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What are logic gates

Hi, Are logic gates only considered physical? if they are a physical object, what are they exactly? are they only the model? a clear and easy explanation of what logic gates actually entail would be very helpful. I'm having difficulty understanding them based on the articles I've found. I'm looking for further my understanding of how computers work. Thank you for your help.

8/2/2021 3:44:12 PM

Mich

34 Answers

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Mich , this something you can find in the web easily. i have just used google for it and the first hit was this: https://en.m.wikipedia.org/wiki/Logic_gate

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thanks to Calvin Thomas and Mich for asking questions and Martin Taylor for answering it .. really learned a lot .... 😂i too had a myth that it all runs through some nanoscale BJTs. after watching nand to tetris . @Martin Taylor are you electrical engineer?! you have a good knowledge about all these stuffs .. if not then from where you learn all these stuffs?! (probably some random articles and experiences i guess...) Willing to share this with ChillPill sir too ...

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Transmitting in the blind... My apologies if anyone is still posting in this thread. I am not ignoring you. There is a bug when viewing discussions using a web browser that prevents me from viewing more than 20 answers. The thread is always sorted by votes and does not display anything if sorted by date. I will have to send a bug report. Thanks for having my back Mehran. Prashanth Kumar, yes I'm an electronics hardware guy originally. This is the first computer terminal I ever used... https://en.wikipedia.org/wiki/Teletype_Model_33#/media/File:Teletype-IMG_7287.jpg My heart goes out to you guys learning this stuff. It was easy for someone to get their head around all the hardware and software in an 8-bit computer with a few KBytes of ram and rom when I started out. As things progressed I simply had to learn a little bit more in stages over the years. You guys are thrown in at the deep end with multi core processors, GBs of RAM, Terabyte hard drives, GPUs, and multithreaded operating systems from day one. It's enough to make anyone's head spin. See you all in another thread.

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Boolean Logic is a branch of mathematics. A logic gate is a physical semiconductor device, though they can also be simulated or emulated for circuit analysis. https://en.m.wikipedia.org/wiki/Logic_gate

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Calvin Thomas RTL is still used in analogue to digital interface circuits and for low speed switching using discrete circuits. It just isn't very common or fast. For higher speeds we use emitter coupled logic (ECL).

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Calvin Thomas, Field Effect Transistors (FETs) work differently than bipolar transistors. The most common type of FET is the metal-oxide-semiconductor field-effect transistor or MOSFET. These are used in pairs in complimentary metal oxide semiconductor (CMOS) devices. The 4000 series being some of the earliest examples. Due to the internal capacitance these devices were typically slower but used less power and had a wider voltage range. Due to this internal capacitance there is a maximum rate at which the voltage can be slewed from Vdd to Vss. By reducing the potential difference between Vdd and Vss higher clock speeds can be used. This combined with improvements in gate drive circuits and thinner wafers (reduced capacitance) led to the development of high speed cmos (HCMOS), low voltage cmos (LVCMOS). Consequently chip supply rails have dropped from 5V to 3.3V and are now around 1.3V in some mobile devices. When the fet is turned off only minimal leakage current flows through the drain-source junction. When the fet is driven to saturation the junction resistance is low. In either of these states the power dissipation across the junction is negligible. The problem is that during the transition period from one state to the other the junction resistance and current flow pass through an intermediate state that causes power to be dissipated as heat across the junction. Multiply this by the millions of junctions in the device switching at GHz frequencies and things get hot. Which is why modern PCs require extensive cooling. Mobile devices with their lower voltages and clock speeds do not. This is why powerful laptops are so expensive. Putting a lot of thermal management in a confined space takes serious engineering. These thermal problems also plague the gpu. Look at the size of the cooling system on a high end PC graphics card then try to engineer that into a laptop case. The second generation Mac Pro "Trash Can" and the XBox 360 both had serious thermal issues.

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Prashanth Kumar thanks for bringing me here. Wow Martin Taylor is on fire. Some great explanations indeed. Yes logic gates can be made with coupled individual transistors. and as martin said, The gates can also be all made up inside a chip. And also gates can be "programmed into existence" in a FPGA.

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Think of a logic gate as an on and off switch... When voltage passes through, it's on and when potential difference is 0 then it's off. Logic gates are mainly used in Boolean problems which are like on/off, 1/0 or True/ False statements. Let's say you are writing a program and it's work is to count numbers in a loop so each time an number is added e.g +1 , the gate will take that as on, 1 or True and it will let the current pass through... I think you should read more from here : https://www.tutorialspoint.com/computer_logical_organization/logic_gates.htm

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@Kiwwi#, yes the logical operators are used to perform Boolean arithmetic to determine true or false. The bitwise operators are for performing bitwise logical operations on values. See: https://www.tutorialspoint.com/cplusplus/cpp_operators.htm https://www.tutorialspoint.com/cplusplus/cpp_bitwise_operators.htm

