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Universal asynchronous receiver-transmitter

Universal asynchronous receiver-transmitter is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Universal asynchronous receiver-transmitter rather than just read about it. In short: A universal asynchronous receiver-transmitter (UART ) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least to most significant, framed by start and stop bits so that precise timing is handled by the communication channel.

Universal asynchronous receiver-transmitter — main illustration
Universal asynchronous receiver-transmitter — illustration

Key takeaways

  • Universal asynchronous receiver-transmitter belongs to astronomy; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Universal asynchronous receiver-transmitter to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Universal asynchronous receiver-transmitter from memory before moving on to harder problems.

Reference excerpt

A universal asynchronous receiver-transmitter (UART ) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least to most significant, framed by start and stop bits so that precise timing is handled by the communication channel. The electric signaling levels are handled by a driver circuit external to the UART. Common signal levels are RS-232, RS-485, and raw TTL for short debugging links. Early teletypewriters used current loops. It was one of the earliest computer communication devices, used to attach teletypewriters for an operator console. It was also an early hardware system for the Internet. A UART is usually implemented in an integrated circuit (IC) and used for serial communications over a computer or peripheral device serial port. One or more UART peripherals are commonly integrated in microcontroller chips. Specialised UARTs are used for automobiles, smart cards and SIMs. A related device, the universal synchronous and asynchronous receiver-transmitter (USART), also supports synchronous operation. In OSI model terms, UART falls under layer 2, the data link layer.

History Some early telegraph schemes used variable-length pulses (as in Morse code) and rotating clockwork mechanisms to transmit alphabetic characters. The first serial communication devices (with fixed-length pulses) were rotating mechanical switches (commutators). Various character codes using 5, 6, 7, or 8 data bits became common in teleprinters and later as computer peripherals. The teletypewriter made an excellent general-purpose I/O device for a small computer. Gordon Bell of DEC designed the first UART, occupying an entire circuit board called a line unit, for the PDP series of computers beginning with the PDP-1. According to Bell, the main innovation of the UART was its use of sampling to convert the signal into the digital domain, allowing more reliable timing than previous circuits that used analog timing devices with manually adjusted potentiometers. To reduce the cost of wiring, backplane and other components, these computers also pioneered flow control using XON and XOFF characters rather than hardware wires. DEC condensed the line unit design into an early single-chip UART for their own use. Western Digital developed this into the first widely available single-chip UART, the WD1402A, around 1971. This was an early example of a medium-scale integrated circuit. Another popular chip was the SCN2651 from the Signetics 2650 family. An example of an early 1980s UART was the National Semiconductor 8250, which was used in the original IBM PC's Asynchronous Communications Adapter card. In the 1990s, newer UARTs were developed with on-chip buffers. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the popular National Semiconductor 16550 has a 16-byte FIFO, and spawned many variants, including the 16C550, 16C650, 16C750, and 16C850. Depending on the manufacturer, different terms are used to identify devices that perform the UART functions. Intel called their 8251 device a "Programmable Communication Interface" because it was actually a USART and capable of synchronous operation as well as asynchronous. It was introduced in 1979. MOS Technology 6551 was known under the name "Asynchronous Communications Interface Adapter" (ACIA). The term "Serial Communications Interface" (SCI) was first used at Motorola around 1975 to refer to their start-stop asynchronous serial interface device, which others were calling a UART. Zilog manufactured a number of Serial Communication Controllers or SCCs. Starting in the 2000s, most IBM PC compatible computers removed their external RS-232 COM ports and used USB ports that can send data faster. For users who still need RS-232 serial ports, external USB-to-UART bridges are now commonly used. They combine the hardware cables and a chip to do the USB and UART conversion. Cypress Semiconductor and FTDI are two of the significant commercial suppliers of these chips. Although RS-232 ports are no longer available to users on the outside of most computers, many internal processors have UARTs built into their chips to give hardware designers the ability to interface with other chips or devices that use RS-232 or RS-485 for communication.

