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Read from Serial Port -> Only zeroes
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Frank Neuhaus
Guest
PostPosted: Fri Oct 31, 2008 12:45 am    Post subject: Read from Serial Port -> Only zeroes Reply with quote
Hi

I am trying to read some data from a serial port. I tried to do so via
gtkterm with 4800 baud, no parity, 8 bits, 1 stopbit, no flowcontrol -
and it worked as intended. Then i tried to write a program to do the
same. I am using the following initialization (i have used the same
code for another device and it worked perfectly there... just in this
case it doesnt...):

tcgetattr ( aFd,&tio );
cfsetispeed ( &tio,B4800 ); // bitrate for input
cfsetospeed ( &tio,B4800 ); // bitrate for outputMsg

tio.c_cflag= ( tio.c_cflag & ~CSIZE ) | CS8; // data bits = 8bit

tio.c_iflag&= ~ ( BRKINT | ICRNL | ISTRIP );
tio.c_iflag&= ~ IXON; // no XON/XOFF
tio.c_cflag&= ~PARENB; // no parity
tio.c_cflag&= ~CRTSCTS; // no CTS/RTS
tio.c_cflag&= ~CSTOPB; // stop bit = 1bit

// Other
tio.c_lflag &= ~ ( ISIG | ICANON | ECHO );

// Commit
tcsetattr ( aFd,TCSANOW,&tio ); // this succeeds

Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

Any idea what I am doing wrong?

Thanks alot
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Frank Neuhaus
Guest
PostPosted: Fri Oct 31, 2008 7:36 am    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On 31 Okt., 06:49, Jasen Betts <ja...@xnet.co.nz> wrote:
Quote:
On 2008-10-31, Frank Neuhaus <dta...@gmail.com> wrote:

Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

I suspect it's the call to read();

In what way? According to the man pages, read returns a positive
integer when it succeeds... and thats what it does for me?
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Frank Neuhaus
Guest
PostPosted: Fri Oct 31, 2008 7:36 am    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
Quote:
Not offhand.  Can you post your complete code, if it isn't excessively long?

Ok here it is (even though I doubt it helps as its not much more than
what I already posted):

#include "MyReceiver.h"

#include <sys/termios.h>
#include <sys/ioctl.h>

#include <fcntl.h>
#include <assert.h>
#include <errno.h>
#include <algorithm>


#define BAUD B4800

namespace Devices
{
//-----------------------------------------------------------------------------
MyReceiver::MyReceiver()
{
m_port=0;
}
//-----------------------------------------------------------------------------
MyReceiver::~MyReceiver()
{
if (m_port)
close(m_port);
}
//-----------------------------------------------------------------------------
bool MyReceiver::init(const char* dev)
{
m_port = open ( dev, O_RDWR );
if (!m_port)
{
LogError("Couldnt connect to Device!");
return false;
}
if (!setupSerial(m_port,BAUD))
{
LogError("setupSerial failed!");
return false;
}
return true;
}
//-----------------------------------------------------------------------------
void MyReceiver::read()
{
char buffer[256];
memset(buffer,0,256);
int reada=::read((int)m_port,&buffer, 200);
std::cout << reada << "X" << buffer << std::endl;
for (int i=0;i<reada;i++)
printf("%02lX ",buffer[i]); // everything 00 here
}
//-----------------------------------------------------------------------------
bool MyReceiver::setupSerial (int aFd, speed_t speed)
{
struct termios tio;

tcgetattr ( aFd,&tio );
cfsetispeed ( &tio,speed ); // bitrate for input
cfsetospeed ( &tio,speed ); // bitrate for outputMsg

tio.c_cflag= ( tio.c_cflag & ~CSIZE ) | CS8; // data bits = 8bit

tio.c_iflag&= ~ ( BRKINT | ICRNL | ISTRIP );
tio.c_iflag&= ~ IXON; // no XON/XOFF
tio.c_cflag&= ~PARENB; // no parity
tio.c_cflag&= ~CRTSCTS; // no CTS/RTS
tio.c_cflag&= ~CSTOPB; // stop bit = 1bit

// Other
tio.c_lflag &= ~ ( ISIG | ICANON | ECHO );

