Users can delay configuration by holding the INIT pin low. This causes the I/O of the device to be placed in a tristate condition (with the exception of Dout).
When the user holds PROG or INIT low before configuration, Dout will be driving a "1". The "1" will be driven until configuration begins at which time the 40 bit header will be passed or the device starts to pass on data in a daisy chain. For more information on configuration, please see the data book.