix.py", line 443, in read
    if not self._isOpen: raise portNotOpenError
ValueError: Attempting to use a port that is not open

If un-commented the line "neo.open()" it is throwing another error:

connected to: /dev/ttyUSB0
Traceback (most recent call last):
  File "pyusb.py", line 13, in <module>
    neo.open()      #opens port
  File "/usr/lib/python2.7/dist-packages/serial/serialposix.py", line 271, in open
    raise SerialException("Port is already open.")
serial.serialutil.SerialException: Port is already open.

I have looked at the similar issue here. But there is a issue with overriding of "serial.Serial()" method. I am unable to identify exactly what is the wrong with code above. Can anyone help me with, what I am doing wrong there?

Answer 1

If you have a look at the docs for pyserial, http://pyserial.readthedocs.io/en/latest/shortintro.html#readline it says you must specify a timeout when using readline(). This is because readline() waits for a EOL character at the end of each transmission. Try increasing the timeout to 1 using something like this:

import serial
neo = serial.Serial(
    port='/dev/ttyUSB0',
    baudrate=9600,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS,
    timeout=1
)
print("connected to: " + neo.portstr)

while True:
    dataline = neo.readline();
    print dataline