问题
I am using a script in Python to collect data from a PIC microcontroller via serial port at 2Mbps.
The PIC works with perfect timing at 2Mbps, also the FTDI usb-serial port works great at 2Mbps (both verified with oscilloscope)
Im sending messages (size of about 15 chars) about 100-150x times a second and the number there increments (to check if i have messages being lost and so on)
On my laptop I have Xubuntu running as virtual machine, I can read the serial port via Putty and via my script (python 2.7 and pySerial)
The problem:
- When opening the serial port via Putty I see all messages (the counter in the message increments 1 by 1). Perfect!
- When opening the serial port via pySerial I see all messages but instead of receiving 100-150x per second i receive them at about 5 per second (still the message increments 1 by 1) but they are probably stored in some buffer as when I power off the PIC, i can go to the kitchen and come back and im still receiving messages.
Here is the code (I omitted most part of the code, but the loop is the same):
ser = serial.Serial('/dev/ttyUSB0', 2000000, timeout=2, xonxoff=False, rtscts=False, dsrdtr=False) #Tried with and without the last 3 parameters, and also at 1Mbps, same happens.
ser.flushInput()
ser.flushOutput()
While True:
data_raw = ser.readline()
print(data_raw)
Anyone knows why pySerial takes so much time to read from the serial port till the end of the line? Any help?
I want to have this in real time.
Thank you
回答1:
You can use inWaiting() to get the amount of bytes available at the input queue.
Then you can use read() to read the bytes, something like that:
While True:
bytesToRead = ser.inWaiting()
ser.read(bytesToRead)
Why not to use readline() at this case from Docs:
Read a line which is terminated with end-of-line (eol) character (\n by default) or until timeout.
You are waiting for the timeout at each reading since it waits for eol. the serial input Q remains the same it just a lot of time to get to the "end" of the buffer, To understand it better: you are writing to the input Q like a race car, and reading like an old car :)
回答2:
You need to set the timeout to "None" when you open the serial port:
ser = serial.Serial(**bco_port**, timeout=None, baudrate=115000, xonxoff=False, rtscts=False, dsrdtr=False)
This is a blocking command, so you are waiting until you receive data that has newline (\n or \r\n) at the end: line = ser.readline()
Once you have the data, it will return ASAP.
回答3:
From the manual:
Possible values for the parameter timeout: … x set timeout to x seconds
and
readlines(sizehint=None, eol='\n') Read a list of lines, until timeout. sizehint is ignored and only present for API compatibility with built-in File objects.
Note that this function only returns on a timeout.
So your readlines will return at most every 2 seconds. Use read() as Tim suggested.
回答4:
A very good solution to this can be found here:
Here's a class that serves as a wrapper to a pyserial object. It allows you to read lines without 100% CPU. It does not contain any timeout logic. If a timeout occurs,
self.s.read(i)returns an empty string and you might want to throw an exception to indicate the timeout.
It is also supposed to be fast according to the author:
The code below gives me 790 kB/sec while replacing the code with pyserial's readline method gives me just 170kB/sec.
class ReadLine:
def __init__(self, s):
self.buf = bytearray()
self.s = s
def readline(self):
i = self.buf.find(b"\n")
if i >= 0:
r = self.buf[:i+1]
self.buf = self.buf[i+1:]
return r
while True:
i = max(1, min(2048, self.s.in_waiting))
data = self.s.read(i)
i = data.find(b"\n")
if i >= 0:
r = self.buf + data[:i+1]
self.buf[0:] = data[i+1:]
return r
else:
self.buf.extend(data)
ser = serial.Serial('COM7', 9600)
rl = ReadLine(ser)
while True:
print(rl.readline())
来源：https://stackoverflow.com/questions/19908167/reading-serial-data-in-realtime-in-python