unknown devices in JTAG chain

Hi,

I designed a FPGA prototyping board with a Spartan XC2S200E and a XC18V02
PROM.
To configure these devices I put a JTAG header on the board and routed it as
described in
the following Xilinx datasheet:
http://toolbox.xilinx.com/docsan/xilinx4/data/docs/pac/designing7.html

At the software-side I use impact and the PC3 from Xilinx. When I start
impact in boundary-scan mode,
the PC3 will be detected (as [EMAIL PROTECTED]) and  the boundary-scan chain will
be identified. But there's the problem:
impact always detects 11 devices in the chain, but there are only two.
And: Even though the two devices are original Xilinx parts, no ID codes were
returned (or better: the returned codes
don't match with Xilinx parts).

I checked the configuration connections and the power supply, but
everything seems to be o.k. Has anybody of you an idea what the problem
could be?

thanks, Kay