are logic gates abtract entities or real ones?

[PG1995]

Hi

Would you please tell me if logic gates are abstract entities or real physical ones? How do they look? How are they made? I mean what electronics components, e.g diode, transistors, they contain inside them? Could you please link me to some picture which shows a 'physical' gate? I have googled but came across only schematic diagrams. Please help me with it.

Regards
PG

 

[carbonzit]

Well, if you've found a schematic, then you've found the plan for a "physical" gate. That's as real as you're going to get.

For instance, here's an OR gate made from diodes:

**broken link removed**

Now, you won't see real OR gates built like this nowadays. But it is, in fact, the plan for a real, physical, working logic gate.

 

[MrAl]

Hi

Would you please tell me if logic gates are abstract entities or real physical ones? How do they look? How are they made? I mean what electronics components, e.g diode, transistors, they contain inside them? Could you please link me to some picture which shows a 'physical' gate? I have googled but came across only schematic diagrams. Please help me with it.

Regards
PG

Hi there PG,

Im a little confused by the apparent contradiction in your question. You say you are looking for what is inside a logic gate, then you say you found 'only' schematic diagrams, then you seem to indicate that you still dont know what is inside. What more can you ask for than for a schematic?

Logic gates come in many forms, from resistor logic to transistor logic and even using op amps. Carbonzit posted a nice schematic of a three input resistor diode logic OR gate (negative logic AND gate) so you can probably start from there to get a basic idea how these things work.

Historically logic gates have changed over the years. RTL (resistor transistor logic) used to be common, but then things moved to TTL (transistor transistor logic) and that took over for quite a while until CMOS came around, which was then called simply CMOS logic even though it also used transistors (they were CMOS transistors instead of bipolar as used in TTL). It really changed a lot after that too, and to list all of the different types of logic here would be hard to do so maybe you could look this up on the web also.

To name just a few logic types here:
DTL, RTL, TTL, CMOS logic, ECL, and the list goes on.

and they also go by the basic part number like for example the 7400 family TTL, or the 74LS00 family TTL to name a couple.

Did you need something other than a schematic for a particular type of logic?

 

[PG1995]

Im a little confused by the apparent contradiction in your question. You say you are looking for what is inside a logic gate, then you say you found 'only' schematic diagrams, then you seem to indicate that you still dont know what is inside. What more can you ask for than for a schematic?

I'm extremely sorry for the confusion. Actually I should have said "circuit diagram" instead of "schematic". I hope it's clear now.

Regards
PG

 

[carbonzit]

I'm extremely sorry for the confusion. Actually I should have said "circuit diagram" instead of "schematic". I hope it's clear now.

Butbutbut ... a schematic diagram is a circuit diagram.

After looking at your link, I think I see your difficulty. What you see is a symbol for a logic gate (a NAND gate in this case). But the symbol hardly gives us a clue as to what's actually inside the gate. The answer, as I think you're seeing, is lots and lots of transistors, diodes and resistors.

This is actually something that's of interest to me. For example, check out this somewhat obsessive guy's page where he sets out to build a "D" flip-flop (a basic logic gate used extensively in such things as computer memory) out of discrete components (here is the complete circuit). Very impressive complexity there. (Yes, I plan on building this myself someday, just to prove that it can be done.)

 

[PG1995]

Butbutbut ... a schematic diagram is a circuit diagram.

After looking at your link, I think I see your difficulty. What you see is a symbol for a logic gate (a NAND gate in this case).

Once again I offer my apologies for the confusion. What I learned from this confusion is the distinction between a schematic and symbol.

I think these main logic gates are building blocks which are used to make other things such as ROM just like transistors and other components are used to make amplifiers etc.

1: Does the designation "CMOS" in 'CMOS NAND gate' reflects the technology used to create the NAND gate? FET and BJT are both transistors but they have been manufactured using different technologies. By the way, I think the term "CMOS" also has something to do with digital camera because mostly cameras specify whether they use CMOS or CCD sensor.

2: Do the logic gates always use two inputs and one output. In the linked diagram you can see all the gates use no more than two inputs and just one output.

3: What is etymological background of term "truth table"? The table uses two kinds of values where "1" is called 'true' value and "0" false. So, it could be as valid to be called 'falsity' table as is 'truth' table.

Please help me with the queries above. Many thanks.

