Two-way C++ communication over serial connection - c++

Question

I am trying to write a really simple C++ application to communicate with an Arduino. I would like to send the Arduino a character that it sends back immediately. The Arduino code that I took from a tutorial looks like this:
void setup()
{
Serial.begin(9600);
}
void loop()
{
//Have the Arduino wait to receive input
while (Serial.available()==0);
//Read the input
char val = Serial.read();
//Echo
Serial.println(val);
}
I can communicate with the Arduino easily using GNU screen, so I know that everything is working fine with the basic communication:
$ screen /dev/tty.usbmodem641 9600
The (broken) C++ code that I have looks like this:
#include <fstream>
#include <iostream>
int main()
{
std::cout << "Opening fstream" << std::endl;
std::fstream file("/dev/tty.usbmodem641");
std::cout << "Sending integer" << std::endl;
file << 5 << std::endl; // endl does flush, which may be important
std::cout << "Data Sent" << std::endl;
std::cout << "Awaiting response" << std::endl;
std::string response;
file >> response;
std::cout << "Response: " << response << std::endl;
return 0;
}
It compiles fine, but when running it, some lights flash on the Arduino and the terminal just hangs at:
Opening fstream
Where am I going wrong?

Answer 1

There are three points:
First: You don't initialize the serial port (TTY) on the Linux side. Nobody knows in what state it is.
Doing this in your program you must use tcgetattr(3) and tcsetattr(3). You can find the required parameters by using these keywords at this site, the Arduino site or on Google. But just for quick testing I propose to issue this command before you call your own command:
stty -F /dev/tty.usbmodem641 sane raw pass8 -echo -hupcl clocal 9600
Especially the the missing clocal might prevent you opening the TTY.
Second: When the device is open, you should wait a little before sending anything. By default the Arduino resets when the serial line is opened or closed. You have to take this into account.
The -hupcl part will prevent this reset most of the time. But at least one reset is always necessary, because -hupcl can be set only when the TTY is already open and at that time the Arduino has received the reset signal already. So -hupcl will "only" prevent future resets.
Third: There is NO error handling in your code. Please add code after each IO operation on the TTY which checks for errors and - the most important part - prints helpful error messages using perror(3) or similar functions.

