Picked up a Vernier GoTemp, a $40 USB temperature probe designed for
the science education market (there are bulk discounts, and courseware
examples on their site) with an eye towards having the Chumby record
and forward local temperatures to a central logging and analysis
server (I'm still looking for ideas and/or solutions for that part -
the popular first guess, rrdtool, isn't really the right
answer for accumulating long-baseline data...)
Turns out to be a quite solidly-made device, ie. "will survive extensive student use" rather than just "cheap enough for schools to afford". I'm pleased, given the extent to which I bang things around :-)
It is recognized under Linux via the LDUSB driver, and it looks like
that's enough to read packets from it; turns out that an 8-byte read
(from /dev/ldusb0) gets you a one-byte sample count, a one-byte
sequence number, and three two-byte little-endian temperature samples
which appear to be 100ths of a degree celsius EDIT 128ths! see later post. It appears to generate
around 2.5 samples per second, which is probably overkill.
gotemp-ldusb.py has the terribly simple code.
One can also talk to it with pyusb as well (0.3.5-4 from Ubuntu
Gutsy), and I started with the code I used for the Foam Rocket
Launcher; the only real issues were that detachKernelDriver
caused an error if LDUSB was already detached by a previous run of the
script, and that if I read too soon, interruptRead would raise
usb.USBError: No error. On the positive side, this path gets
consistent zero values for unused sample slots (there is space for
three samples in every packet, but the leading count tells you how
many of them are actually supplied), where the LDUSB path appears to
leak values from the previous packet; as far as getting samples out
goes, this doesn't matter at all, since you've got to go by the sample
count, not the value, anyway. gotemp-pyusb.py
The LDUSB approach probably has the edge, in that one can configure
udev to make the device accessible, where as the direct usb
driver needs root. pyusb might be more useful in distinguishing
among multiple probes on the same machine, based on USB topology, but
/sys/class/usb/ldusb0 appears to have equivalent information.
Amusingly, a good source of information on talking to this probe from Linux was Greg Kroah-Hartman himself - at the 2005 Ottawa Linux Symposium, he gave a "Write a Real Linux Driver" tutorial, and used the GoTemp as the example hardware, providing a single "temperature now" value from it. (It's not really worth the trouble for my application, it still layers on top of LDUSB after all - but it's an excellent skeleton for figuring out what the USB pieces are all about.)
Footnotes:
ols_2005_driver_tutorial_example_code.tar.gz is linked from http://kroah.com/linux/
08f7:0002 Vernier EasyTemp"