Reading serial data in realtime in Python

Question

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

User · Accepted Answer

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

User · Answer

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  gt   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  gt   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

User · Answer

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

User · Answer

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

[python] Reading serial data in realtime in Python

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