The Serial$-$In Serial$-$Out $($SISO$)$ shift register is a circuit that accepts serial data on its input pin and shifts it out serially on its output pin. The $4-$bit SISO consists of four D flip$-$flops which are connected in a serial manner. All these flip$-$flops are synchronous with each other since the same clock signal is applied to each flip$-$flop.

$($i$)$Draw the schematic of the $4-$bit SISO shift register using $1-$bit TSPCR registers. You may design $1-$bit register and then use it as an instance to draw the schematic of shift register.

$($ii$)$Under what condition it is allowed to connect the output of one register to the input of another register.