Having implemented the previously described circuit and associated code to reset the GSM modem via a USB power cycle when communications with it shut down, there were still occasions when the beaglebone board would lock up, and cease further communications with either the network or the USB-based terminal gadget.
So, I decided that a watchdog timer would provide a last line defense against this sort of "crash." The beaglebone has a special watchdog timer that can be used for just this purpose. When the file /dev/watchdog is open, absence of a write to it for more then about one minute will cause the entire board to reset and reboot.
I added code to beagelgsm.tcl that opened the file just after startup, and then wrote to it once each main loop, which seemed from inspection to provide much more frequent writes than required to keep the watchdog timer from timing out and resetting the board. I added code to measure loop time and log any times greater than about 30 seconds, to get a sense of what the loop times were.
The system is now up and running, as of about 6 PM November 5, 2012.
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