These Schottky-clamped TTL MSI circuits are designed to be used in high-performance
memory decoding or data-routing applications requiring very short propagation
delay times. In high-performance memory systems, these decoders can be used
to minimize the effects of system decoding. When employed with high-speed
memories utilizing a fast enable circuit, the delay times of these decoders
and the enable time of the memory are usually less than the typical access
time of the memory. This means that the effective system delay introduced
by the Schottky-clamped system decoder is negligible.
The 'LS138, SN54S138, and SN74S138A decode one of eight lines dependent
on the conditions at the three binary select inputs and the three enable inputs.
Two active-low and one active-high enable inputs reduce the need for external
gates or inverters when expanding. A 24-line decoder can be implemented without
external inverters and a 32-line decoder requires only one inverter. An enable
input can be used as a data input for demultiplexing applications.
All of these decoder/demultiplexers feature fully buffered inputs, each
of which represents only one normalized load to its driving circuit. All inputs
are clamped with high-performance Schottky diodes to suppress line-ringing
and to simplify system design.
The SN54LS138 and SN54S138 are characterized for operation over the full
military temperature range of -55°C to 125°C. The SN74LS138 and SN74S138A
are characterized for operation from 0°C to 70°C.
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