After trawling the mBed forum and trying out lots of different HTTP client type examples, I was shocked earlier today to actually see some HTML appear in a terminal window. Success!
The mBed successfully talked to the router, got an IP address via DHCP and made a request to my own server (I've set up a web page to broadcast messages when a username/password combination is sent).
Unlike previous attempts, I used the Twitter example which uses HTTP POST instead of HTTP GET to retrieve a response from a web server (it actually posts data to the server too, but I'm not interested in that side of things yet).
I was utterly amazed to see a response from the server appear in the terminal window. Finally, something was working and I could concentrate on building my web-enabled device. True, it was still a niggling concern that previous examples hadn't worked, but by now I just wanted a working solution (see how this whole library-based approach to development just makes you abandon understanding for the sake of getting something to work?!)
I was so thrilled, I wanted to see my web page contents appear again, so I pressed the reset button on the board and waited for it to get the data again.
It was assigned an IP address, then just sat there.
Very much like the previous attempts - it got to the HTTP POST command and just locked up!
I reset the board again.
This time, some HTML appeared in the terminal window.
Exactly the same code running on exactly the same hardware, calling exactly the same web page (nothing changes between attempts) sometimes works and sometimes doesn't. A more frustrating scenario I can't imagine just at this time!
After repeating the experiment, I now have a sort-of-working HTTP client which can return data from a website, but only once every six or seven attempts.
Straight away, I thought hardware - dodgy soldering and loose wires - after all, if the software/firmware doesn't change between it working and not working, it must be something else.
I replaced my home-made RJ45 socket (which still works with other development boards, so I'm convinced it's not the problem, but just to be sure) with a pre-built breakout board, with all the connectors in place.
The result was the same. Resetting the device (or removing power from it completely) resulted in more locking up and occasionally a response written to screen.
All in all, a really frustrating week - I feel it would have been better spent building an ethernet device to connect to a PIC-based project myself! At least then, when it didn't work properly, I'd at least have some understanding about what was going on and maybe even why it wasn't working.
I've never been a fan of plug-n-play type development, as it discourages full understanding. And my experiences with the mBed development system haven't done much to change that view just yet!