5

I want to use Python to send commands to an EV3 brick. In C++ things work as expected - for instance, this starts the motor on port A and then stops it after a while:

#include <unistd.h>
#include <fcntl.h>

int main()
{

    // command to start motor on port A at speed 20
    char start_motor[] = "\x0C\x00\x00\x00\x80\x00\x00\xA4\x00\x01\x14\xA6\x00\x01";

    // command to stop motor on port A
    char stop_motor[] = "\x09\x00\x01\x00\x80\x00\x00\xA3\x00\x01\x00";

    // send commands to EV3 via bluetooth
    int bt = open("/dev/tty.EV3-SerialPort", O_RDWR);
    write(bt, start_motor, 14);
    usleep(500000);
    write(bt, stop_motor, 11);
    close(bt);
}

When I try the same thing in Python, however, only the first command is executed (the motor starts but it doesn't stop):

import time

# command to start motor on port A at speed 20
start_motor = '\x0C\x00\x00\x00\x80\x00\x00\xA4\x00\x01\x14\xA6\x00\x01' + '\n'

# command to stop motor on port A
stop_motor = '\x09\x00\x01\x00\x80\x00\x00\xA3\x00\x01\x00' + '\n'

# send commands to EV3 via bluetooth
with open('dev/tty.EV3-SerialPort', mode = 'w') as bt:
    bt.write(start_motor)
    time.sleep(5)
    bt.write(stop_motor)

I have also tried: suppressing the \n; using os.open('dev/tty.EV3-SerialPort', os.O_RDWR); using bt.flush(); using os.fsync(bt); using bt = open(... (instead of with); and changing the mode and buffering arguments of the open method. Nothing worked.

Well, it does work if I close the tty and open it again, but this sounds inefficient:

# ...
with open('dev/tty.EV3-SerialPort', mode = 'w') as bt:
    bt.write(start_motor)
time.sleep(5)
with open('dev/tty.EV3-SerialPort', mode = 'w') as bt:
    bt.write(stop_motor)

So, what am I missing here?

(OS X 10.9.3, Python 2.7.6, EV3 [31313])

4

Works for me if you set buffering to 0 and get rid of the newlines.

import time

# command to start motor on port A at speed 20
start_motor = '\x0C\x00\x00\x00\x80\x00\x00\xA4\x00\x01\x14\xA6\x00\x01'

# command to stop motor on port A
stop_motor = '\x09\x00\x01\x00\x80\x00\x00\xA3\x00\x01\x00'

# send commands to EV3 via bluetooth
with open('/dev/tty.EV3-SerialPort', 'w', 0) as bt:
    bt.write(start_motor)
    time.sleep(5)
    bt.write(stop_motor)
  • It worked! I could swear I had tried buffering=0, but clearly I was wrong. – Parzival Jun 21 '14 at 21:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.