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The IBM PC compatibles used National Semiconductor 8250, not the Intel serial chip - because the National chips had built-in programmable baud rate generators. The sucessors were also made by National. This article needs a block diagram of a UART, too. -- Wtshymanski 05:58, 13 Apr 2005 (UTC)
I made a diagram of the encoding scheme. I don't know if that's what you meant, though. I'd like it if someone integrated it into the article for me - I fear I would not be able to do it properly. The diagram is here. RT Jones ( talk) 00:18, 5 November 2008 (UTC)
These standards are discussed in their own articles; the UART may interface to the control lines of an RS 232 interface but doesn't directly generate the voltage levels that go out on the wire. -- Wtshymanski 17:49, 17 May 2005 (UTC)
"INT 14" is hopelessly IBM-PC specific and not appropriate for a general article on UARTS. I've moved the following lines here but I don't think they belong in this article at all. -- Wtshymanski 18:57, 24 February 2006 (UTC)
In most UART chips, registers are grouped into 3 categories:
Also, in some hardware manuals, DLab refers to the last bit (bit 7, or the most significant bit) of the Line Control Register (LCR)
The software will need to initialise the serial port first. After successful initialisation, all operations on the serial port will be passed to and handled by the UART chipset. The basic steps are summarised below:
From the article:
Speeds for UARTs are in bits per second (bit/s or bps), although often incorrectly called the baud rate.
From the baud article:
In telecommunications and electronics, baud [...] is a measure of the symbol rate [...] Early modems operated only at one bit per symbol, and so baud rate and bit rate for those devices were equivalent.
Doesn't the second quote contradict the first quote? UARTS are 1 bit per symbol and their baud should therefor be equal to their bps. The first quote should therefor be changed. (I'm just passing by.) 129.241.129.67 20:40, 15 June 2007 (UTC)
Someone gave a pronunciation of OO-wuh AT. I thought this was odd, so I edited it out. Please replace if I'm wrong - there's an English IPA key at {{ IPA-en}}. kwami 06:16, 16 October 2007 (UTC)
An asynchronous transmission sends nothing over the interconnection when the transmitting device has nothing to send; but a synchronous interface must send "pad" characters to maintain synchronism between the receiver and transmitter. The two modes need to be swapped, don't they? Sevcsik 20:12, 27 October 2007 (UTC)
I've tweaked the article to now say "An asynchronous transmission sends no characters over the interconnection when the transmitting device has nothing to send".
Take the case where a USART ran out of things to send, but then 3 normal bit-times later, it was given a "W" to send. All the asynchronous transmitters I've studied send nothing when it had nothing to send -- in some cases "disconnecting" hi-Z from the transmission media, literally sending nothing; in other cases continuously transmitting the "logical 1 stop bit" until it has something else to send. In this case, the UART would send 3 stop bits (rather than the normal 1 stop bit between characters), then when the "W" character suddenly arrived, it would send the 1 start bit, then start sending the data bits that make up the "W". Those "extra" 2 stop bits do help maintain synchronism, but they are not considered an entire "character".
I've seen a few synchronous transmission protocols that would simply stop not transmit those 3 bit clock pulses when it had nothing to send, which seems to contradict the article.
However, all the USARTs I've studied (which is the focus of this article, right?) conform to the article's description. They all continuously transmit bit clock pulses at a constant rate whether or not it has anything to send. If such a synchronous transmitter immediately started sending the "W" 3 bit clocks after the last byte sent, then the receiver would lose the byte alignment. So, in practice, -- in synchronous mode -- the USART immediately must start sending the "pad" character after sending the last real data bit; then it delays sending the "W" until all 8 bits of that pad character have been transmitted, and so maintains byte alignment.
Some higher-level protocols -- especially when communication goes over noisy radio links, rather than hard-wired links -- do send extra "pad" characters to maintain synchronism between the receiver and transmitter, whether or not the underlying hardware uses synchronous or asynchronous transmission. These pad characters -- even when the underlying hardware uses asynchronous transmission, and so does not need them to maintain byte alignment -- helps the higher-level protocol maintain packet alignment (which byte is the start of the packet?), and can also help underlying hardware maintain bit alignment (is this exactly 7 zeros in a row, or 8 zeros?).
Does that answer your question? -- 68.0.124.33 ( talk) 17:07, 16 July 2008 (UTC)
My experience with USARTs when they are operating synchronously is that "pad" data is unnecessary. Also, when operating synchronously, they will still include the framing data--the start,stop and (optional) parity bits.
The "pad" characters are unnecessary since, just as when operating asynchronously, the transmission line will be held in the idle state. And, just as when operating asynchronously, a start bit is used to indicate a start send condition.
