Python pyserial one write delay

Question

I'm having a weird issue with pyserial, using Python 3.6.9, running under WSL Ubuntu 18.4.2 LTS

I've set up a simple function to send GCODE commands to a serial port:

def gcode_send(data):
    print("Sending: " + data.strip())
    data = data.strip() + "\n"  # Strip all EOL characters for consistency
    s.write(data.encode())  # Send g-code block to grbl

    grbl_out = s.readline().decode().strip()
    print(grbl_out)

It sort of works, but every command I send is 'held' until the next is sent.

e.g.

1. I send G0 X0 > the device doesn't react
2. I send G0 X1 > the device reacts to G0 X0
3. I send G1 X0 > the device reacts to G0 X1
4. and so on...

My setup code is:

s = serial.Serial(com, 115200)

s.write("\r\n\r\n".encode())  # Wake up grbl
time.sleep(2)  # Wait for grbl to initialize
s.flushInput()  # Flush startup text in serial input

I can work around the delay for now, but it's quite annoying and I can't find anyone else experiencing the same. Any idea what could be causing this?

Answer 1

There might be a lot of problems here, but rest assured that the pyserial is not causing it. It uses the underlying OS's API to communicate with the UART driver. That being said you first have to test your code with real Linux to see whether WSL is causing it. I.e. whether a Linux and Windows UART buffers are correctly synced. I am sorry that I cannot tell whether a problem is in your code or not because I do not know the device you are using, so I cannot guess what is happening on its end of communication channel. Have in mind that Windows alone can act weirdly in best of circumstances, so, prepare yourself for some frustrations here. Check your motherboard or USB2Serial converter drivers or whatever hw you are using.

Next thing, you should know that sometimes, communication gets confusing if timeouts aren't set. Why? Nobody really knows. So try setting timeouts. Check whether you need software Xon/Xoff turned on or not, and other RS232 parameters that might be required by the device you are communicating with.

Also, see what is going on with s.readline(), I wouldn't personally use it. Timeouts might help or you can use s.read(1024) with timeouts. I do not remember right now, but see whether pyserial supports asynchronous communication. If it does, you can try using it instead of standard blocking mode.

Also, check whether you have to forcefully flush the serial buffer after s.write() or add a sleep after it. It might happen that the device doesn't get the message but the read request is activated. As the device didn't receive the command it doesn't respond. After you send another command, IO buffer is flushed and the previous one is delivered and so forth. Serial communication is fun, but when it hits a snag it can be a real P in the A, believe me.

Ow, a P.S. Check whether the device sends "\r\n\r\n" or "\r\n" only, or "\r" or "\n" in response. s.readline() might get confused. For a start, try putting there two s.readline()s one after another and print out each output. If the device sends double EOL then the one s.readline() is stopping on the empty line and your program receives an empty response, when you send another command s.readline() goes through the buffer and returns a full line that is already there but not read before.

Answer 2

Here it goes. The code promissed in the comment. Big portions of it removed and error checks too.

It is a typing terminal for using PyS60 Python console on Nokia smartphones in the Symbian series via bluetooth. Works fantastically.

from serial import *
from thread import start_new_thread as thread
from time import sleep
import sys, os

# Original code works on Linux too
# The following code for gettin one character from stdin without echoing it on terminal
# has its Linux complement using tricks from Python's stdlib getpass.py module
# I.e. put the terminal in non-blocking mode, turn off echoing and use sys.stdin.read(1)
# Here is Win code only (for brevity):
import msvcrt

def getchar ():
    return msvcrt.getch()

def pause ():
    raw_input("\nPress enter to continue . . .")

port = raw_input("Portname: ")
if os.name=="nt":
    nport = ""
    for x in port:
        if x.isdigit(): nport += x
    port = int(nport)-1

try:
    s = Serial(port, 9600)
except:
    print >> sys.stderr, "Cannot open the port!\nThe program will be closed."
    pause()
    sys.exit(1)

print "Port ready!"
running = 1

def reader():
    while running:
        try:
            msg = s.read()
            # If timeout is set
            while msg=="":
                msg = s.read()
            sys.stdout.write(msg)
        except: sleep(0.001)

thread(reader,())

while 1:
    try: c = getchar()
    except Exception, e:
        running = 0
        print >> sys.stderr, e
        s.write('\r\n\x04')
        break
    if c=='\003' or c=='\x04':
        running = 0
        s.write('\r\n\x04')
        break
    s.write(c)

s.close()
pause()