 ## The history of latch

The following is a pseudo-history
The so-called pseudo-history is not to verify whether the inventor is in this way, but this is definitely a way of thinking.

Pseudo-history started like this. People first invented the inverter and showed its symbol as shown in the picture above.
Then, people began to think about making a loop:

Now we need to look at this loop in detail.
Connect the two inverters in series, and then connect the output of the second to the input of the first.
Hypothesis. . . At A, God created a signal X (1 or 0) and maintained it for a long time.
Then in enough time, the signal at B will also become X.
Then, God left. The data of X is stored in this system.

The purpose of storage was achieved, and then people began to think.
How can we change the data inside?
1. Let’s reconsider the above system first:
Let’s first guess how God might do it:

It might be done like this:
(1) At a certain moment, it replaced the first inverter and replaced it with a device that can send out a signal G (1 or 0).

And, it is maintained for a long enough time so that the inverter 2 outputs G NOT.

(2) After completing the above things, he almost instantly changed this mysterious device back to the inverter:
Next, inverter 2 will output G NOT, then input inverter 1, inverter 1 then output G.
This cycle is realized again.
2. What we have to do is to simulate this process, the process of changing in an instant, for this, let’s first explore the properties of some common doors
Here, I will only explore the NOR gate.
The logical algebraic formula of the NOR gate is given below:

(1) In addition, here, the first nature of the NOR gate is examined.
The NOR gate has two terminals, one A and one B. When one inputs 0, for example, here is when A=0.
At this time, it is actually equivalent to an inverter, isn’t it?
So, the simple loop above, we can make it seem boring to make it more complicated:

(2) Of course, a breakthrough has been made to examine the second nature:
When the input of one terminal is always 1?
For example, assume that A=1
At this time, no matter what B is, this NOR gate has only one function, that is, a device that outputs 0.

(3) In summary:

We simply call A the control terminal and B the input terminal.
A can control this thing. Is it an inverter or a device that only outputs 0?
Now, we can start trying to assemble.

Well, you can return to the common knowledge in the textbook, and come to a bigger picture:
At the output terminals of the two NOR gates, two wires are drawn. So, this is already obvious.
This is a common SR latch (Latch), let’s compare it:

the end.

Introduction of latch: https://www.utmel.com/blog/categories/integrated%20circuit/what-is-latch 