Encontrado en una pagina de xilinx, que el enlace esta en el puesto antes para el jtag finder:

General Description:
How do iMPACT and JTAG Programmer perform an "Initialize Chain" command without knowing what devices are in the chain and the IR lengths of the devices?


Solution 1:
When the TAP (Test Access Port) of a JTAG-compliant device is reset, the default instruction (either "Bypass" or "IDCODE") is pushed into the data register. By moving the TAP of all devices in the chain to SDR and clocking "0's" on TMS, the JTAG Programmer can read in a long string that consists of all of the IDCODES in a chain. Xilinx recommends holding TDI high during this operation.

For example:
Suppose a chain consists of an 18V02PC44 and a Virtex 300EPQ240.

1. Reset the TAP of all devices. Clock {11111} on TMS (one TCK pulse for each bit) to push the IDCODE of each device into its Data Register (DR)
2. Move the TAP to Shift Data Register (SDR). Clock {0100} on TMS.
3. Clock "0's" on TMS until all IDCODES are read. This shifts out the IDCODES of both devices, starting with the last bit of the last device:
110010010000101000000000101000001100100100000000010001010000XXXX
4. Since all IDCODES end with "1" and the IDCODE must be 32 bits long, iMPACT/JTAG Programmer can discern the individual IDCODEs:

XXXX0000101000100000000010010011 (V300E)
00000101000000000101000010010011 (18V02)