// Commit
if ( tcsetattr ( aFd,/*TCSADRAIN*/TCSANOW,&tio ) ==0 ) return true;

return false;
}
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
};
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Jasen Betts
Guest
PostPosted: Fri Oct 31, 2008 7:36 am    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On 2008-10-31, Frank Neuhaus <dtag00@gmail.com> wrote:

Quote:
Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

I suspect it's the call to read();

Bye.
Jasen
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Nate Eldredge
Guest
PostPosted: Fri Oct 31, 2008 7:36 am    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
Frank Neuhaus <dtag00@gmail.com> writes:

Quote:
Hi

I am trying to read some data from a serial port. I tried to do so via
gtkterm with 4800 baud, no parity, 8 bits, 1 stopbit, no flowcontrol -
and it worked as intended. Then i tried to write a program to do the
same. I am using the following initialization (i have used the same
code for another device and it worked perfectly there... just in this
case it doesnt...):

tcgetattr ( aFd,&tio );
cfsetispeed ( &tio,B4800 ); // bitrate for input
cfsetospeed ( &tio,B4800 ); // bitrate for outputMsg

tio.c_cflag= ( tio.c_cflag & ~CSIZE ) | CS8; // data bits = 8bit

tio.c_iflag&= ~ ( BRKINT | ICRNL | ISTRIP );
tio.c_iflag&= ~ IXON; // no XON/XOFF
tio.c_cflag&= ~PARENB; // no parity
tio.c_cflag&= ~CRTSCTS; // no CTS/RTS
tio.c_cflag&= ~CSTOPB; // stop bit = 1bit

// Other
tio.c_lflag &= ~ ( ISIG | ICANON | ECHO );

// Commit
tcsetattr ( aFd,TCSANOW,&tio ); // this succeeds

Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

Any idea what I am doing wrong?

Not offhand. Can you post your complete code, if it isn't excessively long?
Back to top
JH Trauntvein
Guest
PostPosted: Fri Oct 31, 2008 11:42 am    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On Oct 30, 6:45 pm, Frank Neuhaus <dta...@gmail.com> wrote:
Quote:
Hi

I am trying to read some data from a serial port. I tried to do so via
gtkterm with 4800 baud, no parity, 8 bits, 1 stopbit, no flowcontrol -
and it worked as intended. Then i tried to write a program to do the
same. I am using the following initialization (i have used the same
code for another device and it worked perfectly there... just in this
case it doesnt...):

tcgetattr ( aFd,&tio );
cfsetispeed ( &tio,B4800 ); // bitrate for input
cfsetospeed ( &tio,B4800 ); // bitrate for outputMsg

tio.c_cflag= ( tio.c_cflag & ~CSIZE ) | CS8; // data bits = 8bit

tio.c_iflag&= ~ ( BRKINT | ICRNL | ISTRIP );
tio.c_iflag&= ~ IXON;    // no XON/XOFF
tio.c_cflag&= ~PARENB;   // no parity
tio.c_cflag&= ~CRTSCTS;  // no CTS/RTS
tio.c_cflag&= ~CSTOPB;   // stop bit = 1bit

// Other
tio.c_lflag &= ~ ( ISIG | ICANON | ECHO );

// Commit
tcsetattr ( aFd,TCSANOW,&tio ); // this succeeds

Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

Any idea what I am doing wrong?

The fact that characters are being returned but they are only zeroes
seems to indicate a serial framing error. Either the baud rate,
character width, or parity values are wrong.
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Frank Neuhaus
Guest
PostPosted: Fri Oct 31, 2008 12:56 pm    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On Oct 31, 12:26 pm, Jasen Betts <ja...@xnet.co.nz> wrote:
Quote:
On 2008-10-31, Frank Neuhaus <dta...@gmail.com> wrote:

Not offhand.  Can you post your complete code, if it isn't excessively long?
Ok here it is (even though I doubt it helps as its not much more than

There was nothing obviously wrong with your discription of the code.

namespace Devices
{
//-----------------------------------------------------------------------------
MyReceiver::MyReceiver()
{
   m_port=0;

0 is a valid file handle, should probably use -1 for error value.

}
//-----------------------------------------------------------------------------
MyReceiver::~MyReceiver()
{
   if (m_port)
           close(m_port);

as above.

}
//-----------------------------------------------------------------------------
bool MyReceiver::init(const char* dev)
{
   m_port = open ( dev, O_RDWR );
   if (!m_port)

see above: you should definately use -1 (or <0) here.

