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
On Friday, November 29, 2013 6:27:44 PM UTC-5, Dave Hylands wrote:
>
> Hi Mike,
>
> On Fri, Nov 29, 2013 at 9:56 AM, Mikester <[email protected]<javascript:>>
> wrote:
> >
> >
> > I have a USB FTDI serial device that connects to the BBB and it shows up
> under /dev/ttyUSB0.
> >
> > After bootup If I try connecting to the device via a c program I get an
> error "11 - Resource temporarily unavailable".
> >
> > However, if I first connect to the device using something like "screen"
> or "minicom" then kill the process, I am able to use my c program to access
> the serial device without any problems.
> >
> > Is there some sort of initialization script I need to run in order to
> access the USB serial device?
> >
> > If I use stty -F /dev/ttyUSB0 -a I get
> >
> > speed 230400 baud; rows 0; columns 0; line = 0;
> > intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
> eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z;
> > rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
> > -parenb -parodd cs8 -hupcl -cstopb -cread -clocal -crtscts
> > -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon
> -ixoff -iuclc -ixany -imaxbel -iutf8
> > opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0
> vt0 ff0
> > isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop
> -echoprt echoctl echoke
>
> I seem to recall that modemmanager and/or brltty (on your host computer)
> may cause access problems.
> So one of the first things I do is to uninstall those 2 packages (this may
> not be what you're running into, but I've been burned by it several times,
> so I figured I'd mention it).
>
> It looks like CLOCAL is turned off, which means that it expects modem
> control lines to take effect (including carrier detect).
>
> I've written a small sample program I call sertest which you can find here:
> https://github.com/dhylands/projects/blob/master/host/sertest/sertest.c
>
> It opens a serial port and takes any characters you type and sends them to
> the serial port, and prints any received characters to stdout.
>
> If you short RxD and TxD on your USB serial dongle, then you should be
> able to see what you type.
>
> Opening the serial port using O_NONBLOCK is desirable (otherwise you may
> hang if the carrier detect isn't asserted), but it will cause the error
> that you're seeing when you try to read data from the serial port.
>
> You'll see here:
> https://github.com/dhylands/projects/blob/master/host/sertest/sertest.c#L225
> that I turn off the non-blocking behaviour after opening the serial port,
> which means that the read will block instead of returning EAGAIN.
>
> If you're still having issues, it would be useful if you could post the
> code that you're using. You didn't mention which exact call was returning
> the EAGAIN error.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
--
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