General Description: When I synthesize a design with 3-state control, XST does not replicate the logic for each IOB; one 3-state control logic is created for a bus of bidirectional ports.
Solution
To avoid this problem, change the synthesis pack option as follows:
1. Right-click on Synthesize -> Properties. 2. Choose "Xilinx-Specific Options". 3. Select "Pack Registers into IOBs". 4. Change the selection from "Auto" to "Yes".