When initializing the chain on the AFX board consisting of the user PROM and user FPGA, iMPACT identifies many unknown devices. However, iMPACT can properly identify and successfully perform JTAG operations on the service PROM and service FPGA.
This indicates TDO is grounded between the last device in the chain and the parallel cable. The most common cause of this problem is having the DOWNSTREAM_TDO_TERM bit set incorrectly.
TDO of the last device in the user JTAG chain will be connected either to the cable header, or to the downstream JTAG connector depending on how one sets the DOWNSTREAM_TDO_TERM bit. DOWNSTREAM_TDO_TERM will be either a jumper or a dip switch bit depending on which AFX board is being used.
Setting the DOWNSTREAM_TDO_TERM bit to 1 means it is terminating the TDO line by connecting it to the header. Otherwise, it is connected to the downstream JTAG header.