Serial to Ethernet Device Servers on Linux

Hello there!

I am new to low level programing. I have a Moxa 5232 Device server. I
have a rs485 device that I like to be able to access from a Linux
computer. It is a 2 wire 1/2 duplex rs485 enabled device.

After installing the Moxa drivers I have a "/dev/ttyr00 & /devttyr01"
devices/ports on my Linux server.

In order for me to access this device, I wrote a C program. It
basically opens the port and then writes some ASCII text to the port.
If I send H-E-L-L-O the device should respond with a H-I and vice
verse.

The trick is, since it is a 1/2 duplex I need to be able to send the
initial text and then flip the line so that I could receive the data.
If I don't receive the data with in 100ms then I have to resend the
initial text again.

Is there any sample codes available for C on the net. I have been
searching for the last 3 days and I am having no luck with it.
Unfortunately Moxa does not have any sample codes on their web site and
their tech support is not the greatest. I think they are more into the
windows side than Linux.

Please let me know of any sample codes or ideas that I could make use
of. Thanks in advance for all of your helps :)

CM23

> The trick is, since it is a 1/2 duplex I need to be able to send the
> initial text and then flip the line so that I could receive the data.
> If I don't receive the data with in 100ms then I have to resend the
> initial text again.
I don't know your kernel version, but maybe the comments here help:
http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;h=96cb1f07332b68e5fb5b77dfb68a74d6356c6b64;hb=5cfc35cf79d46af998346e3d5cc66fa344d1af0e;f=drivers/char/moxa.c

If they don't, how about flipping DTR and RTS via moxa_tiocmget()?
        $ man termios
could help, too.


Regards,

Phil

On 2006-10-23, pesinc@gmail.com <pesinc@gmail.com> wrote:
> Hello there!
>
> I am new to low level programing. I have a Moxa 5232 Device server. I
> have a rs485 device that I like to be able to access from a Linux
> computer. It is a 2 wire 1/2 duplex rs485 enabled device.
[...]
> The trick is, since it is a 1/2 duplex I need to be able to send the
> initial text and then flip the line so that I could receive the data.
> If I don't receive the data with in 100ms then I have to resend the
> initial text again.

The hardware ought to be doing that for you.  All of the other
network-attached RS-485 devices I've used handle the line
direction automatically.  If the Moxa doesn't, toss it in the
bin and get something decent.

> Is there any sample codes available for C on the net.

http://www.easysw.com/~mike/serial/serial.html

> I have been searching for the last 3 days and I am having no
> luck with it. Unfortunately Moxa does not have any sample
> codes on their web site and their tech support is not the
> greatest.

Another good reason to toss it and buy something decent.

> I think they are more into the windows side than Linux.
>
> Please let me know of any sample codes or ideas that I could
> make use of. Thanks in advance for all of your helps :)

If the device server doesn't turn the line around
automatically, you've got little hope of doing it yourself.
The Moxa device _might_ be using RTS to control the line
direction.  If so, you can control RTS using TIOCM* ioctl calls
shown in the reference above.  But, that's not reliable even on
locally attached devcices due to latency issues.  It's going to
be even less reliable over Ethernet.

-- 
Grant Edwards                   grante             Yow!  Is it NOUVELLE
                                  at               CUISINE when 3 olives are
                               visi.com            struggling with a scallop
                                                   in a plate of SAUCE MORNAY?

Thank you all. I will look into your suggestions and see if I could get
anywhere with the moxa if not I should go and look for something else.
Any suggestions on the device side?

On Oct 23, 11:05 am, Grant Edwards <gra...@visi.com> wrote:
> On 2006-10-23, pes...@gmail.com <pes...@gmail.com> wrote:
>
> > Hello there!
>
> > I am new to low level programing. I have a Moxa 5232 Device server. I
> > have a rs485 device that I like to be able to access from a Linux
> > computer. It is a 2 wire 1/2 duplex rs485 enabled device.
> [...]
> > The trick is, since it is a 1/2 duplex I need to be able to send the
> > initial text and then flip the line so that I could receive the data.
> > If I don't receive the data with in 100ms then I have to resend the
> > initial text again.The hardware ought to be doing that for you.  All of the other
> network-attached RS-485 devices I've used handle the line
> direction automatically.  If the Moxa doesn't, toss it in the
> bin and get something decent.
>
> > Is there any sample codes available for C on the net.http://www.easysw.com/~mike/serial/serial.html
>
> > I have been searching for the last 3 days and I am having no
> > luck with it. Unfortunately Moxa does not have any sample
> > codes on their web site and their tech support is not the
> > greatest.Another good reason to toss it and buy something decent.
>
> > I think they are more into the windows side than Linux.
>
> > Please let me know of any sample codes or ideas that I could
> > make use of. Thanks in advance for all of your helps :)If the device server doesn't turn the line around
> automatically, you've got little hope of doing it yourself.
> The Moxa device _might_ be using RTS to control the line
> direction.  If so, you can control RTS using TIOCM* ioctl calls
> shown in the reference above.  But, that's not reliable even on
> locally attached devcices due to latency issues.  It's going to
> be even less reliable over Ethernet.
>
> --
> Grant Edwards                   grante             Yow!  Is it NOUVELLE
>                                   at               CUISINE when 3 olives are
>                                visi.com            struggling with a scallop
>                                                    in a plate of SAUCE MORNAY?

