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eugenehhk,
"LS" stands for low power Shottky. It is a technology that uses bipolar transistors. The first stage of the gate has the collector "clamped" to the base via a Schottky diode. The diode prevents the first stage from saturating, thus ensuring that the turn-off time of this stage will be minimal.
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"HC" stands for hign speed CMOS. It uses complementary MOS transistors, along with low value resistors to achieve high speed switching.
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"HC" is the faster of the two technologies.
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Compatibility depends on the application. An HC device can adrive a TTL device, but not vice-versa. The reason is that the input switching point of a TTL device is typically around 1.6 Volts. The CMNOS output typically swing s almost rail to rail. However, the guaranteed "1" state output voltage of a TTL device is typically around 2.4 volts (For a 5V supply). The input switching point of a CMOS device is typically around 1/2 the supply voltage (2.5V for a 5 V supplY). As a result, you can not get reliable performance by driving a CMOS input directly from a TTL output. You can partially mitigate this problem by providing a pull-up resistor from the TTL output to the supply.
Regards,
Kral
Hi,
There are many familys in the 74XXX series besides LS and HC.
Like Kral stated the proper type to use depends on what is driving
the input of the gate, and also what the output of the gate is driving.
regarding LS and HC you can get more from datasheet from any vendor with
technical specification, or go through any "logic family" topic in any digital book.
you will get additional info. on more families too.
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