Answer 2

I found a nice example by Jeff Gray of how to make a simple minicom type client using boost::asio. The original code listing can be found on the boost user group. This allows connection and communication with the Arduino like in the GNU Screen example mentioned in the original post.
The code example (below) needs to be linked with the following linker flags
-lboost_system-mt -lboost_thread-mt
...but with a bit of tweaking, some of the dependence on boost can be replaced with new C++11 standard features. I'll post revised versions as and when I get around to it. For now, this compiles and is a solid basis.
/* minicom.cpp
A simple demonstration minicom client with Boost asio
Parameters:
baud rate
serial port (eg /dev/ttyS0 or COM1)
To end the application, send Ctrl-C on standard input
*/
#include <deque>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/asio/serial_port.hpp>
#include <boost/thread.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/date_time/posix_time/posix_time_types.hpp>
#ifdef POSIX
#include <termios.h>
#endif
using namespace std;
class minicom_client
{
public:
minicom_client(boost::asio::io_service& io_service, unsigned int baud, const string& device)
: active_(true),
io_service_(io_service),
serialPort(io_service, device)
{
if (!serialPort.is_open())
{
cerr << "Failed to open serial port\n";
return;
}
boost::asio::serial_port_base::baud_rate baud_option(baud);
serialPort.set_option(baud_option); // set the baud rate after the port has been opened
read_start();
}
void write(const char msg) // pass the write data to the do_write function via the io service in the other thread
{
io_service_.post(boost::bind(&minicom_client::do_write, this, msg));
}
void close() // call the do_close function via the io service in the other thread
{
io_service_.post(boost::bind(&minicom_client::do_close, this, boost::system::error_code()));
}
bool active() // return true if the socket is still active
{
return active_;
}
private:
static const int max_read_length = 512; // maximum amount of data to read in one operation
void read_start(void)
{ // Start an asynchronous read and call read_complete when it completes or fails
serialPort.async_read_some(boost::asio::buffer(read_msg_, max_read_length),
boost::bind(&minicom_client::read_complete,
this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
void read_complete(const boost::system::error_code& error, size_t bytes_transferred)
{ // the asynchronous read operation has now completed or failed and returned an error
if (!error)
{ // read completed, so process the data
cout.write(read_msg_, bytes_transferred); // echo to standard output
read_start(); // start waiting for another asynchronous read again
}
else
do_close(error);
}
void do_write(const char msg)
{ // callback to handle write call from outside this class
bool write_in_progress = !write_msgs_.empty(); // is there anything currently being written?
write_msgs_.push_back(msg); // store in write buffer
if (!write_in_progress) // if nothing is currently being written, then start
write_start();
}
void write_start(void)
{ // Start an asynchronous write and call write_complete when it completes or fails
boost::asio::async_write(serialPort,
boost::asio::buffer(&write_msgs_.front(), 1),
boost::bind(&minicom_client::write_complete,
this,
boost::asio::placeholders::error));
}
void write_complete(const boost::system::error_code& error)
{ // the asynchronous read operation has now completed or failed and returned an error
if (!error)
{ // write completed, so send next write data
write_msgs_.pop_front(); // remove the completed data
if (!write_msgs_.empty()) // if there is anthing left to be written
write_start(); // then start sending the next item in the buffer
}
else
do_close(error);
}
void do_close(const boost::system::error_code& error)
{ // something has gone wrong, so close the socket & make this object inactive
if (error == boost::asio::error::operation_aborted) // if this call is the result of a timer cancel()
return; // ignore it because the connection cancelled the timer
if (error)
cerr << "Error: " << error.message() << endl; // show the error message
else
cout << "Error: Connection did not succeed.\n";
cout << "Press Enter to exit\n";
serialPort.close();
active_ = false;
}
private:
bool active_; // remains true while this object is still operating
boost::asio::io_service& io_service_; // the main IO service that runs this connection
boost::asio::serial_port serialPort; // the serial port this instance is connected to
char read_msg_[max_read_length]; // data read from the socket
deque<char> write_msgs_; // buffered write data
};
int main(int argc, char* argv[])
{
// on Unix POSIX based systems, turn off line buffering of input, so cin.get() returns after every keypress
// On other systems, you'll need to look for an equivalent
#ifdef POSIX
termios stored_settings;
tcgetattr(0, &stored_settings);
termios new_settings = stored_settings;
new_settings.c_lflag &= (~ICANON);
new_settings.c_lflag &= (~ISIG); // don't automatically handle control-C
tcsetattr(0, TCSANOW, &new_settings);
#endif
try
{
if (argc != 3)
{
cerr << "Usage: minicom <baud> <device>\n";
return 1;
}
boost::asio::io_service io_service;
// define an instance of the main class of this program
minicom_client c(io_service, boost::lexical_cast<unsigned int>(argv[1]), argv[2]);
// run the IO service as a separate thread, so the main thread can block on standard input
boost::thread t(boost::bind(&boost::asio::io_service::run, &io_service));
while (c.active()) // check the internal state of the connection to make sure it's still running
{
char ch;
cin.get(ch); // blocking wait for standard input
if (ch == 3) // ctrl-C to end program
break;
c.write(ch);
}
c.close(); // close the minicom client connection
t.join(); // wait for the IO service thread to close
}
catch (exception& e)
{
cerr << "Exception: " << e.what() << "\n";
}
#ifdef POSIX // restore default buffering of standard input
tcsetattr(0, TCSANOW, &stored_settings);
#endif
return 0;
}

Answer 3

You should check if you have access to /dev/tty.usbmodem641. The usual way in Linux is to add the user to the proper group with adduser.
By the way, I know that to access the serial port, one needs to open /dev/ttyS0 (for COM1), until /dev/ttyS3. See for example this example in C.