Modbus

[Zenon Szalata <zms@slac.stanford.edu>]

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