Serial connection return NULL after first call - linux

Question

I'm trying to talk to an arduino via a rs485 serial link. I have a usb to serial rs485 adapter plugged in my pc and a max485 in the arduino side. To get started i simply uploaded a sketch that send back what it receives.
#include <SoftwareSerial.h>
#define SSerialRX 10 //Serial Receive pin
#define SSerialTX 11 //Serial Transmit pin
#define SSerialTxControl 3 //RS485 Direction control
#define RS485Transmit HIGH
#define RS485Receive LOW
#define Pin13LED 13
SoftwareSerial RS485Serial(SSerialRX, SSerialTX); // RX, TX
void setup()
{
pinMode(Pin13LED, OUTPUT);
pinMode(SSerialTxControl, OUTPUT);
digitalWrite(SSerialTxControl, RS485Receive);
RS485Serial.begin(300);
}//--(end setup )---
void loop()
{
if (RS485Serial.available()){
delay(100);
char x = RS485Serial.read();
digitalWrite(Pin13LED, HIGH);
digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit
Serial.write(x);
RS485Serial.write(x);
Serial.flush();
delay(10);
digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
digitalWrite(Pin13LED, LOW);
delay(200);
}
}//--(end main loop )---
Then, to test the connection, i run a python script in the pc that writes a character to the serial port and listens to the response:
import serial
import time
PORT1 = "/dev/ttyUSB0"
try:
rs485 = serial.Serial(PORT1, 300)
while True:
print 'loop'
rs485.write('r')
time.sleep(0.5)
data = rs485.read()
if data == '\0':
print 'null'
else:
print data
time.sleep(1)
except:
rs485.close()
The first time I launch the script it happens something like this:
giulio#giulio-vaio:~$ python rs485io.py
loop
r
loop
r
loop
^C
If i try to launch again the script, it writes nothing more than:
giulio#giulio-vaio:~$ python rs485io.py
loop
null
loop
null
loop
null
^C
It starts working again only if i reboot my pc (with ubuntu by the way). If i unplug and plug again the usb to serial converter, nothing changes, if i reboot the arduino, same story. I tried the same configuration with a raspberry pi and the result is identical. Changing usb port doesn't work, upload again the same sketch in the arduino, nothing happens.
The led on the pin 13 blinks, so the arduino is receiving and sending something, the serial.read() function returns, so something is arriving, but is (after the first time) a null carachter '\x00'. In one positive case, after rebooting, I tried to let the script go on for a few time and everything went fine, until i hitted ctrl-c and started again the script.
This is dmesg after i plug in the serial converter:
[ 6116.508264] usb 1-2.1.3: new full-speed USB device number 27 using ehci-pci
[ 6116.617247] usb 1-2.1.3: New USB device found, idVendor=1a86, idProduct=7523
[ 6116.617257] usb 1-2.1.3: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[ 6116.617264] usb 1-2.1.3: Product: USB2.0-Serial
[ 6116.617694] ch341 1-2.1.3:1.0: ch341-uart converter detected
[ 6116.621498] usb 1-2.1.3: ch341-uart converter now attached to ttyUSB0
I don't know what to do after this, any help would be appreciated.
EDIT :
the pc doesn't need to be rebooted, it works if i unload and then load again the module ch341, the one that handles the usb converter.
I added two lines at the top of the code:
subprocess.Popen("modprobe -r ch341".split()).wait()
subprocess.Popen("modprobe ch341".split()).wait()
I know it doesn't really solve the problem, but it works.
EDIT 2 :
Looking at the documentation http://lxr.free-electrons.com/source/Documentation/serial/serial-rs485.txt , I tried to send an ioctl signal to the driver, using this code in c (i'm not good with C, so please forgive me)
#include (all the libraries needed)
/* Driver-specific ioctls: */
#define TIOCGRS485 0x542E
#define TIOCSRS485 0x542F
/* Open your specific device (e.g., /dev/mydevice): */
int main(){
const char *file1 = "/dev/ttyUSB0";
const char *file2 = "output_cprog.txt";
int fd = open (file1,O_WRONLY);
if(fd < 0){
printf("%s: %s\n","error opening ttyusb",strerror( errno ) );
}
int fq = open(file2,O_WRONLY);
if( fq < 0){
printf("%s: %s\n","errore apertura file",strerror( errno ));
}
struct serial_rs485 rs485conf;
// Enable RS485 mode:
rs485conf.flags |= SER_RS485_ENABLED;
// Set logical level for RTS pin equal to 1 when sending:
rs485conf.flags |= SER_RS485_RTS_ON_SEND;
// or, set logical level for RTS pin equal to 0 after sending:
rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
// Set rts delay before send, if needed:
rs485conf.delay_rts_before_send = 10;
// Set rts delay after send, if needed:
rs485conf.delay_rts_after_send = 10;
int ioc = ioctl (fd, TIOCSRS485, &rs485conf);
if(ioc < 0){
printf("%i\n",ioc);
printf("ioctl error: %s\n", strerror( errno ));
}
char *character = "f\n";
write(fd,character,2);
char *buffer;
buffer = (char *) malloc(10);
read(fd, buffer, 1);
write(fq, buffer, 1);
printf("received character: %s\n",buffer[0]);
if( close(fd) < 0){
printf("%s: %s\n","error closing port",strerror( errno ));
}
if (close(fq) < 0){
printf("%s: %s\n","error closing file",strerror( errno ));
}
return 0;
}
but compiling and running the program what i see is only this:
giulio#giulio-vaio:~$ ./ioctlRS485.o
-1
ioctl error: Inappropriate ioctl for device
received character: (null)
I think, by looking at the driver https://github.com/torvalds/linux/blob/master/drivers/usb/serial/ch341.c that it doesn't have the appropriate function to handle an ioctl at all.