# Python serial between Arduino and Raspberry Pi: Data is modified upon reception

I'm using an Arduino for sensor readings and sending them to a Raspberry Pi via USB, using PySerial for data reception.

It works great, except for the fact received data is awkwardly modified (And set as constant). For example, I'm reading voltages and calculating currents. The results on the Arduino serial are as follows:

Volt   Current
4.93   0.38
4.92   0.37
4.92   0.37
4.92   0.36
...    ...


However, on the Raspberry Pi, it's constantly read as follows (Notice how the digits are changed to zero):

  Volt   Current
4.99   0.30
4.99   0.30
4.99   0.30
4.99   0.30
...    ...


I've tried several turnarounds, but with no luck. I'm not sure where the problem lies, as I am very confident my code is flawless. I even converted the readings to string before sending and yet the constant readings and zero'd digits keep appearing. I appended a counter integer which was sent correctly with no problems.

Has anyone ever tried this before? Any thoughts on how to solve this?

Raspberry Pi Code:

from time import gmtime, strftime
import time
import serial
import struct
ser = serial.Serial('/dev/ttyACM1', 19200)
f = open('results.txt','w')

while 1:
print(temp)
f.write(temp)
f.close()
f = open('results.txt','a')
time.sleep(5)


Arduino Code:

...

double current = 5.18 - temp;   //Resistance ~= 1 Ohm if you are wondering

buffer += d2s(volt,2)+'\t'+d2s(current,2)+'\t'+ d2s(count,0) +'\t' + d2s(minCount,0);

Serial.println(buffer);

...

//I got this from the web

String d2s(double input,int decimalPlaces){

String string;

if(decimalPlaces!=0){
string = String((int)(input*pow(10,decimalPlaces)));

if(abs(input)<1){
if(input>0)
string = "0"+string;
else if(input<0)
string = string.substring(0,1)+"0"+string.substring(1);
}

return string.substring(0,string.length()-
decimalPlaces)+"."+string.substring(string.length()-decimalPlaces);
}

else {
return String((int)input);

}
}

• Can you show the relevant code snippets in both ends? Mar 10, 2013 at 17:36
• Done... Take a look Mar 10, 2013 at 17:57
• A general attack on problems of this sort is to test doing everything to and from text files rather than live streaming data sources. So for example you would capture the arduino output to a text file, verify visually that it is correct, then process that with the python program and verify that the output matches the input. Likely you have a parsing or printing problem with floating point values... Mar 10, 2013 at 18:55
• Where are you configuring the the Arduino's UART? Mar 11, 2013 at 2:27
• @Chris I currently don't have access to an SD card adapter for the arduino, but I'll definitely give a try Mar 11, 2013 at 5:47

The arduino is not well suited to doing floating point math, nor is it partiularly well suited to doing string manipulation.

You would be better off by sending the value read from the analog input directly to the python code on the Pi and do the math and string manipulation in Python.

On the arduino side just do something like this:

int value;

Serial.println(value);  // converts the integer to a string and sends it over the serial port


Then on the Pi side:

str = ser.readline()  # read a string from the serial port

value = float(str)    # convert a string to a floating point number

volt = 5.0 * value / 1023.0  # compute the voltage

• Why didn't I think of that?! I knew the integers were sent correctly, all I needed was just to convert them on the other side. Quick, simple, and works. Thanks a lot! Mar 17, 2013 at 8:49
• Although I still got to find out about the issue of the Pyserial not reading the floating-point strings correctly in the first place. Mar 17, 2013 at 8:50

I found a way to send the information correctly using Serial.write(), but apparently it works byte-by-byte. Thus, I'l have to write some more code to translate each and every byte.

I'm sure there's a better and easier way though.

• Note: Sending an array of bytes didn't work Mar 14, 2013 at 8:10

If you're suspecting the PySerial library, I have to say I'd be really suprised if the issue is there.

I've used PySerial pretty extensively, for both binary and ascii data without issues (though not on a Raspberry Pi).

One thing I can think of is it's possible the port is opening in a weird mode. Try manually specifying the parity and stop-bits mode:

ser = serial.Serial('/dev/ttyACM1', baudrate=119200, bytesize=8, parity='N', stopbits=1)

The other thing I can think of is that you're somehow sending an ASCII control-code, which PySerial is interpreting properly, and the Arduino terminal isn't.
If possible, try an additional serial terminal, and see if it agrees with one or the other.

Honestly, this really smells like a RAM issue to me. You're using a lot of C++ functions (std::string), which are very RAM hungry. You should really think of the arduino as a C device, and avoid C++ abstractions, if possible.

Also, Why the heck are you using doubles? The ADC precisions is 12 bits! A plain-old float is 32 bits on the arduino.

Also you're confident your code is "flawless", which smacks of inexperience to me (the only perfect code is code that doesn't exist).
Together with that (very sloppy) hack of a double to string conversion function (that you found on the internet!), makes me suspect the arduino code more.

Another thing to try is just to store and print some of your results as pure strings:

Serial.println("4.93   0.38")
Serial.println("4.92   0.37")
Serial.println("4.92   0.37")
Serial.println("4.92   0.36")


If that fails, it is indeed likely it's a problem in PySerial. If it doesn't, it's the arduino code.

Actually, pyserial reads data from the Arduino as bytes, e.g.: b234\r\n. So you need to strip those unnecessary values before you can use them:

 valuebtw0to1023= serialObj.readline().strip('\r\n').strip()


if you plan to read serial data from al APC220 RF antenna with python, you will have to add the line :

ser = serial.Serial('/dev/ttyACM1', 19200)
ser.setRTS(0) #  <------------------- this line solve the problem


I found it after a whole day of try-error.