python question with serial communication

[Red Squirrel]

I've been trying to make a very simple program that when run, outputs the output of a command to a serial port. For some reason I can't get this to work. I gave up with C++ as serial communication in C++ is very complex then read it's easier in Python. Keep in mind I barely know this language, actually my first time using it, but here goes.

Why is it that this does not work:

#!/usr/bin/python

import time
import serial

ser = serial.Serial('/dev/arduino-0',9600,timeout=1)

ser.isOpen()

time.sleep(1)

input = 'allstates'

ser.write(input + '\r\n')

time.sleep(2)

out = ser.read(1000)

print out

But if I replace input = 'allstates' with input = raw_input(": ") and type the command manually, then it works?

Why is it not accepting a static string?

 

[EagleKeeper]

Should allstates be surrounded with a double quote instead of a single quote?

 

[Cerb]

What happens, in either case, if you omit '\r'?

 

[Red Squirrel]

Tried single and double quotes, same result. If I ommit \r it does not work in any case. (the serial app looks for \r\n for a command)

 

[Cerb]

I was wondering if maybe \r was being evaluated, rather then sent as a CRLF.

Are you using Python 3.x, by any chance?

 

[Red Squirrel]

Using 2.6 I believe.

I also tried to use a command line argument, and it does not work either. It seems the raw_input function must be formatting it a special way or something that makes it work.

Also if I type my program manually in the interpreter, then it does work.

 

[Red Squirrel]

Got it!

#!/usr/bin/python

import time
import serial
import sys

ser = serial.Serial('/dev/arduino-0',9600,timeout=1)

time.sleep(2)

input = sys.argv[1]

ser.write(input)
ser.write('\r\n')

out = ser.read(1000)

print out

Seems that sleep(1) was not enough, had to sleep for 2 seconds. Does this seem right though? If I use something like picocom, I can issue a command right away and not wait 2 seconds.

 

[EagleKeeper]

2 vs 1 millisecond could explain the issue. The system is unable in the shorter time to properly configure and get a response back from the hardware.

 

[tfinch2]

2 vs 1 millisecond could explain the issue. The system is unable in the shorter time to properly configure and get a response back from the hardware.

time.sleep(2) sleeps for 2 seconds, not milliseconds.

Blindly sleeping for some arbitrary amount of time seems like a bad idea. Not really familiar with the pySerial API, but you may want to try something like:

while not ser.isOpen():
   sleep(0.1) # 100 ms

 

[Red Squirrel]

I tried that and it did not work, but yeah it would be better if I could do something like that instead of sleeping and hoping it does not take longer to initialize at one point and muck up the app.