The SN54 / 74LS398 and SN54 / 74LS399 are Quad 2-Port Registers. They are the logical equivalent of a quad 2-input multiplexer followed by a quad 4-bit edge-triggered register. A Common Select input selects between two 4-bit input ports (data sources). The selected data is transferred to the output register on the LOW-to-HIGH transition of the Clock input. The SN54/ 74LS398 features both Q and Q inputs, while the SN54/ 74LS399 has only Q outputs.