Python Serial reading a USB Arduino, all I get is 0s

Question

I am trying to read an ads1115 monitoring a current shunt so it's reading in millivolts. All I need is to be able to read the serial output from the arduino serial. When I bring the arduino up with the IDE in macos sierra I get changing one or two digit values. I'm confused, am I reading the serial wrong in the Python? I know I have the correct port and only one arduino is plugged in.

Arduino code:

Adafruit_ADS1115 ads; /* Use this for the 16-bit version */
void setup(void)
{
 Serial.begin(9600);
 ads.setGain(GAIN_SIXTEEN); // 16x gain +/- 0.256V 1 bit = 0.125mV 0.0078125mV
 ads.begin();
}
void loop(void)
{
 int16_t results;
 results = ads.readADC_Differential_0_1(); 
 Serial.println(results);
 delay(1000);
}

Python code on RPi:

import io
import sys
import serial
import time
logfilename = 'temp' + time.strftime("%Y-%m-%d")
file = open('/home/pi/' + logfilename + '.log', 'w+')
ser = serial.Serial('/dev/ttyACM0', 9600)
ser.flushInput()
ser.flushOutput()
while True :
 try:
 bytesToRead = ser.inWaiting()
 state=ser.read(bytesToRead)
 file.write( "%s\n" % (state) )
 time.sleep(1)
 except ValueError:
 print "error"
file.close()

What is contained in the file

pi@pi:~$ more ~/temp2017-04-01.log
0
0
0
0
0
0

What I see from the IDE serial monitor:

11
12
0
18
2
0
3
0
14
6

Edit: I didn't figure it out, but I did get an ethernet shield and webserver to work. I'm still curious why this didn't work.

Answer 1

The Arduino outputs each result with a line terminator.

I suggest you use a blocking read line at the Pi.

while True:
 line = ser.readline()
 file.write("%s\n" % (line))

• Rich: I switched to the ser.inWaiting because I was also getting 0s with the ser.readline and thought it might be a buffer issue.
• scruss: Should probably also set timeout if you use readline()

Answer 2

First, are you sure you connected the cable right? Also, I think the Raspi comes with the serial configured for terminal emulation, you have to do something to make it available for your programs. See this: How do I make serial work on the Raspberry Pi3