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Software, Hardware, Firmware. The need to program custom logic devices gave rise to various hardware description languages. The development of these languages and the compilers involved followed the same progression as software programming languages. The logic for early PLDs were described using simple languages such as PALASM, which was a simple language. This was the equivalent of assembly language for programmable logic. These languages, along with their development environments, simulation and debugging tools increased exponentially. Other hardware description languages were Advanced Boolean Expression Language (ABEL), Compiler for Universal Programmable Logic (CUPL), Verilog (standardized as IEEE 1364), and Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) which was standardised as IEEE 1076. https://en.m.wikipedia.org/wiki/PALASM https://en.m.wikipedia.org/wiki/Hardware_description_language https://en.m.wikipedia.org/wiki/VHDL Most of the IDEs for FPGAs require Gigabytes of storage due to the size and complexity of the gate arrays. Some even include intellectual property such as definitions for cpu cores and standard peripheral devices. Some development environments use schematic capture to generate the VHDL descriptions. The electronics industry has long since standardized the output of all of these hdl tools into what is known as a Joint Electron Device Engineering Council (JEDEC) file. This is the programmable logic equivalent of a hex file used for programming non volotile memory. http://www.pldtool.com/pld-file-formats To actually program the physical devices required insertion of the chip into a device programmer, a specialist piece if test equipment. It didn't take long for the industry to standardise on the Joint Test Action Group (JTAG) interface for in system programming. All programmable devices on a board are chained together by a serial link on the pcb and programmed over a JTAG link from a single on board connector.

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Martin Taylor is an electrical engineer. he said 4 months ago in Q/A: " My speciality is embedded real time systems and aerospace equipment. I'm retired in all but name these days. I started out on computers with 8K bytes of ferrite core memory. The processing power in a mobile phone was the stuff of science fiction when I started out. I still remember Bill Gates commenting on the 1 MB limitation of MS-DOS that "1MB should be enough for anyone.". Yeah, that didn't age well Bill. " i always enjoy his answers

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Kiwwi# in old days terminal devices were just printers (yeah typewriter like) ..there werent any display like monitor it came later ... on every command you will have to wait for it to print the result ... really the history of computers is incredibly amazing and there was art in everything ... Martin and Mehran were too lucky i can say that they are spectators of the era..

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I mean, is logic physical? It doesn't seem to be, but in order to convey logic, something in the physical world has to move from one location to another. My understanding is that the term "logic gates" refer to the model, and its common physical implementation in computers involves transistors. http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/trangate.html Of course, that hasn't stopped creative minds from attempting to build fully mechanical computing units: like via marbles https://youtu.be/8BOvLL8ok8I or even water... https://youtu.be/IxXaizglscw

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Martin Taylor I see, so I guess that there are some expensive possibilities to cancel out this capacitive reactance. Thank you very much for answering my questions.

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Martin Taylor thats really fascinating .. amazed to see that they had shift , control and alt keys too...

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thank you

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I have only ever known the term "logic gates" to mean physical transistor circuits or digital microchip components. In software it is called Boolean logic, Boolean expressions or just logic.

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Mich Logic gates are physical semiconductor devices, made using diodes and transistors and sometimes, resistors (this needs to be clarified). However, the representations aren't. Edit: I don't know much about the logic technologies used nowadays. From what I've Googled, TTL is actually faster (the ECL configuration is even faster) than CMOS (possibly due to the fact that it doesn't pose that much of capacitive reactance at high frequencies) but it consumes more power, while the CMOS logic consumes lesser power as the only one contributing to the power intake is the capacitive transient at the start, which is pretty small for low frequencies. Also, MOSFETs can deliver more output power than BJTs. But they're not so suitable for high-speed applications because of its Gate-Source capacitance. Moreover, CMOS is more expensive than TTL logic.

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Calvin Thomas, I thought I was old! Most modern logic gates are hcmos, Transistor-Transistor Logic (TTL) is quite old. Diode-Transistor Logic (DTL) is ancient but Resistor-Transistor Logic (RTL) is prehistoric, even by my standards. See: https://en.m.wikipedia.org/wiki/Logic_family

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The most widely used logic series are the 4000 series and 7400 series. The 5400 series are military grade equivalents of the 7400 series. These were all introduced in the 1960s. They were low part count devices by modern standards, usually containing only six dual input gates. Gates with a larger number of inputs were available with fewer gates per package. https://en.m.wikipedia.org/wiki/7400-series_integrated_circuits https://en.m.wikipedia.org/wiki/4000-series_integrated_circuits Both of these series of devices are still available today with many manufacturers beng 3rd party suppliers. For greater flexibility programmable logic devices (PLDs) can be used. These contain an interconnect matrix and logic blocks. Fuses within the device are "blown" to route signals from the input pins to the logic blocks an then to the output pins. This allows custom devices to be created by the user with application specific logic and pin arrangements without the expense of designing and manufacturing a masked Application Specific Integrated Circuit (ASIC). https://en.m.wikipedia.org/wiki/Programmable_logic_device As increased logic densities and pin counts were required the Complex Programmable Logic Device (CPLD) was introduced. This in turn lead to the introduction of the Field Programmable Gate Array (FPGA). FPGAs allow entire system on chip devices to be designed by the end user. https://en.m.wikipedia.org/wiki/Programmable_logic_device#CPLDs https://en.m.wikipedia.org/wiki/Field-programmable_gate_array Since these devices needed to be programmed by end users (as opposed to the device foundry) this created two problems. Firstly how to describe the logic designs, and secondly how to physically program the devices. to be continued...