Develop Reference

How to check simultaneously for serial input and input from keyboard (simultaneous use of readchar and serial library)

I am trying to code the following using python3 in a raspberry pi:
1) wait for a 14 digits bar code (barcode scanner connected through usb port and input received as keyboard data)
2) after a barcode is read, wait for serial communication (device connected to usb port and sends serial commands. could be one, or more....) the idea is that all commands received are going to be associated with the scanned barcode
3) the process of waiting for serial commands has to stop when a new barcode is read. THIS IS THE PART I HAVE NOT FIGURED OUT HOW TO DO IT
After some research, I decided to use the "readchar" library for the barcode scanner and the "serial" library for the serial communication received. Both of them work by themselves but the problem is when I try to detect both things at the same time.
In the following code, I managed to read a barcode and then wait for 5 lines of serial communication to finally repeat the process and read a barcode again. The program works as it is right now BUT the problem is that I don't know how many lines of serial communication I will receive so I need to somehow detect a new barcode while also waiting to receive the serial communication.
import readchar
import time
import serial
ser = serial.Serial(
port='/dev/ttyUSB0',
baudrate = 115200,
parity=serial.PARITY_NONE,
stopbits=serial.STOPBITS_ONE,
bytesize=serial.EIGHTBITS,
timeout=1
)
print("Waiting for barcode...")
while 1:
inputStr = ""
while len(inputStr) != 14: #detect only 14 digit barcodes
inputStr += str(readchar.readchar())
inputStr = ''.join(e for e in inputStr if e.isalnum()) #had to add this to strip non alphanumeric characters
currentCode = inputStr
inputStr = ""
print(currentCode)
ser.flushInput()
time.sleep(.1)
# Wait for 5 lines of serial communication
# BUT it should break the while loop when a new barcode is read!
count = 0
while count < 5:
dataRead=ser.readline()
if len(dataRead) > 0:
print(dataRead)
count+=1
print("Waiting for barcode...")
If I add a condition to the while loop that reading the serial communication using (ser.readline()) so that if a character is read from the scanner (readchar.readchar()) then it messes thing up. It is like if readline and reacher can not be in the same while loop.
Doing some research I think I need to use Asynchronous IO, or threads or something like that, but I have no clue. Also I don't know if I could keep using the same libraries (serial and readchar). Please help

I cannot be sure (I don't have your barcode reader and serial port device) but based on what you say I don't think you need threads, you just have to rely on the buffers to keep your data stored until you have time to read them.
Simply change the condition on your second while loop to:
while serial.inWaiting() != 0:
This way you will make sure the RX buffer on your serial port will empty. This approach might or might not work depending on the speed and timing of your devices.
You could also try to add a short delay after the buffer is emptied:
import serial
import time
ser=serial.Serial(port="/dev/ttyUSB0",baudrate=115200, timeout=1.0)
time.sleep(1)
data=b""
timeout = time.time() + 1.0
while ser.inWaiting() or time.time()-timeout < 0.0: #keep reading until the RX buffer is empty and wait for 1 seconds to make sure no more data is coming
if ser.inWaiting() > 0:
data+=ser.read(ser.inWaiting())
timeout = time.time() + 1.0
else:
print("waiting...")
This keeps trying to read from the port for 1 second after the last byte is received, to make sure nothing else is coming. You might want to play with the duration of the delay depending, again, on the speed and timing of your devices.
Again, I don't have your devices, so I'm in no position to judge, but the way you read characters from the barcode/keyboard looks far from optimum. I doubt readchar is the best approach. At the end of the day, your barcode reader is probably a serial port. You might want to dig into that and/or find a more efficient way to read several keyboard strokes in one go.

I found this answer in another question:
How to read keyboard-input?
I have tried it and it works! I´ll also give a try to the method proposed by Marcos G.

Python hangs when using serial.write() on my Raspberry 2

I use pySerial for communication between RaspberryPi 2 and Arduino but after my first 100 write-calls it starts to become very slowly when writing.
My Code looks like this:
import serial
ser = serial.Serial("/dev/ttyACM0", 2000000, write_timeout=0)
while True:
byteData = getData()
sentBytes = ser.write(byteData)
if sentBytes == 4:
print("All Data was sent successfully!")
Everything is fine for the first second but then it hangs and I only send like 4 bytes each second. I also saw this post here but on my Raspbian machine a /dev/serial0 or /dev/ttyS0 doesn't exist. How I get this rushing like in the first second permanently?

You are using a very high baud rate, a buffer may be running full and cause the hick up after a short while.
Try a very conservative baud rate of 9600 and see if you have the same issue.
Also make sure your getData() actually always returns 4 bytes, otherwise your print statement might not get evaluated in every loop.

Performance at serial read python

I am reading string from serial in a loop and realize that the processor is at 100% (RaspberryPI) while waiting for the next serial.read().
I found recommendation to add a few sleeps here and there, but doing this might cause missing serial data. In theorie I am getting a string from serial every 5 seconds, but could be a bit more or less and not in my control.
Is there a way to solve this in python better and with less processor use?
#!/usr/bin/env python
import serial
ser = serial.Serial("/dev/ttyUSB0", 57600, timeout=0)
def sr():
while True:
for line in ser.read():
try:
response = ser.readlines(None)
response = str(response)
print response
except:
print datetime.datetime.now(), " No data from serial connection."
if __name__ == '__main__':
sr
ser.close()

from what i remember (been a while since i used pyserial) i am sure that serial uses buffers, so as long as your message doesn't fill the buffer you shouldn't lose any data.
assuming i'm looking at the docs for the right module the following page:
[Pyserial docs][1]http://pyserial.sourceforge.net/pyserial_api.html
make mention about buffers both on the input and output.
so you should have no problems with putting sleeps into your program as the buffers will collect the data until you read it. (assuming your messages are not big enough to cause an overflow)
James

Reading serial data in realtime in Python

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

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 :)

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())

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.

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.

pyserial's readline not able to read last some lines when the output has many lines

I am trying to use pyserial to talk interact with a board and I am sending some commands and waiting for their reply am using this piece of code for waiting for => prompt
# Waits till it finds the => prompt
def s_uboot_prompt(ser):
value = ""
matcher = re.compile("\=\>")
trymatcher = re.compile("try")
while (not matcher.search(value)):
if( trymatcher.search(value.strip()) ):
print "******* Command failure. Exiting ..... **********"
ser.write("boot\r")
ser.close()
sys.exit(0)
value = ser.readline()
print value
time.sleep(2)
It works for all of the command but for one of the tftp commands where the output is really huge it fails to read the last couple of lines and the above code loops forever. is there any reason this is happening ? I am not able to think of any logical reason. Please note it's not able to read couple of last lines.
thnks