ISE的IMPACT中初始化链接时只能识别PROM，不能识别FPGA

再用ISE的IMPACT初始化链接时只能识别PROM，不能识别FPGA，FPGA下显示的是unknown bypass，而PROM下显示的是XCF04S

错误栏里显示的是
ERROR:iMPACT - A problem may exist in the hardware configuration. Check that the cable, scan chain, and power connections are intact, that the specified scan chain configuration matches the actual hardware, and that the power supply is adequate and delivering the correct voltage.
ERROR:iMPACT - Bsdl reader is not available for device 1.
ERROR:iMPACT - Boundary-scan chain test failed. Please check tdi->tdo connection between  device:'1' ( 'unknown') and device:'2' ( 'xcf04s'). A problem may exist in the hardware configuration. Check that the cable, scan chain, and power connections are intact, that the specified scan chain configuration matches the actual hardware, and that the power supply is adequate and delivering the correct voltage.

请问这是怎么回事，请高手解答，先谢谢了

刚开始一直到不到原因，怀疑是JTAG链没弄好，但配置芯片能识别啊，所以又仔细对照了JTAG链，没有错误，第二天找老师说了原因，老师说以前也遇到过相似问题，和我的问题差不多：就是FPGA上电后电流逐渐变大，通过JTAG下载不进去代码。他怀疑是焊接或者助焊剂问题，于是就重新焊了一下并用酒精清洗了助焊剂。然后重下载一次代码，结果就成功了。所以我的原因就是焊接或者助焊剂问题，不好的助焊剂也会可能影响的。