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#!/usr/bin/pythonimport serial, os, time, sys, datetime, csv#Function to calculate MH-Z19 crc according to datasheetdef crc8(a):    crc=0x00    count=1    b=bytearray(a)    while count<8:        crc+=b[count]        count=count+1    #Truncate to 8 bit    crc%=256    #Invert number with xor    crc=~crc&0xFF    crc+=1    return crc    # try to open serial port    port='/dev/ttyUSB0'sys.stderr.write('Trying port %sn' % port)            try:    # try to read a line of data from the serial port and parse        with serial.Serial(port, 9600, timeout=2.0) as ser:        # 'warm up' with reading one input        result=ser.write("xffx01x86x00x00x00x00x00x79")        time.sleep(0.1)        s=ser.read(9)        z=bytearray(s)        # Calculate crc        crc=crc8(s)         if crc != z[8]:            sys.stderr.write('CRC error calculated %d bytes= %d:%d:%d:%d:%d:%d:%d:%d crc= %dn' % (crc, z[0],z[1],z[2],z[3],z[4],z[5],z[6],z[7],z[8]))        else:            sys.stderr.write('Logging data on %s to %sn' % (port, logfilename()))        # log data        outfname = logfilename()        with open(outfname, 'a') as f:        # loop will exit with Ctrl-C, which raises a KeyboardInterrupt            while True:                #Send "read value" command to MH-Z19 sensor                result=ser.write("xffx01x86x00x00x00x00x00x79")                time.sleep(0.1)                s=ser.read(9)                z=bytearray(s)                crc=crc8(s)                #Calculate crc                if crc != z[8]:                    sys.stderr.write('CRC error calculated %d bytes= %d:%d:%d:%d:%d:%d:%d:%d crc= %dn' % (crc, z[0],z[1],z[2],z[3],z[4],z[5],z[6],z[7],z[8]))                else:                           if s[0] == "xff" and s[1] == "x86":                        print ("co2=", ord(s[2])*256 + ord(s[3]))                co2value=ord(s[2])*256 + ord(s[3])                now=time.ctime()                parsed=time.strptime(now)                lgtime=time.strftime("%Y %m %d %H:%M:%S")                row=[lgtime,co2value]                w=csv.writer(f)                w.writerow(row)                #Sample every minute, synced to local time                t=datetime.datetime.now()                sleeptime=60-t.second                time.sleep(sleeptime)except Exception as e:    ser.close()    sys.stderr.write('Error reading serial port %s: %sn' % (type(e).__name__, e))except KeyboardInterrupt as e:    f.close()    ser.close()    sys.stderr.write('nCtrl+C pressed, exiting log of %s to %sn' % (port, outfname))