I2C problem - can't talk to I2C Sensor

Question

I have a gas sensor connected to my Raspberry Pi 2 (5V, GND, SDA, SCL) I use the Raspbian OS. I try to communicate with the sensor according to this manual:

http://co2meters.com/Documentation/AppNotes/AN142-RaspberryPi-K_series.pdf

i2cdetect delivers the following the right address : 0x68

I used the following test program: ( http://www.byvac.com/downloads/sws/notsmb_1_0.zip )

GNU nano 2.2.6 File: test.py

from notsmb import notSMB
bus = notSMB(I2CBUS)
resp = bus.i2c(0x68,[0x22,0x00,0x08,0x2A],4)
print(resp)

But I get write errors all the time:

pi@raspberrypi ~ $ sudo python test.py
Traceback (most recent call last):
File "test.py", line 5, in <module>
resp = bus.i2c(0x68,[0x22,0x00,0x08,0x2A],4)

EDIT to add:

The complete code I have used:

#!/usr/bin/python

import time
from notsmb import notSMB
I2CBUS = 1
CO2ADDR = 0x68
READ = 0x22
readBytes = [0x00,0x08,0x2A]
bus = notSMB(1)
while True:
   try:
      pi=bus.i2c(CO2ADDR,[0x22,0x00,0x08,0x2A],4)
      time.sleep(0.1)
      co2Val=(pi[1]*256+pi[2])
      print(co2Val);
      f=open("/var/www/bhm/K-30/co2wert.txt","w")
      f.write(str(co2Val))
      f.close
      break
   except:
      blank=0;

And the error message:

pi@raspberrypi:~ $ sudo python /var/www/bhm/K-30/co2messung.py
Traceback (most recent call last):
File "/var/www/bhm/K-30/co2messung.py", line 12, in <module>
pi=bus.i2c(CO2ADDR,[0x22,0x00,0x08,0x2A],4)
KeyboardInterrupt

Answer 1

Unfortunately I don't have the CO sensor to test, but...

From the notsmbmodule.c file it looks like you have to call bus.open(1) before using the .i2c(...) function.

When you create the instance of bus the file descriptor (fd) and address (addr) are both set to -1

static PyObject *notSMB_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    notSMB *self;
    if ((self = (notSMB *)type->tp_alloc(type, 0)) == NULL)
        return NULL;

    self->fd = -1;
    self->addr = -1;

    return (PyObject *)self;
}

When you call bus.i2c(...) it is trying to do an ioctl(...) on the file descriptor without having done an open(...) to get a valid file descriptor first.

So, I think it may work if you change your code like so (add the bus.open(1) line):

#!/usr/bin/python

import time
from notsmb import notSMB
I2CBUS = 1
CO2ADDR = 0x68
READ = 0x22
readBytes = [0x00,0x08,0x2A]
bus = notSMB(1)

bus.open(1)

while True:
   try:
      pi=bus.i2c(CO2ADDR,[0x22,0x00,0x08,0x2A],4)
      time.sleep(0.1)
      co2Val=(pi[1]*256+pi[2])
      print(co2Val);
      f=open("/var/www/bhm/K-30/co2wert.txt","w")
      f.write(str(co2Val))
      f.close
      break
   except:
      blank=0;