The device changes incoming parallel information to serial data which can be sent on a communication line. A second UART can be used to receive the information. The UART performs all the tasks, timing, parity checking, etc. The only extra devices attached are line driver chips capable of transforming the TTL level signals to line voltages and vice versa. To use the UART in different environments, registers are accessible to set or review the communication parameters. Setable parameters are for example the communication speed, the type of parity check, and the way incoming information is signaled to the running software.
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The 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. Serial transmission is commonly used with modems and for non-networked communication between computers, terminals and other devices.
There are two primary forms of serial transmission: Synchronous and Asynchronous. Depending on the modes that are supported by the hardware, the name of the communication sub-system will usually include a A if it supports Asynchronous communications, and a S if it supports Synchronous communications. Both forms are described below.
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 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. This requirement was set in the days of mechanical teleprinters and is easily met by modern electronic equipment.
For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 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 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.
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. 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 Start bit always has a value of 0 a Space. The Stop Bit always has a value of 1 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.
RSC also specifies a signal called a Break , 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 Break. The Break signal must be of a duration longer than the time it takes to send a complete byte plus Start, Stop and Parity bits.
In the days of teleprinters, when numerous printers around the country were wired in series such as news services , any unit could cause a Break 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. If the Break is smaller than 1. Breaks cannot be generated from paper tape or from any other byte value, since bytes are always sent with Start and Stop bit. The UART is usually capable of generating the continuous Spacing signal in response to a special command from the host processor. Across the phone line at the other end of a conversation, the receiving modem is also a DCE device and the computer that is connected to that modem is a DTE device.
The NULL modem electrically re-arranges the cabling so that the transmitter output is connected to the receiver input on the other device, and vice versa. Similar translations are performed on all of the control signals so that each device will see what it thinks are DCE or DTE signals from the other device.
Baud is a measurement of transmission speed in asynchronous communication. Because of advances in modem communication technology, this term is frequently misused when describing the data rates in newer devices. Traditionally, a Baud Rate represents the number of bits that are actually being sent over the media, not the amount of data that is actually moved from one DTE device to the other. This means that seven-bit words of data actually take 10 bits to be completely transmitted.
Therefore, a modem capable of moving bits per second from one place to another can normally only move 30 7-bit words if Parity is used and one Start and Stop bit are present. If 8-bit data words are used and Parity bits are also used, the data rate falls to The formula for converting bytes per second into a baud rate and vice versa was simple until error-correcting modems came along.
These modems receive the serial stream of bits from the UART in the host computer even when internal modems are used the data is still frequently serialized and converts the bits back into bytes. These bytes are then combined into packets and sent over the phone line using a Synchronous transmission method. When these bytes are received by the remote modem, the remote modem adds Start, Stop and Parity bits to the words, converts them to a serial format and then sends them to the receiving UART in the remote computer, who then strips the Start, Stop and Parity bits.
The reason all these extra conversions are done is so that the two modems can perform error correction, which means that the receiving modem is able to ask the sending modem to resend a block of data that was not received with the correct checksum.
This checking is handled by the modems, and the DTE devices are usually unaware that the process is occurring. By striping the Start, Stop and Parity bits, the additional bits of data that the two modems must share between themselves to perform error-correction are mostly concealed from the effective transmission rate seen by the sending and receiving DTE equipment.
For example, if a modem sends ten 7-bit words to another modem without including the Start, Stop and Parity bits, the sending modem will be able to add 30 bits of its own information that the receiving modem can use to do error-correction without impacting the transmission speed of the real data.
The use of the term Baud is further confused by modems that perform compression. A single 8-bit word passed over the telephone line might represent a dozen words that were transmitted to the sending modem. The receiving modem will expand the data back to its original content and pass that data to the receiving DTE.
Modern modems also include buffers that allow the rate that bits move across the phone line DCE to DCE to be a different speed than the speed that the bits move between the DTE and DCE on both ends of the conversation. Because the number of bits needed to describe a byte varied during the trip between the two machines plus the differing bits-per-seconds speeds that are used present on the DTE-DCE and DCE-DCE links, the usage of the term Baud to describe the overall communication speed causes problems and can misrepresent the true transmission speed.
So Bits Per Second bps is the correct term to use to describe the transmission rate seen at the DCE to DCE interface and Baud or Bits Per Second are acceptable terms to use when a connection is made between two systems with a wired connection, or if a modem is in use that is not performing error-correction or compression.
Modern high speed modems , , 14,, and 19,bps in reality still operate at or below baud, or more accurately, Symbols per second. High speed modem are able to encode more bits of data into each Symbol using a technique called Constellation Stuffing, which is why the effective bits per second rate of the modem is higher, but the modem continues to operate within the limited audio bandwidth that the telephone system provides.
Modems operating at 28, and higher speeds have variable Symbol rates, but the technique is the same. Each major version is described below. This original UART has several race conditions and other flaws. It contains the same problems as the original INS Same as NS with a byte send and receive buffer but the buffer design was flawed and could not be reliably be used. Same as NS with the buffer flaws corrected. The A and its successors have become the most popular UART design in the PC industry, mainly due to its ability to reliably handle higher data rates on operating systems with sluggish interrupt response times.
Same as NSA with subtle flaws corrected. This is revision D of the family and is the latest design available from National Semiconductor. National reorganized their part numbering system a few years ago, and the NSAFN no longer exists by that name.
If you have a NSAFN, look at the date code on the part, which is a four digit number that usually starts with a nine. The first two digits of the number are the year, and the last two digits are the week in that year when the part was packaged. The new numbers are like PCDV, with minor differences in the suffix letters depending on the package material and its shape.
A description of the numbering system can be found below. As the supply of NSAFN chips continues to shrink, the price will probably continue to increase until more people discover and accept that the PCDN really has the same function as the old part number. The older NS nnnnnrqp part numbers are now of the format PC nnnnnrgp. The r is the revision field. The current revision of the from National Semiconductor is D.
The g is the product grade field. This is an optional field. Over the years, the , A, and have been licensed or copied by other chip vendors. Other vendors reverse-engineered the part or produced emulations that had similar behavior.
Because of the size of the buffer, these emulations can be as reliable as a A in their ability to handle high speed data.
However, most operating systems will still report that the UART is only a A or , and may not make effective use of the extra buffering present in the emulated UART unless special drivers are used. Some modem makers are driven by market forces to abandon a design that has hundreds of bytes of buffer and instead use a A UART so that the product will compare favorably in market comparisons even though the effective performance may be lowered by this action.
There are differences, and in some cases, outright flaws in most of these A clones. When the NS was developed, the National Semiconductor obtained several patents on the design and they also limited licensing, making it harder for other vendors to provide a chip with similar features.
Because of the patents, reverse-engineered designs and emulations had to avoid infringing the claims covered by the patents. Subsequently, these copies almost never perform exactly the same as the NSA or PCD, which are the parts most computer and modem makers want to buy but are sometimes unwilling to pay the price required to get the genuine part.
Serial UART information
The corrected -A version was released in by National Semiconductor. The part was originally made by National Semiconductor. Similarly numbered devices, with varying levels of compatibility with the original National Semiconductor part, are made by other manufacturers. Exchange of the having only a one-byte received data buffer with a , and occasionally patching or setting system software to be aware of the FIFO feature of the new chip, improved the reliability and stability of high-speed connections. The current version since by Texas Instruments which bought National Semiconductor is called the D. This generated high rates of interrupts as transfer speeds increased. More critically, with only a 1-byte buffer there is a genuine risk that a received byte will be overwritten if interrupt service delays occur.