Asynchronous OPC communication

[Tim_S]

I'm finding out that not all OPC servers are equal.

I'm hoping people will have some advice for me on how I might better solve my dilemma. At worst, this post might help someone else out.

I've got two systems on the shop floor: one has Kepware's KepServerEX and one has Rockwell's RSLinx. I have a two test VIs: one connects to the OPC server and performs reads at (hopefully) 100 msec/10 Hz intervals using the traditional datasocket VIs and the other is the same except it performs a write. I'm using traditional datasocket instead of the primatives as I've got LabVIEW 8.0 installed and the primatives (as per the help files and notes in the shipped example code) don't work [well] with OPC.

I've run the LabVIEW code on both machines. Both PCs are equivalent machines. The RSLinx has no issues maintaining 10 Hz with write times of ~20 msec and read times of < 100 msec (the read timeout value is 100 msec; there is only a read if there is a change in the data). The timed loop doesn't show any missed iterations.

The KepServerEX has issues maintaining 4 Hz. The read VI maintains 100 msec until I start the write VI at which point it averages 180 msec. The write maintains about 230 msec average. Kepware tech support tells me the write takes precedent over the read, so the read gets interrupted for the write to occur. Kepware tech support advise me that I might try asynchronous writes as they would be faster. Kepware also seemed confused as to why I would need to go so fast as 100 msec. I will point out that Kepware tech support was extremely good about helping me and analyzing what was going on down to parsing and analyzing a low-level TCP log for me.

NI tech support spoke with R&D for me. It seems datasocket only performs synchronous operations.

I installed LabVIEW 8.6 on the machines and had the same performance with the same test code.

My current path involves using the OPC ActiveX control. I've downloaded the Kepware example code, which also installs a demo of their OPC server. All has been good until I discovered that some properties and methods the VB example code uses (some of which compiles and works using VB 2008 express) are not available/I have been able to locate in LabVIEW (specifically the method used in the example to add an item to a group).

There have been, fortunately, no large, blunt objects nearby.

My impression is that the two OPC servers are designed with different philosophies in mind: RSLinx for machine control (which is what I'm trying to do) and KepServerEX for production control. Datasocket, as a generic interface, performs synchronous communication as everything it can handle supports that.

So, has anyone run into this or found a way to handle it?

Thanks,

Tim

[Tim_S]

I had the opportunity to test a Siemens OPC server the other day. The Siemens OPC server behaved like the RSLinx OPC server (i.e., good for control) with read timeouts at 100 msec and write times averaging 1 msec. I wasn't able to use my test VIs; the communication routine used performs read then write in series.

The KepServerEX and RSLinx communication to the PLC is over Ethernet. The Siemens communication to remote I/O is over PROFIBUS.