If you use the JTAG boundary scan test methodology to apply production test vectors, you might want to have independent external access to each TAP controller. This avoids the requirement to merge test vectors for more than one block in the device. One solution to this is to adopt a hybrid, using a pin on the package that switches elements of the device into a test mode. This can be used to break the internal daisy chaining of TDO and TDI signals, and to multiplex out independent JTAG ports on pins that are used for another purpose during normal operation.