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[FAQ] What's the difference between TTL and 5V CMOS logic?

Guru 56010 points

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It's very common for people to refer to "TTL logic" and to actually mean "5V logic" -- but how are they different?

The key difference is in the input and output voltages. This graphic from TI's Logic Guide does a great job of showing the differences:

Transistor-Transistor Logic (TTL) is a specific type of integrated logic using resistors, diodes, and bipolar junction transistors (BJTs). TI developed the 7400 series of TTL logic in the 1960's that quickly became an industry standard and was duplicated by many of our competitors. By today's standards, this logic is very power hungry, often requiring 30+mA just for static operation, however it does still have very respectable output current and speeds and is still used in many systems.

See a list of 5V bipolar NAND gates here: http://www.ti.com/logic-circuit/gate/nand-gate/products.html#p776=Bipolar&p1498=Catalog

More modern 5-V logic ICs uses CMOS technology, which is vastly more power efficient than the older TTL devices, however these devices operate with different input thresholds and output voltages than the TTL devices. For this reason, there are TTL-Compatible CMOS devices available that meet the same voltage requirements as the older devices for drop-in compatibility.

See a list of TTL-compatible CMOS NAND gates here: http://www.ti.com/logic-circuit/gate/nand-gate/products.html#p776=TTL-Compatible%20CMOS&p1498=Catalog


Looking for a low voltage translator? Check out the AXC family that supports 0.7V to 3.3V translation!

The Logic Minute training page has videos on many interesting topics that all are kept shorter than 5 minutes.