Here I publish the code for the PCE-HT71 programmer/downloader for libusb. I use it only on GNU/Linux but I imagine it may work also on other systems where libusb works.
The PCE-HT71 device allows recording temperature and humidity for long times, once programming it to do so at a specific rate of seconds between samples, and a limited amount of samples. This software allows programming it and later downloading the data it recorded. I acquired mine from PCE ibérica, because I wanted to monitor temperature and humidity in some places of my house, usually not very accessible. I considered I needed humidity recording too, because I imagine the feeling of cold or warm depends a lot on it.
I used a windows xp in a qemu-kvm with the PCE-HT71 Datalogger windows programs, all running in a Linux 2.6.35 with usbmon. I used wireshark to grab and decode the USB traffic, and I then looked for correspondences between the values I set on the windows program, and the usb frames. Additionally, I noticed that the computers I had with VIA USB controllers did not negotiate USB properly. So it works for me only in computers with USB controllers from other manufacturers.
Programming:
It then allows downloading the data into a text file appropiate for gnuplot
I still have not managed to set properly the LED alarms on temperature and humidity.
Download the latest version (GPLv3+ licensed): pce-0.2.tar.gz - v0.2 - 24/10/2010 - This program comes with ABSOLUTELY NO WARRANTY.
Look at the version repository if you are interested in its development.
Here is the usage help of pce v0.2:
usage: pce [-n nsamples] [-t timepersample] [-i] <-d downloadfile | -s>