Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
The legacy 74XX logic-ICs, i.e. the actual discrete DIP chips you could buy from neighborhood electronics store, use 5 volt TTL signalling.
Modern (since Spartan 3, Virtex 4) Xilinx FPGAs are not compatible with 5V TTL! If you connect your FPGA to 74XX chip directly, you will damage/destroy the FPGA. (You can make the Xiilinx FPGA's 5v tolerant by adding series resistor to the I/O pads. But the FPGA will still send out 3.3v signalling-levels.)
Also, most companies that sell 74XX now sell CMOS (3.3v) versions of those chips. Those are safe to use with the Xilinx FPGA.