Luc Hermans wrote:
>
> > From: Luc Hermans <[EMAIL PROTECTED]>
> > > Hi,
> > > In my embedded linux application I've written a modbus interface (RS485)
> > > on /dev/ttyS1 (cfmakeraw) the RTS is used to enable/disable the RS485
> > > driver.
> > >
> > > I use the ioctl(fd[c], TIOCMSET, &status) fucntion.to change RTS'
> > >
> > > Before sending I enable RTS, send all data, (the RS485 echo's all)
> > > wait until all data is received, and then disable RTS.
> > >
> > > The problem is that the RTS is sometimes dropped to slow, up to 5ms,
> > > witch is to long (because I get an answer from the polled device withing
> > > 3ms)
> > > but sometimes also fas, lesser then 500us.
> > >
> > > What is the cause of this ?
> > > I work with an i386 40MHz cpu at 9600 baud
> >
> > Interupt latency and kernel to application latency...
> >
> > This is the biggest pain in the butt with RS-485. After you send your
> > data you need to flip the direction of the transceivers. The hardware
> > that people try to use for RS-485, is pretty strapped to do the job
> > correctly (ie in hardware).
> >
> > Basically if you want any chance of making this work you will need to
> > handle the ISR yourself and you need to flip the direction control
> > (aka RTS) in the ISR. Note: You can NOT use the transmit interrupt to
> > do the flipping because the TX ISR will happen when the last byte
> > begins to shift out of the UART, not when it's finished shifting
> > out!! The way most people handle this is to listen to what you are
> > transmitting, and flip when you receive your last byte.
> >
> > So we are to the point where you are going to rewrite the serial port
> > driver. That shouldn't be too bad since it's just a minor mod.
> >
> > The good news here is it's a lot easier to write a driver for Linux
> > than it is for say, Micros~1 windows.
> >
> > But you still have the problrm of interrupt latency. You're new
> > driver, although it works a lot better, will still NOT be bullet
> > proof. Why... because you are still at the mercy of ethernet and
> > harddrive drivers that may disable interrupts for longer than you're
> > maximum letency requirement of 500uS.
> >
> > To get around this you might want to look at the RT-Linux stuff. With
> > that you can make you RS-485 ISRs top dog in the processing chain.
> >
> > http://www.rtlinux.org/~rtlinux/
> >
> > Good luck...
> >
For Real-time capability on Linux, you will also want to check out the
RTAI derivative of rtlinux (which offers many enhanced features) at:
http://server.aero.polimi.it/projects/rtai/index.html
with a short descriptive document at:
http://www.zentropix.com/support/document/helpdox/rtai.pdf
Also, the www.realtimelinux.org site includes links and documentation to
all of the known real-time linux alternatives.
-dave
> > Hey by the way, MODBUS ??? That sounds like industrial automation,
> > PLC stuff right. I've been watching with anticipation for a while and
> > so far your one of the first that I've seen playing with Linux in
> > that arena. I'm happy to see that not everyone in the industrial
> > automation world is in bed with WinCE. Have you ever done anything
> > with DeviceNet?
> >
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>
> Hi Thanks all for your tips
>
> I have solved this problem by modifying the driver serial.c,serial.h, ioctls.h
>
> and add a new ioctl( TIOCMRTSOFF, x) call which tells serial.c to
> swtich RTS off after receiving x chars. (done in the ISR)
> This works the following way ex. sending a command of 10 chars.
>
> - put RTS on (RS485 tx enable) ioctl( TIOCMSET)
> - call ioctl( TIOCMRTSOFF, 10) if I had to send a command of 10 chars
> - write ( the 10 data chars)
> - after the 10 chars are echoed (because the RS485 receiver is allways
> enabled)
> the RTS will automaticly switch off.
>
> I you want this modification I will be happy to give it.
>
> Luc
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