   {
           LogError("Couldnt connect to Device!");

perror, or strerror is useful at times like these.

...

void MyReceiver::read()
{
   char buffer[256];
   memset(buffer,0,256);
   int reada=::read((int)m_port,&buffer, 200);

                  --     -----       -
what does the :: do?
why the cast?
does c++ need the address operator here?

   std::cout << reada << "X" << buffer << std::endl;
   for (int i=0;i<reada;i++)
           printf("%02lX ",buffer[i]); // everything 00 here
}
//-----------------------------------------------------------------------------

...

   tio.c_iflag&= ~ IXON;    // no XON/XOFF
   tio.c_cflag&= ~PARENB;   // no parity

I'm assuming you've checked these lines (and their neighbours) and
you're setting the correct flags in the structs members, and the
correct baud rate... if your rate is 10 times or more higher than the
devices rate all you'll read are zero bytes (actually break symbols but
you're not checking for them).

I have compared them with http://ulisse.elettra.trieste.it/services/doc/serial/config.html
and IMHO all my settings are correctly set (4800 Baud, 8 Databit, 1
Stopbit, No Parity, No Flow control - these are also the settings that
work in gtkterm!)
I also tried various combinations of the settings and none of those
worked, it always shows the exact same behavior. When I use incorrect
settings inside gtkterm, it shows at least something - it never shows
0 chars only.
Back to top
Frank Neuhaus
Guest
PostPosted: Fri Oct 31, 2008 1:21 pm    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On Oct 31, 1:56 pm, Frank Neuhaus <dta...@gmail.com> wrote:
Quote:
On Oct 31, 12:26 pm, Jasen Betts <ja...@xnet.co.nz> wrote:



On 2008-10-31, Frank Neuhaus <dta...@gmail.com> wrote:

Not offhand.  Can you post your complete code, if it isn't excessively long?
Ok here it is (even though I doubt it helps as its not much more than

There was nothing obviously wrong with your discription of the code.

namespace Devices
{
//-----------------------------------------------------------------------------
MyReceiver::MyReceiver()
{
   m_port=0;

0 is a valid file handle, should probably use -1 for error value.

}
//-----------------------------------------------------------------------------
MyReceiver::~MyReceiver()
{
   if (m_port)
           close(m_port);

as above.

}
//-----------------------------------------------------------------------------
bool MyReceiver::init(const char* dev)
{
   m_port = open ( dev, O_RDWR );
   if (!m_port)

see above: you should definately use -1 (or <0) here.

   {
           LogError("Couldnt connect to Device!");

perror, or strerror is useful at times like these.

...

void MyReceiver::read()
{
   char buffer[256];
   memset(buffer,0,256);
   int reada=::read((int)m_port,&buffer, 200);

                  --     -----       -
what does the :: do?
why the cast?
does c++ need the address operator here?

   std::cout << reada << "X" << buffer << std::endl;
   for (int i=0;i<reada;i++)
           printf("%02lX ",buffer[i]); // everything 00 here
}
//-----------------------------------------------------------------------------

...

   tio.c_iflag&= ~ IXON;    // no XON/XOFF
   tio.c_cflag&= ~PARENB;   // no parity

I'm assuming you've checked these lines (and their neighbours) and
you're setting the correct flags in the structs members, and the
correct baud rate... if your rate is 10 times or more higher than the
devices rate all you'll read are zero bytes (actually break symbols but
you're not checking for them).

I have compared them withhttp://ulisse.elettra.trieste.it/services/doc/serial/config.html
and IMHO all my settings are correctly set (4800 Baud, 8 Databit, 1
Stopbit, No Parity, No Flow control - these are also the settings that
work in gtkterm!)
I also tried various combinations of the settings and none of those
worked, it always shows the exact same behavior. When I use incorrect
settings inside gtkterm, it shows at least something - it never shows
0 chars only.


