: Pick a device with a USB port and I bet you can find an off-the-shelf : device that does sensor inputs and contact closure outputs, somewhere.
The problem is that PDA:s are typically USB "clients" (to sync with a "host" PC), and while the USB gadgets to add sensor inputs are intended to connect to a PC, they are also "clients". Two clients cannot connect.
How do I know? I use Palm PDA to read and plot data from a logger (with my own application). It works over RS-232 data connection. As serial Palms are getting rare, it would be cool to be able to switch over to USB, but unfortunately seems to be impossible...
This is at least partially incorrect. There are actually several PocketPCs that are USB host capable. For example: Toshiba E740 (which I own) and the Dell Axim series. I believe that several other Toshiba and Dell Series PPCs also have USB host, but I can only personally attest to these two. There are also several iPaqs that I see claim to have USB host. You will need to create a driver for your device though. I have only been able to get USB drives and mice to work so far with my E740.
And even if your PocketPC does not have a USB host, you can buy compactflash adapters that provide that capability.
Of course, if you have a SD or MMC port, you have SPI, and with compactflash, you actually have addressing and I/O. Obviously since these ports were mostly meant for storage, you will need to create drivers for them since the operating system (usually) only supports the storage classes.
I would agree that finding a PDA with serial is a better choice though since it is easier to just send raw serial commands than creating a usb stack and driver.
Another possibility is a palm pilot with the ethernet cradle.
I originally used an HP48GX graphing calculator as it had an RS232 port. HP is coming out with an HP50G with USB/RS232 (available outside US now) and is about $130. 4xAAA batteries should last considerably longer than PDA equivalent. Programming in ARM ASM should be wicked fast. Also, calculator has SD card slot with FAT32 capabiliy, so a big data heap is possible.
I'm looking at ways of interfacing the 50G (and 48GX) to a Fluke 189 logging multimeter for DA.