Well, the difference is really going to be internal to the OPC server, so I can’t give a specific answer. However, I can explain the difference between the two modes, so you have an idea of what might occur.
Default - Subscription based updates: FactorySQL creates “groups” on the server that contain a list of items that it’s interested in, and how often it wants to be notified if anything changes. The OPC server will do what it can to accommodate this, and provide notifications in the given time frame. If nothing changes, nothing is sent. So, for example, if FactorySQL subscribes to a point at a rate of 5 seconds, but the actual data changes every second, it will only be notified of every 5th value. If the value doesn’t change for 5 minutes, FactorySQL won’t receive any notifications for that time.
Polled reads: FactorySQL doesn’t notify the opc server of anything in advance. Each time it wants a value, it sends the tag’s address to the opc server in a read command.
There are a few advantages to the subscription model. First, by knowing of all the tags to update, the OPC server is able to optimize the way it reads. For example, if one group wants n7:1 and a different group wants n7:2, it can just read those together. Secondly, it minimizes the amount of work the applications have to do- instead of transferring the same unchanged value for each read request, it only sends updates as they occur. This point is relatively minor, since you’re just using COM on the local machine between FSQL and the OPC server (I assume, as opposed to DCOM if the programs were on separate machines). Still, it’s more work.
So basically, subscriptions are generally more efficient. HOWEVER, my thought process when I made my suggestion was that subscriptions are also generally a bit “looser”- the server is expected to send back a value as soon as possible after subscription, so it will send back a bad quality until it has better. With an explicit read, it may be more likely to take the time it needs to get a good value, up to some (hopefully settable) timeout. Unfortunately, this is completely up to the OPC server, so it’s just a thought/hope.
Hope this helps a bit,