Logic gates - Page 1
Forum section: electronics / beginners

author: Keosintal
author_rank: Contributor
timestamp: December 10, 2018, 04:51:18 am
content: HiI am confused about logic gates. Are they physical component or what? I mean, do they sell these logic gates as individual components or is it some kind of programming inside an IC?

author: Mr. Scram
author_rank: Super Contributor
timestamp: December 10, 2018, 05:30:38 am
content: A logic gate is an abstract component. They can be built from actual components in different ways using different technologies. If you look at the link at the bottom of my post, you'll find how an AND gate can be built using different types of basic components like transistors or diodes.By abstracting a logic gate you separate the underlying technology and its function. It makes designing an architecture and structure of a chip a little easier, as you don't have to deal with a lot of housekeeping. In earlier years logic gates mostly came packaged individually in ICs. Of course an IC also needs power and a few other things, as it's not an abstracted logic gate any more. These were mostly abstracted logic gates made into a physical thing. Combine different of these physical logic gate ICs in the right way and you can build a simple computer. You still find a lot of similar basic logic gate ICs in equipment today, usually performing glue logic functions. These are commonly chips of the HC74 family.Nowadays logic gates tend to be packaged together in much more integrated ways, like in microprocessors or other chips with a specific function. That way literally millions or billions of logic gates can be stuck together on a single chip. Most chips have an architecture, which is the layout of the logic gates inside the chip and thus also the layout of the actual components in the chip.https://en.wikipedia.org/wiki/AND_gate

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2018, 11:13:57 am
content: Yes a gate is made of many other components. They can be switched and relays, transistors or thermionic tubes.Nowadays gates are mostly made of MOSFETs and fabricated on ICs. Glue logic containing individual gates is still available, but it's not as common as it used to be. Most IC packages contain four gates, but six and eight is common for NOT gates and line drivers and single gate packages exist in surface mount format, to save space.Some data sheets of logic gates in the 74HC(T) series.https://assets.nexperia.com/documents/data-sheet/74HC_HCT00.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT1G00.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT14.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT1G14.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT240.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT02.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT1G02.pdf

author: tautech
author_rank: Super Contributor
timestamp: December 10, 2018, 11:20:52 am
content: Have a play with their functionality here:https://tams.informatik.uni-hamburg.de/applets/hades/webdemos/10-gates/00-gates/chapter.html

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2018, 12:30:32 pm
content: Here's a tutorial on how to make various logic gates from MOSFETs, inside the CD4007 transistor array.https://wiki.analog.com/university/courses/electronics/electronics-lab-28

author: iMo
author_rank: Super Contributor
timestamp: December 10, 2018, 12:52:01 pm
content: Buy 5600 pieces of the CD4007, wire them together and you may land on the Moon..

author: Brumby
author_rank: Unknown
timestamp: December 10, 2018, 12:53:37 pm
content: A logic gate is simply a circuit - just like an audio amplifier is simply a circuit.Like any circuit it is made up of individual components. These components can be discrete or they can be fabricated on an integrated circuit.Integrated circuits are the usual presentation of logic gates, with multiple independent gates in the one chip being quite normal. The limitation is the number of pins on the IC package.Because logic gates have set operating levels and are designed to be interconnected, they are universally treated as a single functional unit where the internal circuitry is practically never examined.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 10, 2018, 02:30:08 pm
content: My first CMOS IC design was in metal-gate technology. In order to save space, I used dynamic logic in all of the counters and PLAs. Not a good practice, in general, these days. Anyway, here is a dynamic DFF from the schematics (yup...still have them).Regarding the CD4007. It is not JUST a logic gate! Back in early '80s, I was designing ICs for implantable pacemakers. I breadboarded the lead driver (which was made up of opamps) using CD4007s configured as amplifiers.If you want to play around simulating logic circuits, here is the coolest little tool to use. No installation. Just download and run.http://www.cburch.com/logisim/

author: rstofer
author_rank: Super Contributor
timestamp: December 11, 2018, 02:27:10 am
content: Grab up some 1N914 or 1N4148 diodes and some 1k resistors (just guessing at the value) and try to build up the diode logic gates shown in the Wiki. There is a 3 input AND gate and a 3 input OR gate. Use jumpers or switches for the input and just measure the output with a DMM.https://en.wikipedia.org/wiki/Diode_logicThere was a time with Diode-Transistor-Logic (DTL) was commonly used and it was available as an IC. This predated Transistor-Transistor-Logic (TTL) which became far more popular. The earliest ICs were Resistor-Transistor-Logic (RTL). I was pretty late to the game but I started playing with RTL in '69.

author: Old Printer
author_rank: Frequent Contributor
timestamp: December 11, 2018, 04:36:15 am
content: I would also reccomend a look at Boolean algebra, if you are not already familiar with it. It's principles are how different logic gates can be connected with each other to perform calculations and process data to solve equations.

author: nick_d
author_rank: Regular Contributor
timestamp: December 13, 2018, 11:37:07 am
content: In answer to OPs question about individual packages, traditionally logic gates were sold in 14 to 20ish pin packages, e.g. NOT gates have 2 pins each so you get 6 in a 14 pin package with 2 pins left for +V and GND, whereas NAND gates have 3 pins so you get only 4 in a 14-pin package. Designers had to be very creative in substituting one gate type for another in order to use up spare gates and not introduce a whole new package just to use one gate. Inevitably there were a few leftover gates whose inputs would be tied to GND and outputs unconnected. However, the cool thing now is with surface mount technology and tiny packages, individual gates ARE now available. For example a NAND in a 5 pin package, the 3 pins of the gate and the +V and GND pins just for that gate. It's awesome. While MOST gates are inside chips like say an Intel Core i7 which has billions of gates in it, connected up like adders, multipliers, sequencers, etc... the individual gates are great for interfacing one large chip to another, e.g. one chip might output a request line that goes low when active, and this might need to be connected to an interrupt line on anothet chip that goes high to request an interrupt. So you plonk down a little 4-pin inverter chip (in, out, +V and GND) and Bob's your uncle! Look inside a smartphone or modern laptop and you'll see that enormous amounts of glue like this is used to tie together the various pre-made systems such as camera, touch controllers and so on. Individual gates rock!cheers, Nick