A USART operating synchronously operates pretty much like a traditional UART with the exception that the two ends of the transmission share the same transmission clock. One end acts as a master and generates a clock signal which it shares with the slave. The slave then uses this external clock signal for synchronization when sending and receiving. The two ends still need to agree on which frame format will be used but no longer need to agree to a baud rate since this is determined by the clock signal generated by the master.
When operating synchronously, the two ends need to agree about which edge of the clock will be used for data sampling and which edge of the clock will be used for changing the data.
The clock signal that the master generates will continue to run even when it (the master) isn't sending data. This is neccessary so that the slave can transmit data to the master. I imagine some implementations may stop the clock if the master doesn't receive data from the slave but I don't know that for certain. -- 99.179.69.42 ( talk) 10:53, 24 February 2010 (UTC)
So is UART similar to RAMDAC (in Graphics card and TV Tuner cards), except they translate Digital-to-Analog vs UART that translate parallel-to-serial? -- Ramu50 ( talk) 17:36, 18 July 2008 (UTC)
roflmao....milk Do they come in Human Breast Milk yogurt form......(slap in the face) joking, jokin -- Ramu50 ( talk) 00:01, 21 July 2008 (UTC)
Except for many if not most systems, the stop bits don't have to be the same, because there is nothing timing the stop level. It all gets re-synchronised at the next start bit. That's the beauty of asynchronous communication.
Is there an example of a system that requires the stop bits to be set the same? Change it back if I'm wrong.
The term "TAE" is mentioned but this term is not defined in this article or in other articles in Wikipedia (in a relevant meaning, anyway). I would love to rectify this but I don't know what's TAE and neither Google "define" nor the free acronyms dictionary had the answer :-( 22:07, 2 September 2008 (UTC)
Bits per second is the payload, not the symbol rate. 9600 baud has 9600 symbols per second. For example for a UART set to 8N1, there is it requires 1 baud of start, 8 baud of data, 1 baud of stop, for a total of 10 baud time. The bit per second is 9600 * (8/10) = 7680 bits per second. If you look at any UART data sheet, they use the term baud everywhere, such as the "baud rate" generator. • Sbmeirow • Talk • 09:26, 15 November 2016 (UTC) https://en.wikibooks.org/wiki/Serial_Programming/Complete_Wikibook#Data_Transmission_Rates
A) In WP:COMPUNITS, it says "the definition most relevant to the article should be chosen as primary for that article". A vast majority of recent UART documents clearly show the term "baud" is very relevant term.
B) I understand that multiple bits can be packed into one "baud" or "time unit" in dial-up modems and a mountain of wireless communication protocols, but this article is NOT about them, instead it is specifically about the UART. The only thing that matters in this article is the UART itself and the serial data that is arriving/leaving in/out of the actual UART.
C) My use of "any datasheet", means RECENT datasheets for RECENT parts. Cherry-picking OBSOLETE ANCIENT datasheets are important from a historical sense, but recent technical nomenclature should be used to describe things in 2016. The following 12 links to newer documents from 12 different companies clearly show how prevalent the term BAUD is used with UARTs. Also, I'm confident that I could post a mountain of similar datasheets since almost every microcontroller has a UART inside of it. This evidence alone is a very strong reason of why the BAUD nomenclature should be used as much as possible in this Wikipedia UART article in 2016.
D) Signed • Sbmeirow • Talk • 01:50, 16 November 2016 (UTC)
What is the purpose of the UART models section? I am sure there are thousands of different chips which support UART and which are not shown there. I think that section should be deleted entirely. -- IngenieroLoco ( talk) 11:15, 29 November 2016 (UTC)
I flagged "UARTs are now commonly included in microcontrollers", the article is 16 years old! It would be nice to cite a reference. The History section stops short of covering this advance. -- Skierpage ( talk) 23:13, 18 April 2018 (UTC)
I don't know exactly when, but intel 8051 included UART-like serial interface at 1980. 2400:4051:4CE0:C400:922B:34FF:FED6:A16D ( talk) 09:17, 6 August 2021 (UTC)
Note that the 8251 is actually a USART, which can run in either asynchronous or synchronous mode. I believe some of the others listed along with it are also USART or similar. Gah4 ( talk) 01:46, 13 April 2023 (UTC)
I work with electrical engineers (I'm not one myself) and they often remind me that "UART" is a data link protocol, not a device. This is also emphasized in other sources. As one example, the UART protocol can be used with an LVDS transmitter / receiver, and it can also be used over an acoustic modem, as anyone who has configured a modem for start, stop, parity and data bits might know. I don't think an acoustic modem operating over the PSTN would qualify as a "UART device", at least not as used in this article. Should this article emphasize that UART is a protocol, rather than take it as granted that it is a device? -- Alan.A.Mick 15:25, 22 May 2024 (UTC)
This article is rated Start-class on Wikipedia's
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The IBM PC compatibles used National Semiconductor 8250, not the Intel serial chip - because the National chips had built-in programmable baud rate generators. The sucessors were also made by National. This article needs a block diagram of a UART, too. -- Wtshymanski 05:58, 13 Apr 2005 (UTC)
I made a diagram of the encoding scheme. I don't know if that's what you meant, though. I'd like it if someone integrated it into the article for me - I fear I would not be able to do it properly. The diagram is here. RT Jones ( talk) 00:18, 5 November 2008 (UTC)
These standards are discussed in their own articles; the UART may interface to the control lines of an RS 232 interface but doesn't directly generate the voltage levels that go out on the wire. -- Wtshymanski 17:49, 17 May 2005 (UTC)
"INT 14" is hopelessly IBM-PC specific and not appropriate for a general article on UARTS. I've moved the following lines here but I don't think they belong in this article at all. -- Wtshymanski 18:57, 24 February 2006 (UTC)
In most UART chips, registers are grouped into 3 categories:
Also, in some hardware manuals, DLab refers to the last bit (bit 7, or the most significant bit) of the Line Control Register (LCR)
The software will need to initialise the serial port first. After successful initialisation, all operations on the serial port will be passed to and handled by the UART chipset. The basic steps are summarised below:
From the article:
Speeds for UARTs are in bits per second (bit/s or bps), although often incorrectly called the baud rate.
