PySerial not talking to Arduino

Question

• Python version: 2.6.6
• PySerial version: 2.5
• Arduino board: Duemilanove 328

I have written some code to simulate some hardware I'm working with and uploaded it to the Arduino board. This code works. I know this, because I get the expected response from HyperTerminal.

However, when I try to connect using PySerial the connection does not error, but I get no response to the commands I send.

Why might this be?

Python code

import serial

def main():
    sp = serial.Serial()
    sp.port = 'COM4'
    sp.baudrate = 19200
    sp.parity = serial.PARITY_NONE
    sp.bytesize = serial.EIGHTBITS
    sp.stopbits = serial.STOPBITS_ONE
    sp.timeout = 0.5
    sp.xonxoff = False
    sp.rtscts = False
    sp.dsrdtr = False

    sp.open()

    sp.write("GV\r\n".encode('ascii'))
    value = sp.readline()
    print value
    sp.write("GI\r\n".encode('ascii'))
    value = sp.readline()
    print value

    sp.close()
 
if __name__ == "__main__":
    main()

NB: the code on the Arduino sends back \r\n at the end of a response to a command.

HyperTerminal configuration:

Edit

I have found that if I increase the timeout to 10 seconds and add a sp.readline() before I send anything, then I get responses to both commands.

How long is the hardware handshake usually between PySerial and an Arduino or USB RS-232 ports?

Answer 1

Can not verify this but it could be that you try and read before there is any data there, thus you get no reply back.

To test this you could try and poll until there is data

value = None
while not value:
   value = sp.readline()
print value

Edit

The Arduino will reset when you open a serial connection, any data written during bootup will likely go to bit heaven. You could use a sleep for 2 seconds (could not find the exact time it takes, will likely vary anyway) before you do any reads/writes.

Alternatively you could write to it until you get a response back, after you get a return you start doing "real work".

Answer 2

For the time being I am using a workaround. I have set the timeout to 1.5 seconds and put a readline call in before the first write.

So now the Python code looks like:

import serial

def main():
    sp = serial.Serial()
    sp.port = 'COM4'
    sp.baudrate = 19200
    sp.parity = serial.PARITY_NONE
    sp.bytesize = serial.EIGHTBITS
    sp.stopbits = serial.STOPBITS_ONE
    sp.timeout = 1.5 #1.5 to give the hardware handshake time to happen
    sp.xonxoff = False
    sp.rtscts = False
    sp.dsrdtr = False

    sp.open()

    sp.readline() #to give the hardware handshake time to happen

    sp.write("GV\r\n".encode('ascii'))
    value = sp.readline()
    print value
    sp.write("GI\r\n".encode('ascii'))
    value = sp.readline()
    print value

    sp.close()

if __name__ == "__main__":
    main()

Answer 3

I've also encountered this problem recently and here's my solution:

import serial

ser = serial.Serial(4, timeout=2)
ser.setRTS(True)
ser.setRTS(False)
while 1:
    line = ser.readline()
    print(line)
ser.close

Turns out this will successfully reset the Arduino board.

Answer 4

Add a delay after you open the port as Arduino is reset and the bootloader starts listening for new firmware. If something is sent at that moment, the MCU remains stuck in bootloader. The delay causes the bootloader to timeout.

sp.open()
time.sleep(2) # 2 seconds or possibly a bit less
sp.write("blahblah")

Answer 5

You can easily and robustly connect and communicate between Python and Arduino boards using the compatible libraries SerialTransfer.h and pySerialTransfer. These libraries automatically packetize and parse serial packets using start/end markers, cyclic redundancy checking, consistent overhead byte stuffing, and dynamic payload lengths.

SerialTransfer.h is installable through the Arduino IDE's Libraries Manager and pySerialTrasnfer is pip-installable.

Example Python:

from pySerialTransfer import pySerialTransfer as txfer

if __name__ == '__main__':
    try:
        link = txfer.SerialTransfer('COM13')

        link.txBuff[0] = 'h'
        link.txBuff[1] = 'i'
        link.txBuff[2] = '\n'

        link.send(3)

        while not link.available():
            if link.status < 0:
                print('ERROR: {}'.format(link.status))

        print('Response received:')

        response = ''
        for index in range(link.bytesRead):
            response += chr(link.rxBuff[index])

        print(response)
        link.close()

    except KeyboardInterrupt:
        link.close()

Example Arduino:

#include "SerialTransfer.h"
#include <SoftwareSerial.h>


SoftwareSerial mySerial(2, 3); // RX, TX
SerialTransfer myTransfer;


void setup()
{
  Serial.begin(115200);
  mySerial.begin(9600);
  myTransfer.begin(mySerial);
}

void loop()
{
  myTransfer.txBuff[0] = 'h';
  myTransfer.txBuff[1] = 'i';
  myTransfer.txBuff[2] = '\n';

  myTransfer.sendData(3);
  delay(100);

  if(myTransfer.available())
  {
    Serial.println("New Data");
    for(byte i = 0; i < myTransfer.bytesRead; i++)
      Serial.write(myTransfer.rxBuff[i]);
    Serial.println();
  }
  else if(myTransfer.status < 0)
  {
    Serial.print("ERROR: ");
    Serial.println(myTransfer.status);
  }
}