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PySerial timeout, CPU usage, and barcode scanning

It's worth mentioning up-front that while I have a background in CS, the number of Python scripts I've written in could likely be counted on the number of toes on a sloth's paw. That said, I started playing with PySerial to read from a USB barcode scanner. One problem I'm having is the timeout. If I set it too low, I miss scans. If I set it too high, the processor utilization is huge. Of course, this is mentioned in the documentation for PySerial:
Be careful when using readline(). Do specify a timeout when opening
the serial port otherwise it could block forever if no newline
character is received. Also note that readlines() only works with a
timeout. readlines() depends on having a timeout and interprets that
as EOF (end of file). It raises an exception if the port is not opened
correctly.
Right. So, here's my simple code:
#!/usr/bin/env python
import serial
ser = serial.Serial('/dev/ttyACM0', rtscts=True, dsrdtr=True, timeout=0.05)
ser.baudrate = 115200
while True:
s = ser.readline()
if s:
print(s)
How do I appropriately read from a serial device without risking missed scans? Sure, the odds are incredibly low with that small of a timeout, but I'm wanting to use this for production purposes at my business, so let's assume that this is mission-critical. What's the proper way to approach this problem (again, assuming that my understanding of Python is nil)?
Thanks, everyone!
EDIT: Possible solution?
I came up with the following that doesn't use a timeout and simply reads a single character at a time until it reaches a newline. It seems like this is pretty light on processor utilization (which was the whole issue I was having). Of course, I need to account for other newline possibilities from different scanners, but is there any reason why this wouldn't work?
#!/usr/bin/env python
import serial
ser = serial.Serial('/dev/ttyACM0', rtscts=True, dsrdtr=True)
ser.baudrate = 115200
string = ""
while 1:
char = ser.read(1)
string += char
if char == '\r':
print(string)
string = ""

From what I know about barcode scanners, you can configure them so that they only trigger scanning when you send them a specific write command over serial, you can use that to your advantage.
ser = serial.Serial('/dev/ttyUSBx',timeout=y)
ser.write('<trigger scan>')
value = ser.readline()
ser.close()
For continuous reading, the best way of doing it is to keep reading bytes in a timeout loop like
time_start = datetime.datetime.now()
time_end = time_start + datetime.timedelta(seconds=timeout)
output = []
while datetime.datetime.now() < time_end:
output.append(ser.read(100))

My experience with setting timeout to a high value is the opposite of your assertion. A high timeout ensures that python is not checking the serial buffer every 1/20,000th of a second. That's the point of a serial buffer, to store input until it is read. The timeout is in thousandths of seconds, so 0.05 * 1/1000 = 1/20,000 or 20,000 checks per second. I set it to 10 seconds below. (a minimum of 6 checks per minute) Of course, if python encounters a new line sooner then the readline() does not timeout.
#!/usr/bin/env python
import serial
ser = serial.Serial('/dev/ttyACM0', rtscts=True, dsrdtr=True, timeout=10000)
ser.baudrate = 115200
while True:
s = ser.readline()
if s:
print(s)
However, if your UART has no buffer and discards anything past one character, you could lose input. This depends on your hardware and setup. If the bar code fits in the buffer, you should not encounter any problems.

pySerial - Only reading one byte

I am trying to read and write to a sensor via serial using pySerial. I have no software or hardware flow control.
I am able to send a string of hex to the device, but I only receive one byte back instead of the two-to-ten bytes I should see. The sensor is working -- I've verified this using Realterm.
I've tried using ser.readline() (instead of the inWaiting loop), and ser.read(2); this just causes the program to hang. I've also tried increasing the sleep time, and experimented with different baud rates (on both the PC and sensor), but nothing seems to work.
Does anyone have any advice?
import time
import serial
# configure the serial connections
ser = serial.Serial(
port='COM1',
baudrate=115200,
parity=serial.PARITY_EVEN,
stopbits=serial.STOPBITS_ONE,
bytesize=serial.EIGHTBITS
)
ser.isOpen()
print 'Enter your commands below.\r\nInsert "exit" to leave the application.'
while 1 :
# get keyboard input
data_in = raw_input(">> ")
if data_in == 'exit':
ser.close()
exit()
else:
# send the character to the device
ser.write(data_in.decode('hex') + '\r\n')
out = ''
time.sleep(1)
while ser.inWaiting() > 0:
out += ser.read(1)
if out != '':
print ">>" + " ".join(hex(ord(n)) for n in out)
(I slightly modified the code from that found on Full examples of using pySerial package)

Your read statement is explicitly requesting 1 byte:
ser.read(1)
If you know how many bytes to read, you can specify here. If you are unsure, then you can specify a larger number. For example, doing
ser.read(10)
will read up to 10 bytes. If only 8 are available, then it will only return 8 bytes (following the timeout, see below).
It is also worth setting a timeout to prevent the program from hanging. Just add a timeout parameter to your Serial constructor. The following will give you a 2 second timeout:
ser = serial.Serial(
port='COM1',
baudrate=115200,
parity=serial.PARITY_EVEN,
stopbits=serial.STOPBITS_ONE,
bytesize=serial.EIGHTBITS,
timeout=2
)
The documentation states:
read(size=1) Read size bytes from the serial port. If a timeout is set it may return less characters as requested. With no timeout it will block until the requested number of bytes is read.
So if you do not know how many bytes to expect, then set a small timeout (if possible) so your code does not hang.
If your code does not return the full number of bytes expected, then it is likely the device you are connected to is not sending all the bytes you are expecting. Since you have verified it should be working separately, have you verified the data you are sending is correct? Perhaps encode to bytes first using struct.pack(). As an example, to send a byte with a decimal value of 33 (hex 0x21)
import struct
bytes_to_send = struct.pack('B', 33)
I have also never found it necessary to append the end of line chars \r\n to a message before sending

