Maybe the device is responding, but it isn’t ending its response with a line feed.(I notice you’re not sending any sort of new line character) Maybe the device does not respond when you send just the byte b'S'.Maybe you’re not sending the byte b'S' correctly, or using the wrong serial port settings.Now let’s think about what’s going wrong. (Though it looks like your indentation got messed up during copy and pasting somewhere for the last two lines!) In principle, it looks like the writing should work, though you could just write ser.write(b'S')