What is the reason of calling serial port ?

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I am learning about computer hardware. But, I don't know why this port is called serial port ?

Other answer:

Serial data is one of the earliest forms of data transmission; serial means one thing (bit or item etc) after another, as with a "TV Serial".

The original standard that was most commonly adopted was RS-232. That used a 25 pin connector, though most of the pins were rarely used.

It was created primarily to interconnect teletype style computer terminals with the telephone modems used to link them up to the remote mainframe computers they worked with, when a single computer filled a large room and the only way for most people to access them was via a remote terminal.

Because of that, a lot of the terminology used with RS232 serial ports (aka COM ports) is still related to the modem end (DCE, data communications equipment) or teletype end (DTE, data terminal equipment) of the link.

You still see the 25 pin ports on some equipment and computers but mainly they have been superseded by the nine pin version, which has the signals from pins 2-8 and 20 of the 25 pin version.

The actual data signal is bipolar, nominally switching between -12V ("Mark" state or logic 1) and +12V ("Space" state, logic 0).

The data format is based on older, purely mechanical teletype systems.
The signal is normally in "mark" state when things are idle.
A Space acts as a start signal, then the character is sent as eg. eight bits, mark or space as appropriate for each bit, then a mark to indicate the end of the character.
(Mark/Space terminology dates back to the Telegraph era).

See the image here for an example:

The data speed is whatever is configured at both ends of the link – if they are not the same, you get random junk..

The early mechanical teletype systems ran at eg. 45 – 50 – 75 – 110 bits per second (baud, in serial terminology).

Most computer ports are used at 9600 baud, or anything up to 115,200 baud.

Other pins on the connector are use for flow control, to pause the sending end while data is processed.
Some relate specifically to modem control.

Overall, RS232 is considered one of the most non-standard "standards" as it's almost never now used for it's original purpose and no two companies seem to agree on exactly how to use all the extra signals…

A lot ot things just use the two data lines and ground & rely on software handshake for flow control.

More info:

And a bit of trivial history, if it's of interest – two mechanical Teleprinters which relate to the basis of serial data; this is a Teletype computer terminal, which probably operated at 110 Baud or around ten characters per second; 1960s "State of the art" equipment:

And in the source link below, it's pre-computer ancestor, a teleprinter typically used from around the 1930s onward for "Telegrams" and also with the "Telex" system, a parallel to the telephone system that allowed businesses to send typed documents to each other. They also had a paper tape option for preparing things offline so they could be sent quickly and minimise the call cost, or saving received documents.

It's called serial because it transmits one bit at a time.
Thank you all very much.