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.
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>