On 22 Oct 2006 22:47:54 -0700, pesinc@gmail.com wrote:

>Hello there!
>
>I am new to low level programing. I have a Moxa 5232 Device server. I
>have a rs485 device that I like to be able to access from a Linux
>computer. It is a 2 wire 1/2 duplex rs485 enabled device.

>The trick is, since it is a 1/2 duplex I need to be able to send the
>initial text and then flip the line so that I could receive the data.

And the problem is ?

Any decent USB-to-serial or Ethernet-to-serial RS-485 device will do
that in hardware at the converter. Due to the varying network delays,
there would be a very large uncertainty about turning the transmitter
off if controlled by the host computer (in fact you would have to
readback the transmitted bytes to see when your transmission actually
stopped, before commanding the line to Rx mode).

That Moxa converter contains what they call "Automatic Data Direction
Control" (ADDC) which is controlling the Tx when there is actually
something to send. You do not have to bother about the line turnaround
in RS-485 mode, just throw the converter with a stream of bytes in the
TCP mode and then start reading bytes from the other system.

Paul

Paul Keinanen wrote:
> On 22 Oct 2006 22:47:54 -0700, pesinc@gmail.com wrote:
>
> >Hello there!
> >
> >I am new to low level programing. I have a Moxa 5232 Device server. I
> >have a rs485 device that I like to be able to access from a Linux
> >computer. It is a 2 wire 1/2 duplex rs485 enabled device.
>
> >The trick is, since it is a 1/2 duplex I need to be able to send the
> >initial text and then flip the line so that I could receive the data.
>
> And the problem is ?
>
> Any decent USB-to-serial or Ethernet-to-serial RS-485 device will do
> that in hardware at the converter. Due to the varying network delays,
> there would be a very large uncertainty about turning the transmitter
> off if controlled by the host computer (in fact you would have to
> readback the transmitted bytes to see when your transmission actually
> stopped, before commanding the line to Rx mode).
>
> That Moxa converter contains what they call "Automatic Data Direction
> Control" (ADDC) which is controlling the Tx when there is actually
> something to send. You do not have to bother about the line turnaround
> in RS-485 mode, just throw the converter with a stream of bytes in the
> TCP mode and then start reading bytes from the other system.
>
> Paul

Thanks Paul for your reply. I have been working on this issue and I
don't know if the ADDC works right on the Moxa. When you are dealing
with a slow serial network @9600 baud it seems to be working fine, As
your speed increases the performance of the ADDC goes down as well.
Well, that is what I have been seeing with my loop back test, it could
be very well related to my ethernet network.

I am looking for a good sample application in C to test this, any
suggestions? I have been using the Linux "cu" utility for my loop back
test.

On 6 Nov 2006 07:30:11 -0800, pesinc@gmail.com wrote:

>Thanks Paul for your reply. I have been working on this issue and I
>don't know if the ADDC works right on the Moxa. When you are dealing
>with a slow serial network @9600 baud it seems to be working fine, As
>your speed increases the performance of the ADDC goes down as well.
>Well, that is what I have been seeing with my loop back test, it could
>be very well related to my ethernet network.

That is quite expected, since the TCP/IP stack delays are in series at
both ends, which drops the half duplex performance. 

With some Moxa converters, you may be able to do a single transaction
(send+receive) in about 5-10 ms at the highest speed and short
messages, while some other converters may have 50-100 ms transaction
times. 

A good PCI card master may be able to perform the transaction in 1-2
ms, since no TCP/IP or Ethernet overhead is involved.

Paul