DESCRIPTION
The LS251 is a logical implementation of a single pole, 8-position switch with the switch position controlled by the state of three Select inputs, S0, S1, S2. Both assertion and negation outputs are provided. The Output Enable input (EO) is active LOW. When it is activated, the logic function provided at the output is: Z = EO ⋅ (I0 ⋅ S0 ⋅ S1 ⋅ S2 + I1 ⋅ S0 ⋅ S1 ⋅ S2 + I2 ⋅ S0 ⋅ S1 ⋅ Z = EO ⋅ S2 + I3 ⋅ S0 ⋅ S1⋅ S2 + I4 ⋅ S0 ⋅ S1 ⋅ S2 + I5 ⋅ S0 ⋅ Z = EO ⋅ S1 ⋅ S2 + I6 ⋅ S0 ⋅ S1 ⋅ S2 + I7 ⋅ S0 ⋅ S1 ⋅ S2). When the Output Enable is HIGH, both outputs are in the high impedance (high Z) state. This feature allows multiplexer expansion by tying the outputs of up to 128 devices together. When the outputs of the 3-state devices are tied together, all but one device must be in the high impedance state to avoid high currents that would exceed the maximum ratings. The Output Enable signals should be designed to ensure there is no overlap in the active LOW portion of the enable voltage.
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