From the baud article:
In telecommunications and electronics, baud [...] is a measure of the symbol rate [...] Early modems operated only at one bit per symbol, and so baud rate and bit rate for those devices were equivalent.
Doesn't the second quote contradict the first quote? UARTS are 1 bit per symbol and their baud should therefor be equal to their bps. The first quote should therefor be changed. (I'm just passing by.) 129.241.129.67 20:40, 15 June 2007 (UTC)
Someone gave a pronunciation of OO-wuh AT. I thought this was odd, so I edited it out. Please replace if I'm wrong - there's an English IPA key at {{ IPA-en}}. kwami 06:16, 16 October 2007 (UTC)
An asynchronous transmission sends nothing over the interconnection when the transmitting device has nothing to send; but a synchronous interface must send "pad" characters to maintain synchronism between the receiver and transmitter. The two modes need to be swapped, don't they? Sevcsik 20:12, 27 October 2007 (UTC)
I've tweaked the article to now say "An asynchronous transmission sends no characters over the interconnection when the transmitting device has nothing to send".
Take the case where a USART ran out of things to send, but then 3 normal bit-times later, it was given a "W" to send. All the asynchronous transmitters I've studied send nothing when it had nothing to send -- in some cases "disconnecting" hi-Z from the transmission media, literally sending nothing; in other cases continuously transmitting the "logical 1 stop bit" until it has something else to send. In this case, the UART would send 3 stop bits (rather than the normal 1 stop bit between characters), then when the "W" character suddenly arrived, it would send the 1 start bit, then start sending the data bits that make up the "W". Those "extra" 2 stop bits do help maintain synchronism, but they are not considered an entire "character".
I've seen a few synchronous transmission protocols that would simply stop not transmit those 3 bit clock pulses when it had nothing to send, which seems to contradict the article.
However, all the USARTs I've studied (which is the focus of this article, right?) conform to the article's description. They all continuously transmit bit clock pulses at a constant rate whether or not it has anything to send. If such a synchronous transmitter immediately started sending the "W" 3 bit clocks after the last byte sent, then the receiver would lose the byte alignment. So, in practice, -- in synchronous mode -- the USART immediately must start sending the "pad" character after sending the last real data bit; then it delays sending the "W" until all 8 bits of that pad character have been transmitted, and so maintains byte alignment.
Some higher-level protocols -- especially when communication goes over noisy radio links, rather than hard-wired links -- do send extra "pad" characters to maintain synchronism between the receiver and transmitter, whether or not the underlying hardware uses synchronous or asynchronous transmission. These pad characters -- even when the underlying hardware uses asynchronous transmission, and so does not need them to maintain byte alignment -- helps the higher-level protocol maintain packet alignment (which byte is the start of the packet?), and can also help underlying hardware maintain bit alignment (is this exactly 7 zeros in a row, or 8 zeros?).
Does that answer your question? -- 68.0.124.33 ( talk) 17:07, 16 July 2008 (UTC)
My experience with USARTs when they are operating synchronously is that "pad" data is unnecessary. Also, when operating synchronously, they will still include the framing data--the start,stop and (optional) parity bits.
The "pad" characters are unnecessary since, just as when operating asynchronously, the transmission line will be held in the idle state. And, just as when operating asynchronously, a start bit is used to indicate a start send condition.
