Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Mon, 5 Aug 2013 17:23:43 +0000

Hi Zen,

What timeout are you using for your TCP read?  The read does not actually put anything on the Ethernet, it just waits for a message to arrive from the Beckoff.  So if you use a timeout of 1 second or so then it should never time out, assuming the module really does answer within 20 ms or so.

You need to turn on asynTrace in the underlying drvAsynIPPort driver to see exactly the timing of what is being sent and received by that driver.

Mark

________________________________________
From: Zenon Szalata [[email protected]]
Sent: Monday, August 05, 2013 11:33 AM
To: Mark Rivers; Ernest Williams; [email protected]
Subject: Modbus

Hi Mark,
I am nearly done with my version of modbus TCP for use with Beckhoff
hardware.  I have a few IOCs using this support module and all seems to
be working well in my test area.
I have deployed an IOC for a couple of stepper motors controlled by
Beckhoff devices in a production area and discovered that the IOC is
generating many errors.
The error 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

Now, modbusAsyn is the name I used for the file which has the two
routines I took from your file drvModbusAsyn.  Looking at the log files
of the IOCs used in the same production area which control Beckhoff
devices and which are built with your modbus support module, I see that
the same errors are generated.  Here is an example:

2013/06/16 13:19:25.918 drvModbusAsyn::doModbusIO port BKH10_CPL_R0
error calling writeRead, error=sioc-esb-bkh10:502 timeout: Resource
temporarily unavailable, nwrite=6/6, nread=0

People who use these IOCs ignored these errors because mostly they did
not matter.

In the case of the stepper motor controller IOC it does matter. First of
all I realized that the difference between my testing environment and
that of production is the computer speed and a different networking
environment.  The production computer used for the Beckhoff IOCs is much
faster.  So it seems that there is some timing issue at work here.
After putting print statements in various places, I see that the routine
writeIt in modbusInterpose.c file always succeeds but the routine readIt
in the same file fails from time to time.  I reread the manual for the
Beckhoff BK9000 Ethernet coupler and came across a statement saying that
the coupler takes from 13 to 15 ms to process a modbus message.  So it
seems that the read comes too soon after the write.  In my version of
the modbus support module for Beckhoff, I have added a 20 ms delay after
the pasynOctet->write in modbusInterpose.c and this has mostly solved
the problem.  I say mostly, because I still see this error message but
very infrequently.
I am writing all this because I don't understand why this error is
generated so I am not sure if the direction I took to fix the problem is
correct.  I also think that your modbus support module may need to be
modified if it is to be used with Beckhoff devices.
I would like to hear what you think about this.
Thanks,
Zen

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System