MC54HC646

The MC54/74HC646 is identical in pinout to the LS646. The device inputs are compatible with standard CMOS outputs; with pullup resistors, they are compatible with LSTTL outputs.

MC54HC646 are bus transceivers with D flipflops. Depending on the status of the DataSource Selection pins, data may be routed to the outputs either from the flipflops or transmitted realtime from the inputs (see Function Table and Application Information).

The Output Enable of MC54HC646 and the Direction pins control the transceiver's function. Bus A and Bus B cannot be routed as outputs to each other simultaneously, but can be routed as inputs to the A and B flipflops. Also, the A and B flipflops can be routed as outputs to Bus A and Bus B. Additionally, when either or both of MC54HC646 are in the highimpedance
state, these I/O pins may be used as inputs to the D flipflops for data storage.

The user  of MC54HC646 should note that because the clocks are not gated with the Direction and Output Enable pins, data at the A and B ports may be clocked into the storage flipflops at any time.