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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.