A USART operating synchronously operates pretty much like a traditional UART with the exception that the two ends of the transmission share the same transmission clock. One end acts as a master and generates a clock signal which it shares with the slave. The slave then uses this external clock signal for synchronization when sending and receiving. The two ends still need to agree on which frame format will be used but no longer need to agree to a baud rate since this is determined by the clock signal generated by the master.
When operating synchronously, the two ends need to agree about which edge of the clock will be used for data sampling and which edge of the clock will be used for changing the data.
The clock signal that the master generates will continue to run even when it (the master) isn't sending data. This is neccessary so that the slave can transmit data to the master. I imagine some implementations may stop the clock if the master doesn't receive data from the slave but I don't know that for certain. -- 99.179.69.42 ( talk) 10:53, 24 February 2010 (UTC)
So is UART similar to RAMDAC (in Graphics card and TV Tuner cards), except they translate Digital-to-Analog vs UART that translate parallel-to-serial? -- Ramu50 ( talk) 17:36, 18 July 2008 (UTC)
roflmao....milk Do they come in Human Breast Milk yogurt form......(slap in the face) joking, jokin -- Ramu50 ( talk) 00:01, 21 July 2008 (UTC)
Except for many if not most systems, the stop bits don't have to be the same, because there is nothing timing the stop level. It all gets re-synchronised at the next start bit. That's the beauty of asynchronous communication.
Is there an example of a system that requires the stop bits to be set the same? Change it back if I'm wrong.
The term "TAE" is mentioned but this term is not defined in this article or in other articles in Wikipedia (in a relevant meaning, anyway). I would love to rectify this but I don't know what's TAE and neither Google "define" nor the free acronyms dictionary had the answer :-( 22:07, 2 September 2008 (UTC)
Bits per second is the payload, not the symbol rate. 9600 baud has 9600 symbols per second. For example for a UART set to 8N1, there is it requires 1 baud of start, 8 baud of data, 1 baud of stop, for a total of 10 baud time. The bit per second is 9600 * (8/10) = 7680 bits per second. If you look at any UART data sheet, they use the term baud everywhere, such as the "baud rate" generator. • Sbmeirow • Talk • 09:26, 15 November 2016 (UTC) https://en.wikibooks.org/wiki/Serial_Programming/Complete_Wikibook#Data_Transmission_Rates
A) In WP:COMPUNITS, it says "the definition most relevant to the article should be chosen as primary for that article". A vast majority of recent UART documents clearly show the term "baud" is very relevant term.
B) I understand that multiple bits can be packed into one "baud" or "time unit" in dial-up modems and a mountain of wireless communication protocols, but this article is NOT about them, instead it is specifically about the UART. The only thing that matters in this article is the UART itself and the serial data that is arriving/leaving in/out of the actual UART.
C) My use of "any datasheet", means RECENT datasheets for RECENT parts. Cherry-picking OBSOLETE ANCIENT datasheets are important from a historical sense, but recent technical nomenclature should be used to describe things in 2016. The following 12 links to newer documents from 12 different companies clearly show how prevalent the term BAUD is used with UARTs. Also, I'm confident that I could post a mountain of similar datasheets since almost every microcontroller has a UART inside of it. This evidence alone is a very strong reason of why the BAUD nomenclature should be used as much as possible in this Wikipedia UART article in 2016.
D) Signed • Sbmeirow • Talk • 01:50, 16 November 2016 (UTC)
What is the purpose of the UART models section? I am sure there are thousands of different chips which support UART and which are not shown there. I think that section should be deleted entirely. -- IngenieroLoco ( talk) 11:15, 29 November 2016 (UTC)
I flagged "UARTs are now commonly included in microcontrollers", the article is 16 years old! It would be nice to cite a reference. The History section stops short of covering this advance. -- Skierpage ( talk) 23:13, 18 April 2018 (UTC)
I don't know exactly when, but intel 8051 included UART-like serial interface at 1980. 2400:4051:4CE0:C400:922B:34FF:FED6:A16D ( talk) 09:17, 6 August 2021 (UTC)
Note that the 8251 is actually a USART, which can run in either asynchronous or synchronous mode. I believe some of the others listed along with it are also USART or similar. Gah4 ( talk) 01:46, 13 April 2023 (UTC)
I work with electrical engineers (I'm not one myself) and they often remind me that "UART" is a data link protocol, not a device. This is also emphasized in other sources. As one example, the UART protocol can be used with an LVDS transmitter / receiver, and it can also be used over an acoustic modem, as anyone who has configured a modem for start, stop, parity and data bits might know. I don't think an acoustic modem operating over the PSTN would qualify as a "UART device", at least not as used in this article. Should this article emphasize that UART is a protocol, rather than take it as granted that it is a device? -- Alan.A.Mick 15:25, 22 May 2024 (UTC)