The MC54/74HC640A is identical in pinout to the LS640. The device inputs are compatible with standard CMOS outputs; with pullup resistors, they are compatible with LSTTL outputs. The HC640A is a 3state transceiver that is used for 2way asynchronous communication between data buses. The device has an activelow Output Enable pin, which is used to place the I/O ports into highimpedance states. The Direction control determines whether data flows from A to B or from B to A.