Modbus ip shared variable network failure

Question

I am using lab view 8.6 DSC module to communicate to a watlow system which contains five watlow 96 controllers and an EM gateway.  I have created shared variables for the process temperatures and setpoints for each of the five controllers using watlow modbus register Numbers with a 400001 offset.  I have also created shared variables for Updating,CommFail,UpdateNow,and UpdateRate which where predefined. I have error when starting the VI if the SV  has been  dragged and dropped into the block diagram. The message is  Error -1967353902 (The Modbus I/O server failed to receive any response from the Modbus slave device.) occurred at SV in vi. If I bind a variable in the VI to this same SV the error does not occur but the variable cycles between Good, Network Failure, No known value, and device failure as stated in the variable manager watched variables.  The Updating, CommFail and UpdateRate all have a consistent Good in the quality column of the variable manager.  UpdateNow has X in value, type, timestamp, and quality columns.  CommFail and Updating does cycle between true and false randomly.  I have tried a third party software called SpecView 32 demo to see if the commincation with the modbus system is not working and I can create five watlow controlers on my screen and direct them to the ip address along with a unit address and the system works without faults.  This leads me to believe the commincation bewteen the SV Engine and the IP address is not correct.  HELP Please. 
Robert Jensen
UND EERC

Answer 1

If your application can deal with it I would recommend staying clear of the 'Networked Published' option.
When I started my Modbus development on cRIO....I left it enabled, and with ~100 shared variables on a 9074, the CPU was railing, and I saw a buffering behavior on the shared variables (which was not desirable in my application).
In my application I am using the old modbus library (as apposed to the new API) for cRIO to slave comms, the cRIO being the master.
I am also using the IOserver making the cRIO a slave to an external SCADA - and it passes essentially the same data arrays as I use on the modbus library for my local HMI [Not an NI product].....Which is two full Modbus frame writes (@ 120 words each, and about 60 words more for ~300 words outbound from the cRIO).
The IOserver slave was a recent addition and did not add much to the CPU load - although only 16 bytes is high speed, the balance of the total word package is at either 1 second or 3 seconds.
So, in my experince, the 'Networked Published' option adds significant CPU loading (on entery level cRIOs) YMMV.
I am huge fan of the shared variable engine (some at NI were pusing the CVT, and TCE etc...). However most of my shared variables are not the Networked Published variety (excepting local module channels) those have remained networked published for DSM (Distributed System Manager) use.