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74 series high connections

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inktomi

New Member
Hi there.

There are some instances where I want to keep an input of a digital logic chip permanently high. Should I include a resistor between the supply and the pin? Also does it make any difference if it is an LS chip or a CMOS HC chip?

I've seen some with and without resistors. Which is the correct design?
I personally would think CMOS chips do not require a resistor (since the gate is insulated), while the LS series require one.

Thank you
 

Mike_2545

Super Moderator
You should be able to hold it high directly.

If you are going to pull it low at any time hold it high with a resistor.
 

colin55

Well-Known Member
Is the "drive line" (the line connected to the input of the chip) at the same voltage as the voltage on the chip you are taking HIGH?
 

audioguru

Well-Known Member
Most Helpful Member
A 74LSxxx input will float high but might pickup interference.
A Cmos input must not float but must be forced high or low with a resistor.
 

Roff

Well-Known Member
Fairchild AN-363 (Designing with TTL) has this to say (see attachment). I personally don't see much merit in either of their arguments for using resistors. Mike2545's reason is sound.
 

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