Experimental Physics and
Industrial Control System

Zenon Szalata <[email protected]> · Mon, 5 Aug 2013 09:33:03 -0700

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