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HIGH or LOW signal in digital system

Very common we hear ON/OFF, HIGH/LOW, 1/0, or something similar in digital system. It is called binary. I would like to know what is the very basic component to generate such that binary system? I have searched to know about it, but I could not find. I know about transistor used to make logic gate or even processor. But that component still need to be triggered or fed with binary, to ON or to OFF the input (either base or gate). I doubt the component to generate such LOW/HIGH or ON/OFF is clock as it is work constant. If it is the thing, then how it change to one state to another state?

So my question, what is the very basic component to generate that binary system? There is one similar post here, but is is about logic gate. What I am asking is component, a level of transistor or Christal clock, not logic gate.

AirCraft Lover
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  • Are you asking what determines the logic voltage thresholds to make a HI or LO? That is just the construction of the transistor itself that decides that. – DKNguyen Jan 03 '20 at 05:38
  • CMOS simple example http://tinyurl.com/umar8pk in slow motion (1ns clock) – Tony Stewart EE75 Jan 03 '20 at 05:45
  • internal Miller capacitance http://tinyurl.com/tzmy7kd – Tony Stewart EE75 Jan 03 '20 at 05:51
  • @DKNguyen, no, it is not. Say you are sending a series of data contain 1011110011000001 (whatever is this). Mean, it first send **HIGH** voltage (over another, of course) over a certain time interval, then followed by **LOW** voltage, then **HIGH-HIGH-HIGH-HIGH** then **LOW-LOW**, and so on... Inside any logic gate or microprocessor, there must any electronic component to decide it. Mean, the logic gate or microprocessor itself we could make it in a very 'raw' way, directly from electronic component. – AirCraft Lover Jan 03 '20 at 09:27
  • @TonyStewartSunnyskyguyEE75, I have seen both the animation in both the links. Very interesting. But, I was not wrong to understand, it is just MOSFET. But what send change in state input to the gate is not clear. To make clearer, say I want to send a series of data as I mentioned to DKNguyen. There must be trigger to trigger the input data to the gate to change the state from HIGH to LOW or vice versa. In every computer explanation mostly depicted as ON/OFF switch. But what is that switch, it is not clear, at least to me. – AirCraft Lover Jan 03 '20 at 09:43
  • Are you asking what component you can use to pull a useful binary signal - say, a video file - out of thin air? You cannot pull useful information out of thin air. What information do you want it to transmit? What is it controlled by? – user253751 Jan 03 '20 at 13:26
  • @user253751, if you like to see how about digital is explained as [here](https://www.youtube.com/watch?v=VBDoT8o4q00), always manual switch is made as input. But how it actually done in real computer? Please see the video, it also explains how transistor inside the logic gate. – AirCraft Lover Jan 03 '20 at 14:21
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    @AirCraftLover In a real computer, signals are generated by something that has some data to transmit. For example, a keyboard. They aren't just made up for no reason. Do you want to ask how a keyboard transmits data? Because a keyboard is a bunch of switches and logic circuits. – user253751 Jan 03 '20 at 14:39
  • @user253751, so, can I say that the keyboard is ultimate source input which determines a data is either **LOW** or **HIGH**? Also, can I say that the data from keyboard will determine the transistor state either it **ON** or **OFF**? – AirCraft Lover Jan 04 '20 at 01:12

3 Answers3

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Usually we do not pull random 1s and 0s "out of thin air". Binary signals exist for reasons. So what kind of signal do you want to generate?

You can make a signal that is always low, or always high, just by connecting a wire to the ground rail, or the power supply rail.

You can make one that alternates between 0 and 1 by using a clock.

Any signals more complex than that are generated using logic circuits.

A computer keyboard, for example, sends 1s and 0s depending on which key you pressed. At least, an old PS/2 one does - USB is quite complicated so let's ignore it for now. The keyboard has a bunch of, guess what, switches (one for each key) and logic gates to work out the correct code to send based on the switches.

