此貼長、有趣、信息量大。發報員按住鍵時收報員聽到聲音，放開鍵時聲音消失。如果按鍵持續1ms代表0, 2ms代表1, ..., 65536毫秒代表65535，這個通信系統里的symbol就是16個位。它的問題：1. 發送65535這一個symbol耗時一分鍾；2. 收報員哪能分清1ms和2ms？
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called dots and dashes or dits and dahs. Morse code is named after Samuel Morse, an inventor of the telegraph. In 1844, a skilled operator could send 4 characters per second (~40 words per minute, 0.021 kbit/s).
Morse code is a 變長碼。The length of a dot is 1 unit; A dash is 3 units; The space between parts of the same letter is 1 unit; The space between letters is 3 units; The space between words is 7 units. 它不像哈夫曼碼那樣有不同的前綴，沒有space的話沒法區分..是I還是EE。
The Baudot code was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of five bits, sent over a communication channel such as a telegraph wire or a radio signal. The symbol rate measurement is known as baud, and is derived from the same name.
波特碼是5位定長碼。如果我們收到了不完整的一段，試上5次，依次丟掉0, 1, 2, 3, 4，頭一個丟了，后面的還是完整的。十進制每位0~9十個數，二進制每位0~1兩個數。一進制固定傳0沒有意義。波特碼里英文字母不區分大小寫。
The baud unit is named after Émile Baudot (1845-1903), the inventor of the Baudot code for telegraphy, and is represented in accordance with the rules for SI [國際單位制] units. That is, the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence. It was defined by the CCITT (now the ITU) in November 1926. The earlier standard had been the number of words per minute. One baud was equal to one pulse per second, a more robust measure as word length can vary.
啥叫一個pulse? 比如兩個人拉着一根繩，用抖繩子的幅度表示1, 2..., 8，每秒抖2下，竊以為pulse per second是2，bits per pulse是3，這是個調幅系統。如果手電前弄個塑料片，紅0，綠1，藍2, 白3，這是個調頻系統(光是電磁波，不同顏色的光頻率不同)，每個pulse 2個bits。
到了1940x，Teleprinter (50 baud)0.05 kbit/s 404 operations per minute, 6.7 operations per second，這機器可能比熟練的人速度慢。
波長乘頻率=波速，真空中的光速是30萬km/s。上圖中VIS即Visual, 可見光。infra: 低於; ultra: 超過; 圖左右顛倒下更順。波長越短，每cm中的波數就越多。
頻率越高，通信速度就越快，帶寬越大嗎？好像是的。語速越快，同樣時間內說的話不是越多？瘋狂地抖繩子不比慢慢抖傳遞的信息更多？那玩啥無線電，上gamma射線啊？首先它不好發射，其次它走直線，最后高能的會搞死人的。
下面列出歷史上著名的和未來的一些標准的帶寬。最后介紹RS-232 or a related standard, such as RS-485 or RS-422.
- GPS receivers typically NMEA 0183 at 4,800 bit/s 程序讀進來的是個字符串
- Macintosh AppleTalk using RS-422 at 230.4 kbit/s
- Ethernet (10BASE-X) 10 Mbit/s 1.25 MB/s 1980 (1985 IEEE Standard)
- ADSL (G.dmt) ITU G.992.1 8192/1024 kbit/s 1024/128 kB/s 1999
- NG-PON2 (G.989) (fiber optic service) 40/10 Gbit/s 5/1.25 GB/s 2015
- GPRS (2.5G) 57.6 kbit/s 7.2 kB/s 28.8 kbit/s 3.6 kB/s
- DS1/T1 (and ISDN Primary Rate Interface) 1.544 Mbit/s 0.192 MB/s 1990 那時有T1=家有礦
- LTE Cat 14 3.9 Gbit/s 487 MB/s 1.5 Gbit/s 187 MB/s 5G不僅僅是帶寬高，還有延時短，例如遠程做手術。
- IEEE 802.11ax (aka Wi-Fi 6) 11 Gbit/s 1375 MB/s 2019
- PC3-8500 DDR3 SDRAM DDR3-1066 133 MHz 533 MHz 1.066 GT/s 68 Gbit/s 8.5 GB/s
- PC5-51200 DDR5 SDRAM DDR5-6400 800 MHz 3200 MHz 6.4 GT/s 409.6 Gbit/s 51.2 GB/s
- Apple 2 cassette tape interface 1.5 kbit/s 200 B/s 1977
- CD Controller (1×) 1.171 Mbit/s 0.146 MB/s 1988
- DVD Controller (1×) 11.1 Mbit/s 1.32 MB/s
- Blu-ray Controller (1×) 36 Mbit/s 4.5 MB/s 注意不是4.5Mbit/s，碼率不高的1080p沒意義
- ATA PIO Mode 4 133.3 Mbit/s 16.7 MB/s 1996
- NVMe over M.2 or U.2 (using PCI Express 4.0 ×4 link) 64 Gbit/s 7.876 GB/s 2017
- iSCSI over 100G Ethernet 100 Gbit/s 12.392 GB/s 2010
- FCoE over 100G Ethernet 100 Gbit/s 12.064 GB/s 2010
- Infiniband HDR 12× 600 Gbit/s 75 GB/s 2017
- AGP 1× 2.133 Gbit/s 266.7 MB/s 1997
- PCI Express 6.0 (×16 link) 1024 Gbit/s 126 GB/s 2020 得啥內存配它？
- NVLink 2.0 1.2 Tbit/s 150 GB/s 2017
- Infinity Fabric (Max. theoretical) 4.096 Tbit/s 512 GB/s 2017
- Thunderbolt 3 40 Gbit/s 5 GB/s 2015
- USB4 40 Gbit/s 5 GB/s 2019
