The SN74LS266N is a quad 2-input XNOR (Exclusive NOR) gate from the 74LS family, with open-collector outputs — meaning the high side is floating and you pull it up externally. That open-collector stage lets you wire-OR multiple outputs onto a shared bus, which is the whole point of this part over a totem-pole XNOR. The 30 ns propagation delay at 5 V with a 15 pF load is typical for the LS family; it's fast enough for legacy 8-bit bus arbitration and comparator ladders, but you'll feel the difference if you're swapping in a 74F or 74ALS part without adjusting timing margins.
Open collector: the gotcha and the advantage
The output high, low current spec tells the story: the low side can sink 8 mA, but there's no rated high-side drive — that's the open-collector signature. You supply the pull-up resistor, and the value sets the rise time. For a 5 V rail at 8 mA sink, a 620 Ω pull-up gets you a clean edge, but the total current through all four gates sharing one resistor adds up fast. The quiescent supply current maxes at 13 mA for the whole package, so the power budget is modest.
Supply voltage and input thresholds
It runs on a 4.75 V to 5.25 V rail — classic 5 V TTL territory. The low-level input threshold is 0.8 V; high-level is 2 V. That means it's compatible with 3.3 V logic on the low side but won't reliably see a 3.3 V high as a logic 1. If you're mixing voltages, level-shift the high side or use a 5 V-tolerant driver.
Package and temperature range for repair and new builds
Housed in a 14-pin DIP with 0.300" body width (14-PDIP), through-hole mounting. If you're reworking a board with a scorched DIP, this is the direct drop-in.
For a 74LS part that's been around for decades, that's a solid sign that TI continues to run this die.