Multiplexer selection — 74FCT family, 4 x 2:1 mux
The CY74FCT257ATQC is a 4-bit 2-to-1 multiplexer from Texas Instruments' 74FCT series, a high-speed logic family. It selects one of two 4-bit input words (A or B) to the output based on a single select line, with an active-low output enable for bus isolation. Operating from a single 4.75V to 5.25V supply, this device drives 32 mA high / 64 mA low per output — enough to sink TTL loads or drive a short backplane trace without an external buffer.
Housed in a 16-SSOP (3.90 mm body width, 0.154" pitch) — a standard surface-mount footprint shared by many 74FCT and 74ACT logic parts. The 1.75 mm seated height clears most mezzanine spacings on a mixed-technology board.
Sourcing and compliance
RoHS non-compliant — the original termination finish does not meet the EU RoHS exemption limits. This matters for new builds requiring full RoHS conformance; the part remains usable in repair, MRO, and legacy designs where the exemption or customer waiver applies.