Al,
A few things:
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Our model recommends a PC, which can be DIN mounted or “industrial”, that is reasonably “close” to the PLC, but can be “distant” from the central SQL database. We recommend the PC for it’s cheap memory capability and processing power. “Close” means that you’re unlikely to lose the network connection between them. The PC and PLC may go down together during a power outage (you’ll inevitably lose the data for any period the PLC is down, but don’t want to lose prior data). “Distant” being acceptable between the PC and SQL database because of FSQL’s local data caching.
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My post was a way to answer the question that I’m constantly asked of “why can’t we cache historical data in the PLC with FactorySQL?” It’s kind of a hack and doesn’t fit our model well. In fact, it is only helpful for the situation where the PLC and computer are both up but the network is down between them. This shouldn’t be a common situation. If it does happen the focus should be on the network not FSQL and the PLC.
The fact that we’re discussing trade offs reinforces the fact that this situation could be improved upon. IMO the best solution would be a cost effective embedded computer (DIN mounted, DC powered, NEMA rated, etc) that ran a full version of FSQL (PMI too for small installations) and interfaced with cheap solid state memory (CF, smartmedia, etc). You might be able to configure a Sixnet PLC or something similar to achieve the same effect with FSQL. In theory, if OPC (servers and spec) dealt with historical data “better” there could be a nice way to implement FSQL where the data was cached in the PLC in a vendor neutral way. Dreaming further, this would be really cool for slow connections since the data being transferred could also be compressed.
There are hardware dataloggers. I haven’t used them, but am under the impression that they’re cheesy compared to the functionality of a PC and decent program. I think they typically store the data as text - using separate files per day and can either be set up to upload them periodically via ftp or have a simple web server built in. I have no idea how much they cost. They could be right for certain applications and may be usable with FSQL/FPMI. If I hear about one that is, I’d be happy to recommend it.