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Some common acronyms are:
UART Universal Asynchronous Receiver/Transmitter
USART Universal Synchronous-Asynchronous Receiver/Transmitter
Synchronous Serial Transmission
Synchronous serial transmission requires that the sender and receiver share a clock with one another, or that the sender provide a strobe or other timing signal so that the receiver knows when to <quote>read</quote> the next bit of the data. In most forms of serial Synchronous communication, if there is no data available at a given instant to transmit, a fill character must be sent instead so that data is always being transmitted. Synchronous communication is usually more efficient because only data bits are transmitted between sender and receiver, and synchronous communication can be more costly if extra wiring and circuits are required to share a clock signal between the sender and receiver.
A form of Synchronous transmission is used with printers and fixed disk devices in that the data is sent on one set of wires while a clock or strobe is sent on a different wire. Printers and fixed disk devices are not normally serial devices because most fixed disk interface standards send an entire word of data for each clock or strobe signal by using a separate wire for each bit of the word. In the PC industry, these are known as Parallel devices.
The standard serial communications hardware in the PC does not support Synchronous operations. This mode is described here for comparison purposes only.
Asynchronous Serial Transmission
Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units.
When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter. These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word. (This requirement was set in the days of mechanical teleprinters and is easily met by modern electronic equipment.)
After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit (LSB) being sent first. Each bit in the transmission is transmitted for exactly the same amount of time as all of the other bits, and the receiver <quote>looks</quote> at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a <literal>1</literal> or a <literal>0</literal>. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a <literal>1</literal> or a <literal>0</literal> after one second has passed, then it will wait two seconds and then examine the value of the next bit, and so on.
The sender does not know when the receiver has <quote>looked</quote> at the value of the bit. The sender only knows when the clock says to begin transmitting the next bit of the word.
When the entire data word has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter.
When the receiver has received all of the bits in the data word, it may check for the Parity Bits (both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiver looks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.
Regardless of whether the data was received correctly or not, the UART automatically discards the Start, Parity and Stop bits. If the sender and receiver are configured identically, these bits are not passed to the host.
If another word is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bit for the previous word has been sent.
As asynchronous data is <quote>self synchronizing</quote>, if there is no data to transmit, the transmission line can be idle.
Other UART Functions
In addition to the basic job of converting data from parallel to serial for transmission and from serial to parallel on reception, a UART will usually provide additional circuits for signals that can be used to indicate the state of the transmission media, and to regulate the flow of data in the event that the remote device is not prepared to accept more data. For example, when the device connected to the UART is a modem, the modem may report the presence of a carrier on the phone line while the computer may be able to instruct the modem to reset itself or to not take calls by raising or lowering one more of these extra signals. The function of each of these additional signals is defined in the EIA RS232-C standard.
The RS232-C and V.24 Standards
In most computer systems, the UART is connected to circuitry that generates signals that comply with the EIA RS232-C specification. There is also a CCITT standard named V.24 that mirrors the specifications included in RS232-C.
RS232-C Bit Assignments (Marks and Spaces)
In RS232-C, a value of <literal>1</literal> is called a <literal>Mark</literal> and a value of <literal>0</literal> is called a <literal>Space</literal>. When a communication line is idle, the line is said to be <quote>Marking</quote>, or transmitting continuous <literal>1</literal> values.
The Start bit always has a value of <literal>0</literal> (a Space). The Stop Bit always has a value of <literal>1</literal> (a Mark). This means that there will always be a Mark (1) to Space (0) transition on the line at the start of every word, even when multiple word are transmitted back to back. This guarantees that sender and receiver can resynchronize their clocks regardless of the content of the data bits that are being transmitted.
The idle time between Stop and Start bits does not have to be an exact multiple (including zero) of the bit rate of the communication link, but most UARTs are designed this way for simplicity.
In RS232-C, the "Marking" signal (a <literal>1</literal>) is represented by a voltage between -2 VDC and -12 VDC, and a "Spacing" signal (a <literal>0</literal>) is represented by a voltage between 0 and +12 VDC. The transmitter is supposed to send +12 VDC or -12 VDC, and the receiver is supposed to allow for some voltage loss in long cables. Some transmitters in low power devices (like portable computers) sometimes use only +5 VDC and -5 VDC, but these values are still acceptable to a RS232-C receiver, provided that the cable lengths are short.
RS232-C Break Signal
RS232-C also specifies a signal called a <literal>Break</literal>, which is caused by sending continuous Spacing values (no Start or Stop bits). When there is no electricity present on the data circuit, the line is considered to be sending <literal>Break</literal>.
The <literal>Break</literal> signal must be of a duration longer than the time it takes to send a complete byte plus Start, Stop and Parity bits. Most UARTs can distinguish between a Framing Error and a Break, but if the UART cannot do this, the Framing Error detection can be used to identify Breaks.
In the days of teleprinters, when numerous printers around the country were wired in series (such as news services), any unit could cause a <literal>Break</literal> by temporarily opening the entire circuit so that no current flowed. This was used to allow a location with urgent news to interrupt some other location that was currently sending information.
In modern systems there are two types of Break signals. If the Break is longer than 1.6 seconds, it is considered a "Modem Break", and some modems can be programmed to terminate the conversation and go on-hook or enter the modems' command mode when the modem detects this signal. If the Break is smaller than 1.6 seconds, it signifies a Data Break and it is up to the remote computer to respond to this signal. Sometimes this form of Break is used as an Attention or Interrupt signal and sometimes is accepted as a substitute for the ASCII CONTROL-C character.


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Source string comment
(itstool) path: sect2/para
Source string location
String age
a year ago
Source string age
a year ago
Translation file
articles/serial-uart.pot, string 30