Hi, what is the technically difference between USB type A and B? I know type A is host and B is slave, but are they any electrical differences (diode prevents use A in tasks of B connector and reverse?)? Why cant we use only USB A? How B helps to prevent short circuit/mess pins (prevent GND go to VCC)? I mean, them both plug only in one direction.
Or is the case that there aren't concrete differences here? If I replace USB B with A connector, it works? Connector is just dump?
It would be easy to connect two hosts together - not a good idea.
So, the core of my question: is the only reason to prevent humans plug things wrong? USB type A and B are electrically exactly the same, but the connector is physically different shape? I can replace USB A with B connector and it works normal?
A & B are only connector types. They don't matter. You can get all kinds of adapters and mix & match adapters.
But you need a host-controller-computer and a device, or with a hub you can connect multiple devices to one host. The A & B connectors (and variations) help to keep that straight.
The Arduino is usually a device that communicates with a computer.
You can get a host-controller board but it's use is limited because you won't have a driver for a printer or most other USB devices. HIDs (human interface devices) like keyboard & mice are standardized and with the right software they can work. There is also software for USB MIDI either way, as a controller or a MIDI instrument.
A & B are only connector types. They don't matter. You can get all kinds of adapters and mix & match adapters.
Are also the cables just wires without any circuits/boards?
But you need a host-controller-computer and a device, or with a hub you can connect multiple devices to one host. The A & B connectors (and variations) help to keep that straight.
Yup, I know that.
Just to make sure, can anyone answer yes/no to these:
So, the core of my question: is the only reason to prevent humans plug things wrong? USB type A and B are electrically exactly the same, but the connector is physically different shape? I can replace USB A with B connector and it works normal?
Also: to meet the standard.
To understand fully, you probably have to have been active during the pre-USB times, when the "RS232 connector/cable" was common. Except that some connectors (on PCs and Devices) were male, some were female, some were serial, some were parallel, some were SCSI, and they'd use varying numbers of actual signal leads. Then the cables would have varying genders of connectors on each end, with somewhere between 3 and 26 wires connecting them, with or without "crossovers" on some of the signal pairs.
It was a mess. When I did training on connecting "Terminal Servers" to devices, I had a prominent slide: "It's ALWAYS the cable!"
USB fixed most of that. Yea! (Until USB-C, but don't get me started 
)
I can replace USB A with B connector and it works normal?
Assuming that it's wired correctly, it should work. There have been occasional products originating in "less standards-conforming areas of the world" that have put USB-A connectors on devices. Aside from needing to find a non-standard cable to go with them, they work fine, AFAIK.
But WHY would you want to do that?
So, essentially what I'm asking is: Is the sole purpose of differentiating between USB type A and B connectors merely to prevent humans from plugging them in the wrong way? Both types are electrically identical, but their physical shapes differ. Can I interchange a USB A connector with a USB B connector and still expect normal functionality?
just to make sure, can anyone answer yes/no to these:
No
USB type A and B are electrically exactly the same, but the connector is physically different shape?
No
I can replace USB A with B connector and it works normal?
No
What are the another reasons? I word this a little different, what is/are the main difference(s)?
USB type A and B are electrically exactly the same, but the connector is physically different shape?
No
What is the differences in eletrically? I have open both of them and what I see is no PCB and ports connect wires in identically way I think (VCC -> VCC, GND -> GND, D+ -> D+, D- -> D-). The only think came my mind is, is there a little diode or something like that?
I can replace USB A with B connector and it works normal?
No
What doesn't work? As per comments here, if I have certain case (e.g. a computer (A port) -> a mouse) and I change the ports (e.g. solder the B port to that computer, so the computer (B port) -> the mouse) it should work. Of course it's against the standard, stupid and if I mistakenly connect things wrong (e.g. connect an another computer (A port) -> (B port) the computer) my devices can frie, but still should work if I have carefully and use it in only the certain cases.
Can you tell, what's in that makes need for USB A and B? Sorry, I don't understand 
And if the historically reasons are left out, is the prevent human mistakes the main reason? Or are there some electric technically reasons that makes need for them? Some places said that ports connects wires vice versa (so wires that comes A goes in reverse order to B side), but as I said, seems that isn't the case. I'm just not sure.
But WHY would you want to do that?
Most of places says that "the reason is to separate devices and peripherals", but not the physical, tehnically difference. I'm just want to know that.
Do you just ask my questions in different wording or try to suggest the answers for them or what?
To prevent you from connecting to hosts together or two devices together.
My mind came a different same kind of separate question:
You probably know Type 2 connector in electric cars and it has male and famale heads. In EU charger end (socket outlet) is female, cable that end (plug) is male, cable another end female (connector and car end (vehicle inlet) is male, so cables are male - female. Okay, sound resonable, but in China both socket outlet and vehicle inlet are female, so cables are male - male. What this difference means in technically? Does China version chagers have need to communicate about host or what?
Another case are plugs in walls. Them never have something like client side so the only needed case is female in wall.
This sounds very stupid thoughts... but I don't get logic in there...
Is the thing that current have always certain direcsion and ends have to prepared about that. In stupid ports we just need connect out to in, but more smarter can negotiate about it. If ports are stupid we want physical shape for them so human see what belongs to what, but in smarter ones ports can be the same because that negotiate. Are USB-C and Type 2 China version examples of this? I have heard that Type 2 doesn't really communicate about host, but is it the only way? The wall plugs doesn't have the case of different direcsion since it always is the host, so it doesn't need two ports because of that?
As you requested I answered all your questions with a YES or NO.
Now you basically as the same questions again.
Exactly what do you want to know about USB?
I would have answered all three questions with "Yes", though.
I still want to know "why?"
See post #15
YES, that is the primary reason. Any other reason is not relevant to your question.
if these are the only type A and type B connectors we are discussing
YES. Why you would want to do that is irrelevant but it would work normally. The USB type A and B connectors pictured above don't include any way to determine whether they are plugged Ito a host or a device so if it worked before you changed the connectors, it will work after. I think your original concern was that there might be a diode or something allowing current to pass in only one direction and there is not.
Right, but answer no is opposite to all other answers here. Have you any reasoning for your answer?
Yup
