"Use polled reads instead of subscriptions"

I have used this option “Use polled reads instead of subscriptions” prior to version 4.2.10 with no issues on CompactLogix but this weekend we are working with some SLC 5/05 and the group failed when writing the Handshake I get an E_Fail from the OPC server. If I disabled the “Use polled reads instead of subscriptions” everything works. Any Ideas the OPC Server is Kepware



Hi Julio,

If you go into Settings->Service Settings, are either “Use DA 2” or “Use Async Writes” enabled?

I would bet that async writes are turned on, since I believe that has been the default for a while. However, even though it’s the default, FactorySQL wouldn’t change it on the fly on an upgrade - meaning existing systems probably continued to use synchronous writes, and the new installation is using async. And, it appears, there is a problem writing asynchronously while using polled reads.

If this is the case, you should be able to just turn off the “async write” option.


Thanks Colby,

I will check in tomorrow when I am back on site


I should also mention that we’ve fixed the problem, so async writing will work with polled reads in 4.2.11. However, that’s currently the only change that’s been made for that version, so it might be a while until it’s released. In the mean time, just turn off the async write option and you should be fine.