Reading time off of a cellular modem with python 2.7 and pyserial

Question

TL;DR:

I need to read the time off of my Hologram Nova USB cellular modem on /dev/ttyACM0 and update the system clock after the pi restarts. I'd like to do it in Python but it isn't reading the serial tty correctly.

My build

• I'm running python 2.7.13 on Raspbian 9
• I'm using pyserial 3.2.1-1
• I have a Pi 3 b+
• The modem is a USB connected U-Blox SARA-U201 (AKA Hologram Nova)

What works so far:

• My cellular modem is sitting on /dev/ttyACM0
  • I can communicate with it using screen /dev/ttyacm0 9600
• I can get a time stamp in the screen session by typing AT+CCLK? and hitting Enter

Here is the output from the screen session

AT+CCLK?

+CCLK: "19/05/16,21:31:47-16"

OK

What doesn't work so far:

• I have tried to use pyserial to interrogate the tty and return the time stamp.
• I have tried to create an expect script using screen
  • I'd prefer not to do this with an expect script when python is a lot better.
  • I'll post my source for expect if this can't be done in Python.

Python Code:

import serial
import time, sys
import datetime
response = ''
SERIAL_PORT = "/dev/ttyACM0"
ser = serial.Serial(SERIAL_PORT, baudrate = 9600, timeout = 5)
ser.write("AT+CCLK?\r")
response = ser.readline()
print response

When using the python console the line ser.write("AT+CCLK?\r") results in a 9 being returned on the next line. response remains empty and ser.inWaiting() is equal to 0. Here is the text returned from the tty:

root@raspberrypi:/home/pi# python
Python 2.7.13 (default, Sep 26 2018, 18:42:22)
[GCC 6.3.0 20170516] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import serial
>>> import time, sys
>>> import datetime
>>> response = ''
>>> SERIAL_PORT = "/dev/ttyACM0"
>>> ser = serial.Serial(SERIAL_PORT, baudrate = 9600, timeout = 5)
>>> ser.write("AT+CCLK?\r")
9
>>> response = ser.readline()
>>> print response

Answer 1

Ok turns out I am an impatient man.

I was simply requesting the data too quickly; screen stays connected and shows it, Python is just too fast when it reads the input.

This code attempts to read the time from the modem at 2/10 of a second intervals for 20 seconds. If unsuccessful it sleeps for 20 seconds and then tries again. It will return the datetime string that is provided to the modem. That can be parsed from the "response" variable.

import serial
import time, sys
import datetime

i=0
time_found=False
response=''
SERIAL_PORT="/dev/ttyACM0"
ser=serial.Serial(SERIAL_PORT, baudrate = 9600, timeout = 15)

while time_found==False:
        ser.write('AT+CCLK?\r')
        response = ser.readline()
        while "CCLK:" not in response:
                response=ser.readline()
                time.sleep(0.2)
                ++i
                if i==200:
                        break
        if "CCLK:" in response:
                time_found=True
        else:
                time.sleep(20)

print response

Answer 2

Question

Setup = Hologram Nova USB cellular modem on /dev/ttyACM0

Can do = Terminal command AT+CCLK? replies +CCLK: "19/05/16,21:31:47-16"

How do in python?

Answer

Suggest 2 steps:

1. Verify serial OK by loop back on '/dev/serial0'.

2. Replace '/dev/serial0' by '/dev/ttyACM0' and try your luck on Hologram Nova.

Update 2019may17hkt1628

In case loopback /dev/serial0 works but /dev/ttyACM0 does not work, then it might be the external USB problem. I usually recommend to use the powerful RealTerm to do troubleshooting. You might might read the details in the second reference below. RealTerm can send/receive long strings of file, store data in files, new line setting, etc. So it is much faster than python and the not so powerful puTTY or TeraTerm. I show below a couple of screen captures for your reference. You might also like to read SparkFun for their recommendation of RealTerm (see reference below). And in case you are trying USB/TTL cables, good quality USB cable is also important. I usually recommend CH340 drivers, though Raspbian accepts PX2xxx drivers OK.

References

UART Serial Debugging Tips

[How to verify serial by loopback check?] Serial Communication from Raspbian using Python to Arduino

[How to debug serial?] Serial to Arduino totally non-responsive

Terminal Emulators

Serial Terminal Basics (TeraTerm) - SparkFun

RealTerm (Windows) - SparkFun Recommendation

TeraTerm is awesome for simple ASCII-only serial terminal stuff, but what if you need to send a string of binary values ranging from 0-255? For that, we like to use RealTerm. RealTerm is designed specifically for sending binary and other difficult-to-type streams of data, ...

USB TTL

How many serial ports are on the Pi 3?

Options to add 20 UART to RPi

Increase number of TTYs

Persistent [USB] Device Names on Raspberry Pi - Vrijdag 2015feb06

Program to setup serial on '/dev/serial0'

from   time import sleep
import serial

serialPort0 = serial.Serial(port = '/dev/serial0',
        baudrate = 9600,
        parity = serial.PARITY_NONE,
        stopbits = serial.STOPBITS_ONE,
        bytesize = serial.EIGHTBITS,
        timeout= 1)

Answer 3

You have supplied insufficient information to provide a definitive answer.

It is most likely your modem is expecting a LF or CR/LF

Replace the \r with \n

read(30) is going to wait for 30 characters. You should use readline() or equivalent