These 4-bit registers feature parallel and serial inputs parallel outputs mode control and two clock inputs The registers have three modes of operation:
Parallel (broadside) load
Shift right (the direction QA toward QD)
Shift left (the direction QD toward QA)
Parallel loading is accomplished by applying the four bits of data and taking the mode control input high The data is loaded into the associated flip-flops and appears at the outputs after the high-to-low transition of the clock-2 input During loading the entry of serial data is inhibited.
Shift right is accomplished on the high-to-low transition of clock 1 when the mode control is low shift left is accomplished on the high-to-low transition of clock 2 when the mode control is high by connecting the output of each flipflop to the parallel input of the previous flip-flop (QD to input C etc ) and serial data is entered at input D The clock input may be applied simultaneously to clock 1 and clock 2 if both modes can be clocked from the same source Changes at the mode control input should normally be made while both clock inputs are low however conditions described in the last three lines of the truth table will also ensure that register contents are protected.