IC compatibility Question

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IC compatibility Question

Post by dacflyer » Thu Nov 15, 2018 3:05 pm

i need a 74HTC125 but in my junk box i have a 74LS125 - they look pin for pin the same except for the labeling of pins of 1,4,10,13 the high OE pins.
The 74LS125 has it's pins listed as 1G - 1G* , 2G - 2G* etc.
so this is where i get confused.

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Re: IC compatibility Question

Post by haklesup » Thu Nov 15, 2018 4:49 pm

They have the same logic function but internally they are very different. The HCT part is CMOS and the LS part is Bipolar technology. The input and output logic levels are different. Consult the DC specifications page of the datasheet and look at the VIL, VIH, VOL and VOH levels. In most cases the CMOS part will consume less supply current and have wider voltage margins at the input and output levels making it easier to make compatible with mixed technologies.

It is possible to mix CMOS and Bipolar parts in a single project but it is not advised and you have to pay careful attention to logic voltage levels and may require level shifting or pull up resistors in worse cases.

One of the key differences is the output pin. The CMOS part can usually sink and source current from this pin in equal amounts at both the low and high levels and many parts can drive 10mA or more and still maintain output levels. The CMOS parts are way weaker and only supply current in one direction and if you try to put another voltage on those outputs (like a summing node or a pull up or pulled down node) Then the output voltage will collapse and track with the load.

The LS part probably has an older date code, especially in DIP. These are not recommended for new designs unless you are doing something special, Stick to CMOS types and you will have fewer headaches from loaded logic nodes.

Here are a couple charts comparing logic levels for various families of logic devices

https://www.quora.com/In-logic-gates-is ... igh-or-low

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