The SN74HC251N is a single 8-to-1 multiplexer in a 16-pin DIP package. It selects one of eight data inputs and routes it to a single output, controlled by a three-bit address.
Output drive — what 7.8 mA means for your load
The high and low output current is rated at 7.8 mA each. That's enough to drive standard CMOS inputs and low-power logic, but don't plan on lighting an LED or pulling a relay coil directly — you'll want a buffer or transistor stage. On a repair bench, if the board's output trace looks clean but the next gate isn't toggling, check that the load doesn't exceed this figure.