Re: [Ftdi-usb-sio-devel] latency on serial port

[Bill R. <bil...@gm...>]

Y

On Wed, Jun 23, 2010 at 5:36 PM, Tao Jin <jin...@gm...> wrote:

> Hi Bill,
>
> Thanks a lot for your quick response!  I set VMIN=1, VTIME=0 so the
> serial device works in block mode.
>
> I just tried the serial echo test on my linksys linux router, which
> has onboard serial port.  setserial works for me and the RTT latency
> can be reduced down to below 1ms
>
> But when I did the same test on my laptop with a FT232RL usb-to-serial
> adapter, the setserial doesn't work.   Probably the usb serial device
> works in different way?  Sorry I am new to Linux kernel
>

Y ou could try using stty instead of setserial.

I can't remember the calls setserial makes. It's possible one is not
supported by the driver.

I think stty does it using supported calls.


If you could send an output of

strace -v setserial what ever options you used

it'll give me a hint.

Also you might need to hold the device open. (in a separate window run
cat > /dev/ttyUSBthenumberoftheport
and just leave it running - it's doing nothing except holding the port open)

then try setserial or stty and your test program






> Thanks again!
>
> jintao
>
> On Wed, Jun 23, 2010 at 1:14 AM, Bill Ryder <bil...@gm...>
> wrote:
> > Have a look at this:
> >
> > http://www.tldp.org/HOWTO/Serial-Programming-HOWTO/index.html
> >
> > You are probably setup to do 0.1s timeout when reading the port which is
> the
> > default.
> >
> > you need to set  VMIN/VTIME properly.
> >
> >
> >
> >
> > On Wed, Jun 23, 2010 at 4:57 PM, Tao Jin <jin...@gm...> wrote:
> >>
> >> Dear all,http://www.tldp.org/HOWTO/Serial-Programming-HOWTO/index.html
> >>
> >> I am trying to get my laptop running Ubuntu to communicate with a
> >> sensor through serial interface.  I used usb-to-serial adapter to
> >> connect the laptop and sensor serial RX/TX pins.  I observed a long
> >> latency when I do a one-byte serial echo test, around 10ms.
> >>
> >> I googled a bit, and seems a lot of people had the same issue. someone
> >> said setserial /dev/ttyS* low_latency can get the serial device work
> >> in low latency mode, with higher CPU load.  I tried, that doesn't work
> >> for me.
> >>
> >> Anyone here know about this latency issue on serial link?  Could you
> >> please provide some guidance to resolve this issue?
> >>
> >> Thanks a lot in advance!
> >>
> >> jintao
> >>
> >>
> >>
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