Python Serial Read on Pi

I am attempting to read serial data with my Pi from an Arduino Mega. Most of the time I get the correct read, but occasionally I do not read the correct number of bytes. My serial monitor from the Arduino shows the correct data being sent.
Arduino Code
String reading = String(analogRead(0));
String out1 = reading + "\n";
Serial.print(out1);
Pi Python Code
ser = serial.Serial('/dev/ttyACM0', 9600 , timeout = 1)
ser.open()
num = ser.readline()
print num
ser.close()
The value of num ranges from 60 to 200.
Here is an example of the output from seven consecutive executions (bold is bad read):
74
74
734
73
734
74
3
I have scoured the forums and cannot find anyone who has asked a question to address my issue. Everything I read says this should be a piece of cake, but I am still having issues.

It's probably a timing issue. The Arduino code presumably runs in a loop, correct? It's just bashing out characters at some rate. It looks like the Python program has no loop, or if there is one, you're continually opening and closing the serial port. The Arduino might be transmitting when the Pi isn't listening.
Try this:
ser = serial.Serial() # whatever you need as arguments
ser.open()
while True:
num = ser.readline()
print(num)
You will have to break out of this with Control-C or something, but at least you can see if it works.

Python serial port read delay

I'm playing around with this serial module in python. I have a little problem with it. I want my script to get a char from the console send it to an AVR board, and read back the response.
Everytime I read from the USB port, and print it out, I see the previous result. Why's that?
For example:
I write 5
I read nothing
I write 6
I read 5
import serial
import sys, time
port=serial.Serial(
port='/dev/ttyUSB0',\
baudrate=9600,\
parity=serial.PARITY_NONE,\
stopbits=serial.STOPBITS_ONE,\
bytesize=serial.EIGHTBITS,\
timeout=0)
i=0
tmp = 0
while True:
tmp=raw_input('send: ')
port.write(tmp)
port.flushOutput()
print port.read(1)
port.flushInput()

From the documentation: "Writes are blocking by default, unless writeTimeout is set. For possible values refer to the list for timeout above." Try setting writeTimeout=0 as well in your constructor.

You are probably receiving a single unexpected byte on startup - either the microcontroller is sending it, or it might be noise from connecting a plug. As you are only reading a single byte for each string transmitted, you will always be off by one.
Instead of port.read(1), try:
while True:
tmp=raw_input('send: ')
port.write(tmp)
port.flushOutput()
print port.read(port.inWaiting())
port.flushInput()
This would also have happened if your typed in more than one character at the input prompt.

python and serial. how to send a message and receive an answer

I have to send ZANE:1:00004:XX_X.X_XXXX_000XX:\r\nvia serial communication with python.
here is my code:
import serial
ser = serial.Serial('/dev/cu.usbserial-A901HOQC')
ser.baudrate = 57600
msg = 'ZANE:1:00004:XX_X.X_XXXX_000XX:\r\n'
If I write:
>>> ser.write(msg)
the answer will be 33, which is the length in byte of the message I'm sending.
How can I receive the answer? The connected device will answer just after he gets the message, but if I type
>>> ser.write(msg); ser.readline()
the result will be that readline never gets any message at all...
any ideas?

Your device is probably not terminating its response with a newline character.
the .readline() method is expecting a newline terminated string.
See here: http://pyserial.sourceforge.net/shortintro.html#readline
for more info.
try setting a timeout on your serial connection
ser.timeout = 10
and replace the ser.readline() with ser.read(n) where n is the number of characters you wish to read.
ser.read(100)
will try to read 100 characters. If 100 characters don't arrive within 10 seconds, it will give up and return whatever it has received.

In order to read you need to open a listening port(with a timeout) first, for example:
ser = serial.Serial('/dev/cu.usbserial-A901HOQC', 19200, timeout=5)
x = ser.read() # read one byte
s = ser.read(10) # read up to ten bytes (timeout)
line = ser.readline() # read a '\n' terminated line
ser.close()
See more details here.

I believe the earlier answers didn't understand that you are using the same port for writing and reading.
I'm having the same problem and solved it using a sleep function. Basically:
import serial
from time import sleep
ser = serial.Serial('/dev/cu.usbserial-A901HOQC', timeout=1)
ser.baudrate = 57600
msg = 'ZANE:1:00004:XX_X.X_XXXX_000XX:\r\n'
ser.write(msg)
sleep(0.5)
ser.readline()
So with that sleep you are giving time to the receiver (a machine?) to send the reply. Also note that you have to add a timeout if you want to use readline.

Here two thinks are important.first one is timeout and second one is EOL charector..
if you are going to use time out in the receiver side then no need EOL from transmitter side.
if you are going to use EOL charector in transmitter side(/n,/r) then no need to put time out in the receiver side.
Ex: serialport=serial.serial(port,baud,timeout) if you are going to use time out
incoming signal over serial port(Ex: hello how are you? nice to meet you man!!) Here new line cherector does not respond well.so you can leave it.
Ex: serialport=Serial.serial(port,baud) if you are not going to put time out in serial port then you should use end of line charector(/n,/r) in the transmitter
Note : Second way is more efficient than first way