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Looking at this doc [click here] I find on the Proliphix website, it looks like you want to use SSL data encryption?
Is your Opto controller hardware-based (like a SNAP-PAC-R or -S) or might you be using SoftPAC?
Our built-in PAC Control HTTP Get command help does have a small mention of using SSL: "Argument 3. Use 0 for a non-secure connection. Use a number other than zero to use SSL (Secure Socket Layer)." However, the rest of the details on how to use it need some fleshing out--we're working on that now.
In terms of what you're actually trying to do with the web interface, have you sniffed (or example, using Wireshark) the communication between your web browser & the device? When programming PAC to do similar things (HTTP page scrapes, etc.), I've found that sometimes the docs for the device are not always helpful/accurate/available. But if you can see an actual exchange (and perhaps compare it to what your PAC is doing as you develop your strategy) that can be very helpful for converging on a working solution.