Logic Levels for 5V TTL Systems

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1024MAK
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Logic Levels for 5V TTL Systems

Post by 1024MAK » Thu Nov 08, 2018 11:23 am

The voltages of the signals in most 5V logic systems is based on the TTL logic levels. As most microprocessor systems used 74xxx or 74LSxxx or related series chips for at least some functions.

Hence when NMOS chips replaced the bipolar transistor logic chips, the NMOS chips were designed to be compatible with TTL logic levels. Later on, as CMOS memory chips were designed, again a lot of these were designed to interface to microprocessors. So it was logical for CMOS microprocessors and other support chips to again be designed to be compatible with TTL logic levels when operated on a 5V supply. Note however that the CMOS chips output drivers (output pins) can drive the output much closer to the +5V supply compared to the figure shown in the diagram below. Also the input switching levels may not be exactly the same as in the diagram below. Always refer to the relevant datasheet for the chips you are using.

This diagram shows the TTL logic levels:-
DC9C1FC6-468F-46ED-839A-BC497CD7F547.png
TTL Logic Levels
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Richard Russell
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Re: Logic Levels for 5V TTL Systems

Post by Richard Russell » Thu Nov 08, 2018 1:01 pm

1024MAK wrote:
Thu Nov 08, 2018 11:23 am
So it was logical for CMOS microprocessors and other support chips to again be designed to be compatible with TTL logic levels when operated on a 5V supply.
It might be helpful to make a distinction between the 74HC and 74HCT ranges of CMOS logic chips; the former do not have TTL-compatible input thresholds whereas the latter do. The 4000-series CMOS logic devices don't have TTL-compatible inputs either.

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BigEd
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Re: Logic Levels for 5V TTL Systems

Post by BigEd » Thu Nov 08, 2018 1:12 pm

I often end up finding this diagram:

Image

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