Ok thanks for your help everyone - I think i solved it... Obviously
the device i was working with has some "timing issues" - i found some
information on that on the web somewhere. After calling setattr, i am
now doing a 200 ms sleep, then a tcflush, another 200 ms sleep, and
another tcflush - then it all works great... weird device... (its a
GPS receiver...)
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Jasen Betts
Guest
PostPosted: Fri Oct 31, 2008 4:26 pm    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On 2008-10-31, Frank Neuhaus <dtag00@gmail.com> wrote:
Quote:
On 31 Okt., 06:49, Jasen Betts <ja...@xnet.co.nz> wrote:
On 2008-10-31, Frank Neuhaus <dta...@gmail.com> wrote:

Then I am trying to use the read() function to read some data from the
port. read returns some positive number indicating that data could be
read successfully, but the buffer that i passed is only filled with
zeroes without any sign of an error.

I suspect it's the call to read();

In what way?

wrong parameters? not having the source it's imposible to do other
than guess.

Quote:
According to the man pages, read returns a positive
integer when it succeeds... and thats what it does for me?

means you gave it a valid file handle... but was it the right one
(etc...)

Bye.
Jasen
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Jasen Betts
Guest
PostPosted: Fri Oct 31, 2008 4:26 pm    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
On 2008-10-31, Frank Neuhaus <dtag00@gmail.com> wrote:
Quote:
Not offhand.  Can you post your complete code, if it isn't excessively long?


Quote:
Ok here it is (even though I doubt it helps as its not much more than

There was nothing obviously wrong with your discription of the code.

Quote:
namespace Devices
{
//-----------------------------------------------------------------------------
MyReceiver::MyReceiver()
{
m_port=0;

0 is a valid file handle, should probably use -1 for error value.

Quote:
}
//-----------------------------------------------------------------------------
MyReceiver::~MyReceiver()
{
if (m_port)
close(m_port);

as above.

Quote:
}
//-----------------------------------------------------------------------------
bool MyReceiver::init(const char* dev)
{
m_port = open ( dev, O_RDWR );
if (!m_port)

see above: you should definately use -1 (or <0) here.

Quote:
{
LogError("Couldnt connect to Device!");

perror, or strerror is useful at times like these.

....

Quote:
void MyReceiver::read()
{
char buffer[256];
memset(buffer,0,256);
int reada=::read((int)m_port,&buffer, 200);
-- ----- -

what does the :: do?
why the cast?
does c++ need the address operator here?

Quote:
std::cout << reada << "X" << buffer << std::endl;
for (int i=0;i<reada;i++)
printf("%02lX ",buffer[i]); // everything 00 here
}
//-----------------------------------------------------------------------------

....

Quote:
tio.c_iflag&= ~ IXON; // no XON/XOFF
tio.c_cflag&= ~PARENB; // no parity

I'm assuming you've checked these lines (and their neighbours) and
you're setting the correct flags in the structs members, and the
correct baud rate... if your rate is 10 times or more higher than the
devices rate all you'll read are zero bytes (actually break symbols but
you're not checking for them).

--

Bye.
Jasen
Back to top
Ulrich Eckhardt
Guest
PostPosted: Sat Nov 01, 2008 7:52 pm    Post subject: Re: Read from Serial Port -> Only zeroes Reply with quote
Jasen Betts wrote:
Quote:
On 2008-10-31, Frank Neuhaus <dtag00@gmail.com> wrote:
void MyReceiver::read()
{
char buffer[256];
memset(buffer,0,256);
int reada=::read((int)m_port,&buffer, 200);
-- ----- -
what does the :: do?

Lookup of 'read' would otherwise have found MyReceiver::read.

Quote:
why the cast?

It isn't necessary and using C-style casts are a damn bad idea anyway.

Quote:
does c++ need the address operator here?

No, but it actually shows quite clearly that you are passing the address of
a buffer to read(), so why not. I'm not sure though why there is not a
single case of 'sizeof buffer' in above code though...

Quote:
std::cout << reada << "X" << buffer << std::endl;
for (int i=0;i<reada;i++)
printf("%02lX ",buffer[i]); // everything 00 here
}

Mixing IOStream with stdio is IMHO a bad idea. Further, the parameters don't
match, 'lX' claims that you pass a long (unsigned long?), but you are
passing a char. So, for a proper result, use a buffer of unsigned char,
static_cast it to unsigned (Note: using unsigned arithmetic avoid sign
extension) and then use '%02X' as format.

BTW: there is no semicolon after every closing curly bracket. In particular,
there must not be one after the closing ones of namespaces or function,
though they are actually okay after the definition of a memberfunction
inside a class definition.

Uli
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