Read from two serial ports asynchronously

Question

I'd like to read from two (or more) serial ports (/dev/ttyUSB0 etc) at the same time in python on Linux. I want to read complete lines from each port (whichever has data) and process the results in the order received (without race conditions). As a simple example could just write the lines to a single merged file.

I assume the way to do this is based on pyserial, but I can't quite figure out how to do it. Pyserial has non-blocking reads using asyncio and using threads. Asyncio is marked as experimental. I assume there wouldn't be any race conditions if the processing is done in asyncio.Protocol.data_received(). In the case of threads, the processing would probably have to be protected by a mutex.

Perhaps this can also be done not in pyserial. The two serial ports can be opened as files and then read from when data is available using select().

Make two threads which read from the serial ports and put data to be processed on a queue. Look up "producer consumer python". — Alex Hall, May 29 '16 at 00:52
@AlexHall Sounds like an easy solution to the problem. Post an answer with threads+queue+pyserial for instant upvote and accept :) — Alex I, May 29 '16 at 06:22

Johann Chang · Answer 1 · 2018-10-01T06:44:59.950

Consider using aioserial.

Here's an example:

import asyncio
import concurrent.futures
import queue

import aioserial


async def readline_and_put_to_queue(
        aioserial_instance: aioserial.AioSerial,
        q: queue.Queue):
    while True:
        q.put(await aioserial_instance.readline_async())


async def process_queue(q: queue.Queue):
    with concurrent.futures.ThreadPoolExecutor(max_workers=1) as executor:
        while True:
            line: bytes = await asyncio.get_running_loop().run_in_executor(
                    executor, q.get)
            print(line.decode(errors='ignore'), end='', flush=True)
            q.task_done()


q: queue.Queue = queue.Queue()
aioserial_ttyUSB0: aioserial.AioSerial = \
        aioserial.AioSerial(port='/dev/ttyUSB0')
aioserial_ttyUSB1: aioserial.AioSerial = \
        aioserial.AioSerial(port='/dev/ttyUSB1', baudrate=115200)

asyncio.run(asyncio.wait([
    readline_and_put_to_queue(aioserial_ttyUSB0, q),
    readline_and_put_to_queue(aioserial_ttyUSB1, q),
    process_queue(q),
]))

score 3 · Accepted Answer · answered Jul 30 '16 at 09:14

As suggested by @AlexHall in a comment, here is a solution that uses one thread for each serial port and a queue to synchronize access:

import serial
import Queue
import threading

queue = Queue.Queue(1000)

def serial_read(s):
    while True:
        line = s.readline()
        queue.put(line)

serial0 = serial.Serial('/dev/ttyUSB0')
serial1 = serial.Serial('/dev/ttyUSB1')

thread1 = threading.Thread(target=serial_read, args=(serial0,),).start()
thread2 = threading.Thread(target=serial_read, args=(serial1,),).start()

while True:
    line = queue.get(True, 1)
    print line

It may be possible to write this more elegantly, but it works.

Giorgio B · Answer 3 · 2016-05-31T18:48:01.403

0

You could try to take the values in order and memorise it in variables:

a = data1.read ()
b = data2.read ()

And after process it in order:

If len (a) != 0 or len (b ) != 0:
             Process a
             Process b

Using this method if one or both of the value has data, process it

edited May 31 '16 at 18:48

answered May 29 '16 at 05:34

Giorgio B

193
6

In the first solution you propose, data2 may have a whole lot of lines ready to read, but data1 has none, so the first read blocks. That's exactly what I want to avoid. In the second solution using threads, how does a single function process the lines read by both threads in order and without race conditions? – Alex I May 30 '16 at 03:21
You only have to insert in your code... threading is multiple activities; at the same time. Notice me if it work. – Giorgio B May 30 '16 at 05:30
No, it does not work. I need "do action" to happen sequentially, not at the same time, but with the code you have it can happen at the same time. – Alex I May 31 '16 at 11:16
So do you want to receive data from 2 serial ports and process in order even if you do not receive in one port? – Giorgio B May 31 '16 at 12:58
Yes, exactly, process in order. – Alex I May 31 '16 at 18:39
As I explained earlier, this will block. I want a solution that doesn't block but also doesn't have race conditions. If it was that easy I'd already know how to do it lol – Alex I May 31 '16 at 18:46
No, this do not block – Giorgio B May 31 '16 at 18:50
I suppose... if you found a easiest method write me please – Giorgio B May 31 '16 at 18:51

Read from two serial ports asynchronously

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