What the open-drain XNOR buys you on the bench
The SN74HC266DR is a quad 2-input XNOR gate with open-drain outputs — meaning each output can be pulled low by the gate but relies on an external pull-up resistor to go high. This lets you wire multiple outputs together on a common bus line (wired-OR) without contention, which is handy for shared interrupt lines or address decoding where any one gate going low should assert the line. Quiescent current maxes out at 2 µA — negligible for battery-powered gear.
Timing and drive — what to expect when swapping
Propagation delay is 23 ns max at 6V with a 50 pF load. That puts the practical toggle rate around 20-30 MHz depending on the pull-up resistor value and trace capacitance — fast enough for most glue logic but not for high-speed clock distribution. The high-level output current is listed as '-, 5.2mA' — open drain means it only sinks current on the low side. The 5.2 mA sink capability is enough to drive a standard TTL input or an LED with a current-limiting resistor, but don't expect to drive a relay coil directly.
Package and board-fit
Housed in a 14-SOIC (0.154-inch wide, 3.90 mm body width) — the standard narrow SOIC footprint. Surface-mount, so it's a hot-air or reflow job; not a field-swap candidate unless you've got a hot-air station in the kit.