Using fstream with USB Serial Port

Jochen Arndt

That is probably because the serial device has echoing enabled. Such is used with serial terminals. When a character is received, it is send back to be displayed in the terminal (which usually does not display the character locally before sending it but waits for the echoed character to display that). To avoid that you have to disable echoing by configuring the serial device initially:

stty -F /dev/ttyACM0 raw pass8 -echo -hup -clocal 9600

Note that it might be necessary to specify also other settings, remove some used above, and change the baud rate. See stty(1): change/print terminal line settings - Linux man page[^] for the available options. Before changing the options I suggest to execute

stty -F /dev/ttyACM0

to check the current settings. As already mentioned by others it is rather uncommon to use C++ streams with serial ports. The common method is using termios and the IO functions from the standard C library, or a library like Serial Ports - 1.65.0[^] from Boost.Asio - 1.66.0[^] . Related reads: Serial HOWTO[^] Serial Programming/Serial Linux - Wikibooks, open books for an open world[^]

OscardelaGrouch

Jochen Arndt wrote:

The common method is using termios and the IO functions from the standard C library

Thank you for the reply. It is ironic that I ran across this yesterday and was able to write something that used this successfully. I will use this in my final code I am sure. My issue now is the write command, it seems to be char based, so I'm working on sending a char buffer. Once again, thanks to you and the forum, I really appreciate it. OdlG

OscardelaGrouch

Thanks for the help so far. I have written a small PC App program to try termios out. I am having two issues. First is the first time I send data to the PIC, the return response is junk. Second is the response is out of sync with the sent command, it takes sending the command a second time for the response string to be correct. Would appreciate a second pair of eyes to see what is the problem. PC app code ..

int main(int argc, char** argv)
{
ComPort MyComPort;
char TXBuf[255];
char RXBuf[255];
char c;

while(1)
{
    cout << "(U)load New Map, (N)ext Map, (Q)uit .." << endl;
    cin >> c;
    c = (char)toupper(c);
    if (c == 'Q') return 0;
    if (c == 'U')
    {
        strcpy(TXBuf, "UPLOAD NEW MAP\\n");
        MyComPort.WriteComPortDataMsg(TXBuf, strlen(TXBuf));
        MyComPort.ReadComPortDataMsg(RXBuf);
        cout << "RXBuf = " << RXBuf << endl;
    }
    else if (c == 'N')
    {
        strcpy(TXBuf, "NEXT MAP\\n");
        MyComPort.WriteComPortDataMsg(TXBuf, strlen(TXBuf));
        MyComPort.ReadComPortDataMsg(RXBuf);
        cout << "RXBuf = " << RXBuf << endl;
    }
    cout << endl;
}
return 0;

}

class defs ..

/***** Class Declaration ******************************************************/
class ComPort
{
public:
ComPort();
~ComPort();
void WriteComPortDataMsg(char *data, char num);
void ReadComPortDataMsg(char *data, char dlimc);
void ReadComPortDataMsg(char *data);
void SetDlimiter(char dlimc);
private:
char dlim;
struct termios config;
int hComPort;
bool Error;
};

void ComPort::WriteComPortDataMsg(char *data, char num)
{
for (int cnt = 0; cnt < num; cnt++)
{
write(hComPort, &data[cnt], 1); // write char to Com Port
}
tcdrain(hComPort);
}

void ComPort::ReadComPortDataMsg(char *data)
{
int cnt = 0;
char c;

data\[0\] = 0x00;
do
{
    read(hComPort, &c, 1);
    if ((c >= 0x20) && (c <= 0x7E)) data\[cnt++\] = c;
} while ((c != dlim) && (cnt < 255));
data\[cnt++\] = '\\0';

}

responses ..

(U)load New Map, (N)ext Map, (Q)uit ..
u
RXBuf = MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM

Jochen Arndt

I did not see any code that initialises the serial port. So it will still use the current settings as shown by the stty command line tool. You have to use tcsetattr to select the same basic settings as on the PIC (probably 8N1; baud rate seems to mach already), disable all kinds of flow control (software and hardware), and all additional features like echoing. Example:

memset(&config, 0, sizeof(config));
config.c_cflag = CS8 | CREAD | CLOCAL; // 8N1, enable receiver, ignore ctrl lines
cfsetospeed(&config, B9600); // baud rate used by the PIC
cfsetispeed(&config, B9600);
tcsetattr(hComPort, TCSANOW, &config);

With new line terminated data transfers, you may also use the canonical mode (ICANON).

OscardelaGrouch

Jochen Arndt wrote:

I did not see any code that initialises the serial port.

There is, I was having trouble with the forum editor, thought maybe my code was too long. The first part is here ..

