Electronics > Beginners
What is the point of 74 series logic OR gate chip
visty:
if you can just use some diodes and a pull down resistor what is the point of having a large 7400 series chip for OR gates?
I'm a newb college student EE major don't roast me too hard.
Nerull:
So you don't have to use some diodes and a pull down resistor.
How many of your diode gates can you chain together before the voltage drops too low to be useful? How many other gates can you drive high or low?
visty:
ok this makes sense. I was just curious.
Tomorokoshi:
How much you get roasted depends on your Rth(j-c).
Resistor and diode logic was used a long time ago. Then they made TTL, Transistor-Transistor Logic.
Just a few advantages on 74 series logic over resistors and diodes:
1. 55+ years of use.
2. Cost for number of units in a package.
3. Size of number of units in a package.
4. Reduction of external pins to install due to internal integration.
5. Higher switching speed.
6. Higher fan-out.
7. Lower fan-in.
8. Lower power.
9. Compatibility with a wide range of parts in the family.
10. Compatibility with many parts in other logic families.
11. Compatibility with processors, etc.
12. Standard design with controlled process.
13. Noise immunity.
Ironically, the threshold to replace logic circuits with very small processors is very low these days.
rstofer:
The totem pole output, where the output transistors pull both up and down, help with rise time in the face of load capacitance. To accomplish the same thing with a pull up resistor would require a fairly low value and waste a lot of heat when pulled down.
You can expand on this by reviewing how RTL (Resistor-Transistor-Logic) and DTL (Diode-Transistor-Logic) work and how those ultimately led to TTL.
I started playing with RTL in 1969... It wasn't new even then.
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