General Description: Why won't my "new" parts program via ISP in a system that I know is good?
Verify that you are using true ISP parts; XPLA 1 CoolRunner 128 macrocell, 960 macrocell, 320 macrocell, "CS" devices, and "AS" devices support ISP operations.
The XPLA2 and XPLA3 series are also fully JTAG/ISP-compatible.
Ensure that you have not programmed the CPLD with a design that has disabled the JTAG pins (used as user I/O). In this situation, you must hold the Port_en pin high to re-enable the JTAG pins' functionality.
For more detail on the Port_en pin and ISP for the XPLA3 family, please see (Xilinx XAPP343): "In-System Programming of XPLA3 Devices."