Sending Float Values from Python to Arduino - Data in buffer but no values read

Question

I am trying to send float values from Python on Windows to an Arduino. The first problem I encounter is that I cannot view the serial monitor when I think I am sending data from my Python script. I have read online that this is becuase only one application can manage the port at once: https://forum.arduino.cc/t/serial-communication-only-working-when-serial-monitor-is-opened/601107/12

However I have seen examples where the user is viewing the serial monitor to see data coming in over serial from Python and serial.print outs from the Arduino. So I am unsure what is the case... not being able to view the serial monitor sure does make debugging this senario hard.

My Python code:

import struct
import serial
import time
print('test123')
x=0.8
y=0.2

ser = serial.Serial('COM5', 9600, timeout=1)

#print(ser)
time.sleep(3)

def sendmess():
    bin = struct.pack('ff',x,y) #Pack float value into 4 bytes
    data = ser.write(bin)

    #print(bin)
    #print(len(bin))
    #print([ "0x%02x" % b for b in bin])

    
    #ser.close()

while True:
    sendmess()

My Arduino Code:

int d = 250;
float incomingByte = 1;
void setup() {
  // initialize the serial communication:
  Serial.begin(9600);
  pinMode(2, OUTPUT);
  
}

void loop() {
  digitalWrite(2, HIGH);
  delay(d);
  digitalWrite(2, LOW);
  delay(d);
 // reply only when you receive data:
  if (Serial.available() > 0) {
    // read the incoming byte:
    d = 1000;
    float incomingByte = Serial.read();

    // say what you got:
    Serial.println(incomingByte);
    
  }
  else{
Serial.println(incomingByte);    
d = 20;
  }
}

I see the LED flash every second so I know the serial buffer is >0, but I cannot get any data out :(.

Thanks in advance!

Mark

I have tried examples online, but I never get any data to display in the serial monitor. Nor can I say have a pin turn HIGH when I think I am getting the data I think I have sent. But without the Serial Monitor how can I debug this?

Serial.read(); return a byte, not a float (which is 4-byte long), to get a floating point number, use float myFloat = Serial.parseFloat(SKIP_ALL, '\n');, Read the doc. — hcheung
– hcheung, Commented Nov 8, 2022 at 0:57

Dacoolinus · Accepted Answer · 2022-11-08 17:03:24Z

0

Unfortunately, you aren't able to connect more than two devices to the serial connection. However, using Python you should be able to read a response from your arduino and then print that to the python Terminal (or a file) and take a look at that.

The easiest way to do that would be to use pySerial and either ser.read(4) to read back 4 bytes of your float or ser.readline() which will look for either a '\n' which you can add to your arduino code, or a timeout. Then just print it to the python terminal with print().

As for actually reading the float, Serial.read() will only read in the next byte in the buffer. Since a float is 4 bytes long, you need to either read the 4 bytes into an array or use Serial.parseFloat()

Python Code

import struct
import serial
import time
x=0.8
y=0.2

ser = serial.Serial('COM5', 9600, timeout=1)

#print(ser)
time.sleep(3)

def sendmess():
    bin = str(x) + str(y) #Pack float value into 4 bytes
    data = ser.write(bin.encode())
    echo = ser.readline()
    print("Echo: " + echo)
    #ser.close()

while True:
    sendmess()

Arduino Code:

int d = 250;
float X;
float Y;
char buffer[40];
void setup() {
  // initialize the serial communication:
  Serial.begin(9600);
  pinMode(2, OUTPUT);
  
}

void loop() {
  digitalWrite(2, HIGH);
  delay(d);
  digitalWrite(2, LOW);
  delay(d);
 // reply only when you receive data:
  if (Serial.available() > 0) {
    // read the incoming byte:
    d = 1000;
    X = Serial.parseFloat();
    Y = Serial.parseFloat();

    // say what you got:
    sprintf(buffer, "X: %f Y: %f", X, Y);
    Serial.println(buffer);
    
  }
  else{
   Serial.println(buffer);  
d = 20;
  }
}

edited Nov 8, 2022 at 17:03

answered Nov 7, 2022 at 22:07

Dacoolinus

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7 Comments

hcheung Over a year ago

echo doesn't give you the floating point that you think to be, you need to struct.unpack('f', data) to get a floating point back and you might lost the precision.

Dacoolinus Over a year ago

@hcheung I believe the println() function from the arduino library returns a human readable version of the float and not the raw data of the float itself. I updated the code to ensure it reads the entire line but the code you provided above will not find a float. You'd need to parse the incoming ascii characters with float(echo). For debugging purposes, it shouldn't really matter.

masmith8 Over a year ago

Thanks for the repsonse, I can now see what python is sending. However it looks like it is just sending 0.00 infrequntly not 0.8 & 0.2. Echo: 0.00 Echo: Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: 0.00 Echo: Echo:

masmith8 Over a year ago

Thanks, I can now view what Python is sending. But it looks like it is just sending 0.00 intermittanly? and not the 0.8 and 0.2 vaules I have? Echo: 0.00 Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: Echo: Echo: Echo: Echo: Echo: 0.00 Echo: Echo: Echo: Echo: 0.00

Dacoolinus Over a year ago

@masmith8 I modified the code above by removing the data packing in python. The Serial.parsefloat() function expects a string and not raw bytes. I ran it through an arduino emulator, wokwi, and it works as expected now. I also cleaned up the print statements in the arduino code so that it will respond with both X and Y in a single string.

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