What this 4 x 2:1 multiplexer/demultiplexer does on the board
The Texas Instruments SN74CBT3257CPW is a single-supply 4 x 2:1 multiplexer/demultiplexer from the 74CBT logic family. It routes one of two 4-bit data sources to a common output, or splits a single 4-bit bus into two paths, depending on the direction. The select line chooses which input pair connects through. This is a straightforward digital switch — no level translation, no latching — just FET switches that pass 5V logic signals with low on-resistance.
Supply rail and temperature — where it fits
Runs from a single 4.5V to 5.5V supply, so it lives comfortably on a 5V rail. No extended automotive temp here, but for a 5V backplane or control-card mux, this is the right grade.
Housed in a 16-TSSOP (4.40mm width) surface-mount package. The 0.173" body pitch is standard for this density. No thermal pad to worry about — the package dissipation is modest for a 4:1 mux.
This part is a standard catalog item in the 74CBT family, so it is straightforward to source through independent distribution.