Benchoff: Hey it's Bil! Bil Bil Bil Herd the computer nerd
Al Williams: Howdy Brian
Bil Herd: Just to be clear I dont get called that to my face.
Sophi Kravitz: :: laughing
Sophi Kravitz: alright: first Q: What are the differences between CMOS and TTL?
Bil Herd: Tobasco drinking contest! It will have to be based on the honor system tho...
jlbrian7: I had to switch to a pc, the tablet was killing me.
Benchoff: oh so peppercon...
Al Williams: Well truthfully, it isn't just CMOS and TTL -- there are tons of families with different characteristics
Sophi Kravitz: is there a chart?
Al Williams: And variations of even those
jlbrian7: The one I was working on is 75T. I think... i was more worried about where the wire were going.
Al Williams: Well there's two ways to look at it: inside and black box
Bil Herd: Some info in the video posts where I also showed the progression of the families as far as speed, etc.
Al Williams: from the black box perspective what you want to know is what level is high and what level is low (on input) and how much you can drive on the output
Philip: The difference is the way the silicon is processed and how the chips are designed. Logically a 7404 (TTL) and a 74C04 (CMOS) do the same basic function
esot.eric: I found a nice graph some where, from TI I think, easier to visualize than a table... bbiaf
Benchoff: That picture above is very much incomplete...
Al Williams: So a TTL gate might take anything below 2V as a zero and anything above 3V as a 1. On output it might output 0.7V and 4.3V. However, usually the amount of current on a "1" at TTL is very low compared to the amount of current it can "sink" when doing a zero
Bil Herd: To me the family difference starts with wether its a current consuming device on its input or a voltage sensing... except that that is a simplification.
Sophi Kravitz: https://cdn.hackaday.io/images/7027181455070253379.png
Al Williams: CMOS is more fair... usually 1/2 threshold (e.g., 2.5V @5V) and usually pretty equal sink/source currents
Al Williams: You can dig into the internals to find out why, but for most people the black box approach is better.
Al Williams: The sink and source currents get important when you fan out (connect lots of inputs to an output) or try to drive an LED or something else directly
Bil Herd: https://cdn.hackaday.io/images/7425201455070364009.png
Al Williams: the threshold voltages get important when you have noise issues although they can play in other things too including interfacing between families
jlbrian7: What is the difference between CMOS and BiCMOS, and how do you identify the two families?
Bil Herd: https://cdn.hackaday.io/images/4435701455070439189.PNG
Al Williams: Now you are in the internals
Bil Herd: TTL Input sinks current from the transistors typically
Bil Herd: https://cdn.hackaday.io/images/7393871455070487081.png
Al Williams: The way I like to explain is... is think of this way... a logic gate output is like two switches... one to + and one to ground (except ECL... nevermind)... only one switch is on at one time.
Bil Herd: CMOS usually means MOSFET which has an insulated gate that senses voltage and doesn't really have something like a sharing of current
Al Williams: Now the families depend on what are the characteristics of those switches
Bil Herd: Love ECL
Sophi Kravitz: what kinds of characteristics are there
Sophi Kravitz: ?
Al Williams: Well look at some old logic... the switch to ground would be a transistor
Bil Herd: And over time the line blurred between CMOS and TTL and the Fast TTL families that CMOS can do almost everything TTL can
Al Williams: and the switch to + would be just a resistor
Al Williams: so the current through the resistor had to be low... but the transistor could sink a lot
Al Williams: Then as Bil says.... what turns the switch on
Al Williams: a bipolar switch looks like a diode from base to emitter (or you have the emitter inputs like in that schematic above)... but a CMOS (or NMOS or PMOS)...
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