Transmitting and receiving serial data

A UART contains the following components:

a clock generator, usually a multiple of the bit rate to allow sampling in the middle of a bit period input and output shift registers, along with the transmit/receive or FIFO buffers transmit/receive control read/write control logic The universal asynchronous receiver-transmitter (UART) takes bytes of data and transmits the individual bits in a sequential fashion. At the destination, a second UART re-assembles the bits into complete bytes. Each UART contains a shift register, which is the fundamental method of conversion between serial and parallel forms. Serial transmission of digital information (bits) through a single wire or other medium is less costly than parallel transmission through multiple wires. The UART usually does not directly generate or receive the external signals used between different items of equipment. Separate interface devices are used to convert the logic level signals of the UART to and from the external signaling levels, which may be standardized voltage levels, current levels, or other signals. Communication may be in three modes:

simplex (in one direction only, with no provision for the receiving device to send information back to the transmitting device) full duplex (both devices send and receive at the same time) half duplex (devices take turns transmitting and receiving) For UART to work the following settings need to be the same on both the transmitting and receiving side:

Voltage level Baud rate Parity bit Data bits size Stop bits size Flow control For the voltage level, two UART modules work well when they both have the same voltage level, e.g 3V-3V between the two UART modules. To use two UART modules at different voltage levels, a level shifting circuit needs to be added externally.

Data framing

A UART frame consists of five elements:

… excerpt ends here. Continue reading the full article.

Illustrations

Universal asynchronous receiver-transmitter: Block diagram for a UART
Block diagram for a UART
Universal asynchronous receiver-transmitter: Raspberry Pi 2 connected to a laptop through a UART to USB adapter. Three wires are used: one for ground and one for data in each direction. Such a setup can be used to access the command-line interface of the Raspberry Pi running Linux from the host computer .
Raspberry Pi 2 connected to a laptop through a UART to USB adapter. Three wires are used: one for ground and one for data in each direction. Such a setup can be used to access the command-line interface of the Raspberry Pi running Linux from the host computer .
Universal asynchronous receiver-transmitter: Example of a UART frame. In this diagram, one byte is sent, consisting of a start bit, followed by eight data bits (D1-8), and two stop bits, for an 11-bit UART frame. The number of data and formatting bits, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed must be pre-agreed by the communicating parties. The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each data frame, the receiver stops briefly to wait for the next start bit. It is this difference that keeps the transmitter and receiver synchronized. BCLK = Base Clock
Example of a UART frame. In this diagram, one byte is sent, consisting of a start bit, followed by eight data bits (D1-8), and two stop bits, for an 11-bit UART frame. The number of data and formatting bits, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed must be pre-agreed by the communicating parties. The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each data frame, the receiver stops briefly to wait for the next start bit. It is this difference that keeps the transmitter and receiver synchronized. BCLK = Base Clock

Worked examples

Example 1 — a first encounter with Universal asynchronous receiver-transmitter

Start with the simplest possible case. Write down what Universal asynchronous receiver-transmitter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Universal asynchronous receiver-transmitter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Universal asynchronous receiver-transmitter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Universal asynchronous receiver-transmitter

In research
Universal asynchronous receiver-transmitter appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Universal asynchronous receiver-transmitter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Universal asynchronous receiver-transmitter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data transmission, so understanding it makes those chapters shorter.
In everyday life
Look for Universal asynchronous receiver-transmitter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Universal asynchronous receiver-transmitter in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Universal asynchronous receiver-transmitter means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Universal asynchronous receiver-transmitter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Universal asynchronous receiver-transmitter in simple terms?

A universal asynchronous receiver-transmitter (UART ) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least to most significant, framed by start and stop bits so that precise timi…

Why does Universal asynchronous receiver-transmitter matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Universal asynchronous receiver-transmitter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Universal asynchronous receiver-transmitter.

Tags

  • Data transmission

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