Question about Xilinx FPGA

[mikedreams]

I have a question about FPGA ,I use boundary scan of ISE to find Xilinx FPGA and its configuration chip and ISE can’t identify them, But I find that ISE can work well as long as I use the probe of oscillograph to contact TDO pin of FPGA, is it because the impedance of probe impact on the signal .how can I deal with it

 

[sbob]

Add 1K pull-up resistors to TDI, TDO, TCK, and TMS.