4 x 2:1 multiplexer for data-path selection
The Texas Instruments CY74FCT257ATD is a 4 x 2:1 multiplexer from the 74FCT logic family — it selects one of two 4-bit input buses onto a single 4-bit output bus, controlled by a common select line.
Output drive and signal integrity
The output stage sources 32 mA and sinks 64 mA — that's enough drive to fan out to multiple CMOS loads on a short backplane trace without a buffer. The asymmetric sink/source ratio matches the typical loading of a TTL-compatible input bus where the low-level current demand is higher. The 0.154-inch pitch leaves room for a probe clip during bring-up.