- Serial MIDI 31.25 kbit/s 3.9 kB/s 1983
- HDMI 36.864 Mbit/s 4.608 MB/s audio
- DisplayPort 36.864 Mbit/s 4.608 MB/s audio
- HDMI 2.1 48 Gbit/s 6 GB/s [b] 2017 addio
- DisplayPort 2.0 (4-lane) 80 Gbit/s 10 GB/s 2019
串口
Operating systems usually create symbolic names for the serial ports of a computer, rather than requiring programs to refer to them by hardware address. Unix-like operating systems usually label the serial port devices /dev/tty*. TTY is a common trademark-free abbreviation for teletype, a device commonly attached to early computers' serial ports, and * represents a string identifying the specific port; the syntax of that string depends on the operating system and the device. On Linux, 8250/16550 UART hardware serial ports are named /dev/ttyS*, USB adapters appear as /dev/ttyUSB* and various types of virtual serial ports do not necessarily have names starting with tty. The DOS and Windows environments refer to serial ports as COM ports: COM1, COM2,..etc. Ports numbered greater than COM9 should be referred to using the \\.\COM10 syntax.
The number of data bits in each character can be 5 (for Baudot code), 6 (rarely used), 7 (for true ASCII), 8 (for most kinds of data, as this size matches the size of a byte), or 9 (rarely used). 8 data bits are almost universally used in newer applications. 通信的最小單位不是character更不是word，而是symbol, symbol一般是bit。
Parity is a method of detecting errors in transmission. When parity is used with a serial port, an extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check. 研究糾錯碼需要數學很好。
Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. If slow electromechanical teleprinters are used, one-and-one half or two stop bits are required.
The data/parity/stop (D/P/S) conventional notation specifies the framing of a serial connection. The most common usage on microcomputers is 8/N/1 (8N1). This specifies 8 data bits, no parity, 1 stop bit. In this notation, the parity bit is not included in the data bits. 7/E/1 (7E1) means that an even parity bit is added to the 7 data bits for a total of 8 bits between the start and stop bits. If a receiver of a 7/E/1 stream is expecting an 8/N/1 stream, half the possible bytes will be interpreted as having the high bit set.
Flow control is used in circumstances where a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a handshaking method, usually distinguished between hardware and software handshaking.
奈奎斯特
In signal processing, the Nyquist rate, named after Harry Nyquist, specifies a sampling rate. In units of samples per second its value is twice the highest frequency (bandwidth) in Hz of a function or signal to be sampled. With an equal or higher sampling rate, the resulting discrete-time sequence is said to be free of the distortion known as aliasing. Conversely, for a given sample rate, the corresponding Nyquist frequency in Hz is the largest bandwidth that can be sampled without aliasing, and its value is one-half the sample-rate. Note that the Nyquist rate is a property of a continuous-time signal, whereas Nyquist frequency is a property of a discrete-time system.
六級/考研單詞: dash, successor, alphabet, advent, derive, accordance, pulse, robust, fibre, optic, ax, thunder, serial, audio, compute, hardware, abbreviation, data, seldom, detect, corrupt, electron, usage, interpret, transmit, distort, converse, correspond IT技術類英文比小說簡單啊。
波特是engineer, 不是scientist. My fellow engineers, 加油！:-)
藍光波長=3e8/500e-9=6e14=600THz. 量子糾纏說不定是因為斯貓波，它波長比gamma射線還短很多，斯貓粒子超小，光子對它來說內部是真空。微粒是它的琴鍵，你使勁看，也看，不，見！