Regards
PG

 

[carbonzit]

1: Does the designation "CMOS" in 'CMOS NAND gate' reflects the technology used to create the NAND gate? FET and BJT are both transistors but they have been manufactured using different technologies. By the way, I think the term "CMOS" also has something to do with digital camera because mostly cameras specify whether they use CMOS or CCD sensor.

CMOS (complementary metal oxide semiconductor) is a semiconductor fabrication process, which really has little to do with the actual construction of the logic gate circuitry.

2: Do the logic gates always use two inputs and one output. In the linked diagram you can see all the gates use no more than two inputs and just one output.

No. Look at the OR gate illustration I posted: it has 3 inputs. Gates can have an arbitrary number of inputs, and some have multiple outputs. A flip-flop, for instance, may have 4 inputs (clock, D, R, S) and two outputs (Q and not-Q).

3: What is etymological background of term "truth table"? The table uses two kinds of values where "1" is called 'true' value and "0" false. So, it could be as valid to be called 'falsity' table as is 'truth' table

I guess since "truth" is considered by many of us humanoids to be more virtuous than falseness, it seemed nicer to call it a "truth table" rather than a "falseness table" (or a "lie table"?).

By the way, a small point: you do understand that 1/0 (true/false, on/off) can have different electrical meanings depending on the device, right? Some devices consider "true" to be a "high" electrical level, while others consider it to be "low" (where "high" and "low" mean "levels referenced to zero", not high voltage as in thousands of volts). Just another potential point of confusion in the digital world.

 

[dougy83]

This is actually something that's of interest to me. For example, check out this somewhat obsessive guy's page where he sets out to build a "D" flip-flop (a basic logic gate used extensively in such things as computer memory) out of discrete components (here is the complete circuit). Very impressive complexity there. (Yes, I plan on building this myself someday, just to prove that it can be done.)

I don't like that circuit.

It's a transparent negative-edge-triggered FF. Here's one I just whipped up that'll work in the same manner as an FF but is obviously less complex.

 

[MrAl]

I'm extremely sorry for the confusion. Actually I should have said "circuit diagram" instead of "schematic". I hope it's clear now.

Regards
PG

Hey no problem. Your picture made it clear what you were originally talking about. What you want is to see what is "inside" the chip, at least in part (transistors, resistors, diodes, etc.)

BTW it is possible to make a SR Flip Flop with two transistors and two resistors, period. Not sure why anyone would want to do this though, unless they want to save an IC chip some day on an already crowded PC board.

 

[alec_t]

That's a neat FF, Dougy. Took me a while to realise it relied on current through the b-c diode of the transistor to clear Q.

 

[PG1995]

Historically logic gates have changed over the years. RTL (resistor transistor logic) used to be common, but then things moved to TTL (transistor transistor logic) and that took over for quite a while until CMOS came around, which was then called simply CMOS logic even though it also used transistors (they were CMOS transistors instead of bipolar as used in TTL). It really changed a lot after that too, and to list all of the different types of logic here would be hard to do so maybe you could look this up on the web also.

To name just a few logic types here:
DTL, RTL, TTL, CMOS logic, ECL, and the list goes on.

and they also go by the basic part number like for example the 7400 family TTL, or the 74LS00 family TTL to name a couple.

Did you need something other than a schematic for a particular type of logic?

Hi MrAl

TTL, DTL, etc. are technologies to manufacture logic gates? That would mean I can get a TTL AND gate or DTL gate. Perhaps, both would have their own particular limitations. Please let me know.

Best wishes
PG

 

[alec_t]

TTL, DTL, etc. are technologies to manufacture logic gates? That would mean I can get a TTL AND gate or DTL gate. Perhaps, both would have their own particular limitations. Please let me know.

You can look that up for yourself, using Google etc.

 

[crutschow]

Two of the most common logic gate types are TTL and CMOS.

TTL gates are constructed using bipolar transistors. It is fairly fast but takes power even when the logic inputs are static. Its approximate logic level states are 0V and 3.5V with a 5V (standard) supply voltage. They are not much used in new design.

CMOS gates are constructed using both NMOS and PMOS transistors. It takes virtually no power when the logic inputs are static. When operating, most of the power goes into charging and discharging the various circuit stray capacitances. The old 4000 series CMOS are quite slow but can operate up to 15V. The newer high speed CMOS (generally using part designations similar to the TTL gates) are much faster but generally operate at 5V maximum. Its approximate logic states are 0V and the supply voltage (5V with a 5V supply).