Dave I appreciate the info.
Unfortunately still no joy. I used
int qsb = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NONBLOCK);
fcntl(qsb, F_SETFL, fcntl(qsb, F_GETFL ) & ~O_NONBLOCK);
fcntl after my open() but the only result is that the process hangs
indefinately and there is no output. If I run then kill "screen" before
executing the code change works albeit much slower.
Did I put the call in the wrong location?
On Monday, December 2, 2013 4:55:55 PM UTC-5, Dave Hylands wrote:
>
> Hi Mike,
>
>
> On Mon, Dec 2, 2013 at 1:45 PM, Mikester <[email protected] <javascript:>>
> wrote:
> >
> > Im using a TTYL encoder/decoder counter device that translates the
> encoder pulses to a USB serial port.
> >
> > the call being used is
> > int qsb = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NONBLOCK);
> > ...
> > ioResult = read(qsb, response, responseSize); //EAGAIN error in here
>
> Exactly. This happens because the serial port was opened using O_NONBLOCK.
>
> Opening with O_NONBLOCK is desirable, so you need to add
>
> fcntl(gsb, F_SETFL, fcntl(gsb, F_GETFL ) & ~O_NONBLOCK);
>
> to turn off non-blocking mode once you've done the open.
>
> Then the read command will block and wait for the data, rather than
> returning EAGAIN if the data isn't ready yet.
>
> Also - In general I highly recommend that you put the port in raw mode.
> Otherwise if you're sending binary data you'll get a bunch of weird
> translations happening and your data will get corrupted. In canonical mode,
> it interprets things like backspaces, and may massage your CRs
>
> I use VMIN = 1 and VTIME = 0 which means that read will return as soon as
> a single byte is available, but it will return more if its available.
> So you may get less data that you asked for, but on the other hand if you
> ask for 6 bytes, it won't actually wait for 6 bytes to arrive.
>
> It kind of boils down to the type of error handling that you want to do.
> Here's a site that explains the VMIN/VTIME configuration:
> http://www.unixwiz.net/techtips/termios-vmin-vtime.html
>
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
--
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