In practice, if you want to generate a signal like this, for testing purposes, if the speed (bit rate) is slow then you would use a microcontroller. For very fast signals you could use a SERDES (fed by several bits in parallel from a slower microcontroller), or an FPGA.

user253751
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  • ***You can make one that alternates between 0 and 1 by using a clock.*** As per my understanding, clock is to give time interval of computation. Say the clock is 1MHz, then the computation will calculate every signal sequence will be calculate every 1 micro second (us). Say if there are 2 us low state, than it is considered as two **0**, and vice versa. But as you comment above, I now realize that the state is a pre-determined thing, which, as you said, come from keyboard, and we press keyboard. Input from keyboard will affect the state of the input to either gate of MOSFET or to base of BJT. – AirCraft Lover Jan 04 '20 at 01:20
  • Then, by including here your comment above, I may accept your answer. ***@AirCraftLover In a real computer, signals are generated by something that has some data to transmit. For example, a keyboard. They aren't just made up for no reason. Do you want to ask how a keyboard transmits data? Because a keyboard is a bunch of switches and logic circuits. – user253751*** – AirCraft Lover Jan 04 '20 at 01:22
  • @AirCraftLover A clock is ***also*** a digital signal that alternates between 0 and 1. Since we're talking about transistors, transistors don't care about your clock rate. A transistor doesn't know whether a 2us low state is 2 bits at 1MHz or 4 bits at 2MHz or 1 bit at 0.5MHz. Most combinational circuits don't care. Only memory elements care about the clock. – user253751 Jan 04 '20 at 20:41
  • @AirCraftLover You can have other input devices. For example, a paper tape reader can be made using mechanical components and a light sensor. Then you could say the light sensor is an input device. Same for a ball mouse - it also uses a few light sensors internally, and those are the inputs to the electronics. A light sensor is an analog device, but it's designed so it's always full light or no light (i.e. binary). – user253751 Jan 04 '20 at 20:44
  • Of course, tape or other peripheral are another type of ultimate input which crate state LOW or HIGH. My point in my question is to know what is the deciding component to low or to high the signal in the computer system. – AirCraft Lover Jan 05 '20 at 05:42
  • @AirCraftLover There isn't one, usually. But each logic gate usually does "a little bit of deciding". You may find it interesting to read about *metastability*. – user253751 Jan 06 '20 at 10:43
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Because there are no discrete signals in nature, there is no component that will output a binary signal by itself.

Moreover, to sustain an alternating (digital) signal, you need to provide energy.

To obtain a continuous alternating binary signal, you need to discretise an analog alternating signal.
There are several ways to make an electronic oscillator that outputs an alternating binary signal.

Huisman
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  • So, how a binary is made? What is trigger one state to another state, say from HIGH to LOW or vice versa from LOW to HIGH? – AirCraft Lover Jan 03 '20 at 09:49
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    I think you should specify what you mean by 'binary' first. Binary is ```relating to, composed of, or involving two things```. It can be *anything*! If you mean typical used binary voltage levels, do read [Logic Signal Voltage Levels](https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/) – Huisman Jan 03 '20 at 10:41
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For inputs it could be a switch wgich makes the first pulse.

Inside complex systems there are also oscillators, which autonomously just make pulse after pulse when the power is on.

Jasen Слава Україні
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  • Say, if I would like to send data **1011110011000001** over a very raw digital system that I make from scratch (from component), then what is the component could trigger the state from one to another? Say from HIGH to LOW or vice versa? Logic gate and microprocessor also build from millions electronic component, right? – AirCraft Lover Jan 03 '20 at 09:45
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    some sort of SERDES could spit out that pattern if supplied with a suiable clock (from an oscillator) a SERDES is a moderately comples system of gates, (thousands of transistors) often available as part of a microcontroller, (eg SPI USART) the timing of the bits is derived from the clock oscillator the pattern of the high and low comes from other inputs – Jasen Слава Україні Jan 03 '20 at 10:26