A PDA is not designed to replace a desktop or laptop, and as a result just simply isn't able to. The USB/Serial cable is for syncing with your laptop, not connecting to other peripherals, such as a printer, scanner, etc. The software, drivers, and connectivity just isn't there.
While there are devices, such as modems, designed to be connected to a PDA, I'm betting that none of what you have is and that you'd rather stick with that than hunting down accessories for a PDA that are less capable and more expensive than what you have.
My advice is to keep the laptop and use your new PDA, which ever you choose, for what it's deigned for...being a Personal Digital Assistant. If you want the small, portable device with full connectivity you'll want to look into a uPC (ultra portable computer) such as the OQO 01+, not a PDA.
I like to buy a handheld/pda but i want that thing can connect to other peripheral such cisco router. I see from the other website that O2 XDA III have adapter from usb to serial cable but the thing that i don't know what program that O2 connect to pc, could it using actifesync or another.
I want know if the O2 or another pda can run hyperterminal or another emulator software when connect to other peripheral using serial cable.
Onother thing i want connect pda using athertnet device so i it can use for ping or connect to LAN.
Could you help me to find out, what kind of pda, adapter device and software. If it can work i would replace my laptop.