Re: Modbus

[Andrew Johnson <anj@aps.anl.gov>]

Hi Zen,

On 08/06/2013 11:47 AM, Zenon Szalata wrote:
> I ran the same IOC on two different computers which have access to the
> same "public restricted" sub-net where the Beckhoff is connected.  In
> both cases the computers are more powerful and IO fails sometimes very
> often at other time not so often.  "public restricted" is a sub-net
> configured such that it is only accessible from two public sub-nets here
> at SLAC.  Other than that, it is supposed to be a normal sub-net.
>
> When the IOC fails to do IO, it is the read in the write-read that
> fails.  As best as I can tell, the read fails immediately after the
> write completes.  This is based on observing the print statements in
> both the write and read routines, which makes rather subjective.
>
> The error printed is:
>
> 2013/08/05 08:55:14.982 modbusAsyn::doModbusIO port BKH15 error
> calling writeRead, error=sioc-esb-bkh15:502 timeout: Resource
> temporarily unavailable, nwrite=6/6, nread=0
>
>
> Is it some kind of networking issue?

That error message does suggest so.  I would suspect that when the 
Beckhoff is on your "public restricted" subnet it may be getting swamped 
by broadcast packets from other devices on that network, all of which 
have to be examined and thrown away by the network stack.  If the device 
doesn't have very much CPU power this could easily explain the problem; 
ask your network guys for the statistics on the volume of traffic being 
fed to it.  Do you have any other similar "public restricted" subnets 
you could move it into to see whether the same effect occurs there?

- Andrew
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