Hi Fletch,
most HMI software will allow you to write TO a database, typically there will be three types: cyclical datalogging, on-change datalogging and event-based datalogging.
But be careful: Not all HMI's have all of the above.
For example RSViewME does not have event-based datalogging.
You have to check if it suits your application.
Also: most HMI's will not allow you to automatically write FROM a database to a PLC. At least they will require you to tweak the system to do that.
For example RSView32 can only do this if you write a lot of VBA code.
(personally I hate that, but somebody seems to love it).
An alternative to HMI software might be a software package like RSSQL that is intended to connect databases and PLC's directly. RSSQL can use OPC (I believe) but is of course focused on AB PLC's.
My recommendation:
If you have to connect to only one PLC brand, then use that PLC brands "own" database software (like RSSQL).
If you are looking a bit forward, and might use PLC's from several different brands (maybe at the same time), then use OPC server software to connect to the various PLC's and OPC database client software to connect to the databases (or even wite to databases directly).
best regards,
Jesper M. Pedersen