Serial Port transmission not working

[hthakar]

I am trying serial communication on Ubuntu 9.04 using C/C++.

I have installed GtkTerm & minicom after unsuccessfully trying various C codes. Both these applications do not transmit the data.There is no write error as such but the data is not transmitted immediately on the bus.
Seems that the write operation only updates the transmit buffer and doesn't transmit the data on serial bus.

This occurs while using a null modem (loop-back) as well as connecting to another PC.

The transmission occurs only after receiving some data from the other terminal.
can someone help us on this?

regards,
Harsh

[jpkotta]

For your C code, try tcdrain(). See also the termios(3) man page.


#include <termios.h>
#include <unistd.h>
int tcdrain(int fd);

tcdrain() waits until all output written to the object referred to by
fd has been transmitted.



I have never had such a problem with minicom. You may want to try screen in tty mode or kermit.

[hthakar]

Hey..Thanks for the answer.
I tried per your suggestions, but didn't work.
Same results with Kermit as well, there is no transmission unless some byte is received.:(
Any idea?
regards,

[jpkotta]

Does it work perfectly after receiving one piece of data? Or does it only transmit a limited number of bytes after receiving one?

How do you know that it isn't transmitting? Are you looking at the TX pin with an oscope or voltage meter or something like that?

You could try simply dumping data into the device file, but this isn't much different than writing to it from a C program.

echo hello > /dev/ttyS0

Sometimes you need to enable a serial port in the BIOS, but that's probably not the problem because you say it works sometimes.

That's about all I can think of.

[hthakar]

After receiving some byte, The string that was typed previously is transmitted. For instance if I type ABCD followed by ENTER key, then the string "ABCD" is transmitted once I receive a byte.
I also verified this with oscilloscope.

The behavior is always like that. The data written to the port is always held in the Tx buffer and actual transmission is triggered by some reception!

[hthakar]

ok! I tried to use the same program with another PC having an embedded RS232 port and everything works fine.

If I use the RS232 connected through a PCI slot then the problem occurs.

Any idea??

[jpkotta]

Probably a bug in the driver. Maybe try a USB to serial converter. They're very cheap and I've had good luck with them.