/***** Class Declaration ******************************************************/
class ComPort
{
public:
ComPort();
~ComPort();
void WriteComPortDataMsg(char *data, char num);
void ReadComPortDataMsg(char *data, char dlimc);
void ReadComPortDataMsg(char *data);
void SetDlimiter(char dlimc);
private:
char dlim;
struct termios config;
int hComPort;
bool Error;
};

ComPort::ComPort()
{
dlim = '\n';
char c;

Error = false;
hComPort = open("/dev/ttyACM0", O\_RDWR | O\_NOCTTY | O\_NDELAY);
if (hComPort == -1)
{
    cout << "failed to open port\\n" << endl;
    cout << "Hit  to Stop .." << endl;
    cin >> c;
    c = (char)toupper(c);
    if (c == 'S') Error = true;
}

if(!isatty(hComPort))
{
    cout << "file descriptor is  not pointing to a TTY device\\n" << endl;
    cout << "Hit  to Stop .." << endl;
    cin.get(c);
    if ((c == 's' || c == 'S')) Error = true;
}

if(tcgetattr(hComPort, &config) < 0)
{
    cout << "configuration of the serial interface not found\\n" << endl;
    cout << "Hit  to Stop .." << endl;
    cin >> c;
    c = (char)toupper(c);
    if (c == 'S') Error = true;
}

//
// Input flags - Turn off input processing
//
// convert break to null byte, no CR to NL translation,
// no NL to CR translation, don't mark parity errors or breaks
// no input parity check, don't strip high bit off,
// no XON/XOFF software flow control
//
config.c\_iflag &= ~(IGNBRK | BRKINT | ICRNL | INLCR | PARMRK | INPCK | ISTRIP | IXON);

// Output flags - Turn off output processing
//
// no CR to NL translation, no NL to CR-NL translation,
// no NL to CR translation, no column 0 CR suppression,
// no Ctrl-D suppression, no fill characters, no case mapping,
// no local output processing
//
// config.c\_oflag &= ~(OCRNL | ONLCR | ONLRET |
//                     ONOCR | ONOEOT| OFILL | OLCUC | OPOST);
config.c\_oflag = 0;

//
// No line processing
//
// echo off, echo newline off, canonical mode off, 
// extended input processing off, signal ch

Jochen Arndt

Use the code from my post. It sets all while you are modifying the existing where something might be still set / cleared. CREAD is probably set but what about CLOCAL?

OscardelaGrouch

Jochen Arndt wrote:

CREAD is probably set but what about CLOCAL?

I had the same code except .. memset

if(tcgetattr(hComPort, &config) < 0)
{
cout << "configuration of the serial interface not found\n" << endl;
cout << "Hit to Stop .." << endl;
cin >> c;
c = (char)toupper(c);
if (c == 'S') Error = true;
}
memset(&config, 0, sizeof(config));

this is baudrate code ..

config.c_cflag = CS8 | CREAD | CLOCAL; // 8N1, enable receiver, ignore ctrl lines

if(cfsetispeed(&config, B9600) < 0 || cfsetospeed(&config, B9600) < 0)
{
    cout << "Baud Rate did not set correctly\\n" << endl;
    cout << "Hit  to Stop .." << endl;
    cin >> c;
    c = (char)toupper(c);
    if (c == 'S') Error = true;
}

I get the same result as before. OdlG

Jochen Arndt

The source is probably here:

read(hComPort, &c, 1);
if ((c >= 0x20) && (c <= 0x7E)) data[cnt++] = c;

See read(2): read from file descriptor - Linux man page[^]:

Quote:

Return Value On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal. On error, -1 is returned, and errno is set appropriately. In this case it is left unspecified whether the file position (if any) changes.

read() will return zero when there are no data available. So you have either to clear your c variable before, or - better - check the return value:

int received = read(hComPort, &c, 1);
if (received == 1 && c >= 0x20 && c <= 0x7E)
data[cnt++] = c;
else if (received < 0)
// handle error here

Another option is setting blocking mode (attributes c_cc[VMIN] to non zero and c_cc[VTIME] to the timeout value). Then you will not get zero return values but still have to check for negative return values indicating timeout or other errors.

OscardelaGrouch

I was beginning to think I needed some way to wait on the USB/Com Port, but wasn't sure how to do that. This works great. My code change ..

void ComPort::ReadComPortDataMsg(char *data)
{
int cnt = 0;
int received;
char c;

data\[0\] = 0x00;
do
{
    received = read(hComPort, &c, 1);
    if ((received == 1) && (c >= 0x20) && (c <= 0x7E))
    {
        data\[cnt++\] = c;
    }
    else if (received < 0)
    {
        
    }
} while ((c != dlim) && (cnt < 255));
data\[cnt++\] = '\\0';

}

Not sure how to deal with received < 0. Do you know where can I read up on

struct termios

and

global errno variable

? Thanks!!! OdlG

Jochen Arndt

For all standard C library features you can get the description from the shell command prompt as man page:

man errno

will shown the content of the errno(3) - Linux manual page[^]. Similar for termios which is covered by a single man page: termios(3) - Linux man page[^]. If you want to get the above links, just use the same "man " with your favorite search engine. Other good sources for the standard C and C++ libraries are cplusplus.com - The C++ Resources Network[^] and cppreference.com[^].

OscardelaGrouch

Question on how to error handle. I am falling into the received < 0 but if I just ignore it and keep looking for chars, I get a complete, correct message. Is it safe to do that or should I clear the error? And How? Thanks in advance ;) OdlG

Jochen Arndt

What is the error (errno) code then?

OscardelaGrouch

Looks like it is 11

Jochen Arndt

That is EGAIN (you can look it up in errno.h and get the corresponding error message with strerror(3) - Linux manual page[^] ) and in most cases (as here) not an error. You are just calling read again too fast (the next character has not been received so far). To avoid this you can set a timeout value. Then the read() will block until data are available or a timeout occured.