74LS257/54LS257A pdf datasheet

54LS257A/DM54LS257B/DM74LS257B,
54LS258A/DM54LS258B/DM74LS258B
TRI-STATEé Quad 2-Data Selectors/Multiplexers
General Description
These Schottky-clamped high-performance multiplexers
feature TRI-STATE outputs that can interface directly with
data lines of bus-organized systems. With all but one of the
common outputs disabled (at a high impedance state), the
low impedance of the single enabled output will drive the
bus line to a high or low logic level. To minimize the possibility
that two outputs will attempt to take a common bus to
opposite logic levels, the output enable circuitry is designed
such that the output disable times are shorter than the output
enable times.
This TRI-STATE output feature means that n-bit (paralleled)
data selectors with up to 258 sources can be implemented
for data buses. It also permits the use of standard TTL registers
for data retention throughout the system.
Features
Y TRI-STATE versions LS157 and LS158 with same pinouts
Y Schottky-clamped for significant improvement in A-C
performance
Y Provides bus interface from multiple sources in highperformance
systems
Y Average propagation delay from data input 12 ns
Y Typical power dissipation
LS257B 50 mW
LS258B 35 mW
Y Alternate military/aerospace devices (54LS257A/
54LS258A) are available. Contact a National Semiconductor
Sales Office/Distributor for specifications.