OPC gateway high performance
We are looking for an ABB solution that allows customer to exchange 45.000 opc properties with 16 OPC DA clients using just "one" machine. This 45.000 opc items can be duplicated between different clients so total load can be greather than 55.000 opc items communication.
OPC DA Surrogate allow 4 connection with maximum of 10.000 opc items each, but it's performance is not good enough.
We are going to upgrade to 800xA 126.96.36.199 and could use MOM, History 6.0 or any other ABB solution to concentrate all the connections to one single Server, but it's mandatory that maximum two servers concentrate this 16 clients and system have a good performance communicating with clients.
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Please see attached PDF for an explanation to the traditional OPC DA functions used to retrieve data from a server.
The most convenient method to obtain a daily newspaper is to subscribe for it. The whole delivery chain is tuned to provide you with a daily issue to your doorstep.
Scanning OPC clients need to visit the kiosk (OPC server) even on days when there is no issue (holidays, etc). This involves more job for all parties.
For optimum performance ABB recommend either of these two methods:
1) Asynchronous, Advise-based subscription with for the OPC server optimal update rate
2) Synchronous, Read from cache with active items and for the OPC server optimal update rate
The optimal rate varies with the OPC server. Read up!
I guess 45000 items exceed OPC server limits anyway. (AC800M or Surrogate OPC)
Maybe it is too complicated idea, but there is a way to bypass AC800M OPC server.
I have experience with direct reading of ethernet MMS packets transmitted by MMSWrite function block and with storing data to MySQL real-time database. This solution has been working in the plant for 17 years.
Data from database table can be converted again to OPC for example by Matrikon OPC server for SQL.
If client is using Asynchronous OPC DA, the surrogate should not cap at 45000 items.
Notice: some tunneler softwares convert asynchronous operations into synchronous!
In most configurations with OPC connection to 800xA, the bottleneck is the source OPC server and its limitation to communicate with the controller.
Most OPC servers in the System 800xA family handle subscribing clients far better than scanning (cyclic reading) clients. The write performance is most often several magnitudes lower than the subscribe performance. Some controllers have additional performance recommendations, eg 800xA for Advant Master. Study your choice of controller(s) and adapt to them instead of vice versa.
A well tuned historian could act as a ”moderator”, speaking controller optimized OPC DA with the OPC Server and controllers while allowing clients to read data efficiently in bulk using OPC HDA.
Interesting case, can you tell us more about the purpose of this 16 OPC DA clients?
I assume these are operator workplaces with another SCADA product? As Stefan suggests is to use subscription for just part of the tags instead of scanning them all. This should be an option since I don't think an operator is able to interpretate 45.000 properties at once.
We use an alternative SCADA product connected to the 800xA OPC server and have the following implemented:
1. Some important tags are permenantly subscribed (scanned) by the SCADA server for archiving.
2. Most other tags are only subscribed if the operator opens the process panel or faceplate, and are unsubscribed if no longer needed.