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Introduction to Serial Communications
Learn the basic principles of serial communication. This page also contains basic connector pinouts, recommended cable lengths and other useful information.
Many PCs and compatible computers are equipped with two serial ports and one parallel port. Although these two types of ports are used for communicating with external devices, they work in different ways.
A parallel port sends and receives data eight bits at a time over 8 separate wires. This allows data to be tran however, the cable required is more bulky because of the number of individual wires it must contain. Parallel ports are typically used to connect a PC to a printer and are rarely used for much else. A serial port sends and receives data one bit at a time over one wire. While it takes eight times as long to transfer each byte of data this way, only a few wires are required. In fact, two-way (full duplex) communications is possible with only three separate wires - one to send, one to receive, and a common signal ground wire.
Bi-Directional Communications
The serial port on your PC is a full-duplex device meaning that it can send and receive data at the same time. In order to be able to do this, it uses separate lines for transmitting and receiving data. Some types of serial devices support only one-way communications and therefore use only two wires in the cable - the transmit line and the signal ground.
Communicating by Bits
Once the start bit has been sent, the transmitter sends the actual data bits. There may either be 5, 6, 7, or 8 data bits, depending on the number you have selected. Both receiver and the transmitter must agree on the number of data bits, as well as the baud rate. Almost all devices transmit data using either 7 or 8 databits.
Notice that when only 7 data bits are employed, you cannot send ASCII values greater than 127. Likewise, using 5 bits limits the highest possible value to 31. After the data has been transmitted, a stop bit is sent. A stop bit has a value of 1 - or a mark state - and it can be detected correctly even if the previous data bit also had a value of 1. This is accomplished by the stop bit's duration. Stop bits can be 1, 1.5, or 2 bit periods in length.&
The Parity Bit
Besides the synchronization provided by the use of start and stop bits, an additional bit called a parity bit may optionally be transmitted along with the data. A parity bit affords a small amount of error checking, to help detect data corruption that might occur during transmission. You can choose either even parity, odd parity, mark parity, space parity or none at all. When even or odd parity is being used, the number of marks (logical 1 bits) in each data byte are counted, and a single bit is transmitted following the data bits to indicate whether the number of 1 bits just sent is even or odd.
For example, when even parity is chosen, the parity bit is transmitted with a value of 0 if the number of preceding marks is an even number. For the binary value of
the parity bit would be 0. If even parity were in effect and the binary number
were sent, then the parity bit would be 1. Odd parity is just the opposite, and the parity bit is 0 when the number of mark bits in the preceding word is an odd number. Parity error checking is very rudimentary. While it will tell you if there is a single bit error in the character, it doesn't show which bit was received in error. Also, if an even number of bits are in error then the parity bit would not reflect any error at all.
Mark parity means that the parity bit is always set to the mark signal condition and likewise space parity always sends the parity bit in the space signal condition. Since these two parity options serve no useful purpose whatsoever, they are almost never used.&
RS-232 stands for Recommend Standard number 232 and C is the latest revision of the standard. The serial ports on most computers use a subset of the RS-232C standard. The full RS-232C standard specifies a 25-pin "D" connector of which 22 pins are used. Most of these pins are not needed for normal PC communications, and indeed, most new PCs are equipped with male D type connectors having only 9 pins.
DCE and DTE Devices
Two terms you should be familiar with are DTE and DCE. DTE stands for Data Terminal Equipment, and DCE stands for Data Communications Equipment. These terms are used to indicate the pin-out for the connectors on a device and the direction of the signals on the pins. Your computer is a DTE device, while most other devices are usually DCE devices.
If you have trouble keeping the two straight then replace the term "DTE device" with "your PC" and the term "DCE device" with "remote device" in the following discussion.&
The RS-232 standard states that DTE devices use a 25-pin male connector, and DCE devices use a 25-pin female connector. You can therefore connect a DTE device to a DCE using a straight pin-for-pin connection. However, to connect two like devices, you must instead use a null modem cable. Null modem cables cross the transmit and receive lines in the cable, and are discussed later in this chapter. The listing below shows the connections and signal directions for both 25 and 9-pin connectors.
Male RS232 DB25
Pin Number
Direction of signal:
Protective Ground
Transmitted Data (TD) Outgoing Data (from a DTE to a DCE)
Received Data (RD) Incoming Data (from a DCE to a DTE)
Request To Send (RTS) Outgoing flow control signal controlled by DTE
Clear To Send (CTS) Incoming flow control signal controlled by DCE
Data Set Ready (DSR) Incoming handshaking signal controlled by DCE
Signal Ground Common reference voltage
Carrier Detect (CD) Incoming signal from a modem
Data Terminal Ready (DTR) Outgoing handshaking signal controlled by DTE
Ring Indicator (RI) Incoming signal from a modem
Male RS232 DB9
Pin Number
Direction of signal:
Carrier Detect (CD) (from DCE) Incoming signal from a modem
Received Data (RD) Incoming Data from a DCE
Transmitted Data (TD) Outgoing Data to a DCE
Data Terminal Ready (DTR) Outgoing handshaking signal
Signal Ground Common reference voltage
Data Set Ready (DSR) Incoming handshaking signal
Request To Send (RTS) Outgoing flow control signal
Clear To Send (CTS) Incoming flow control signal
Ring Indicator (RI) (from DCE) Incoming signal from a modem
The TD (transmit data) wire is the one through which data from a DTE device is transmitted to a DCE device. This name can be deceiving, because this wire is used by a DCE device to receive its data. The TD line is kept in a mark condition by the DTE device when it is idle. The RD (receive data) wire is the one on which data is received by a DTE device, and the DCE device keeps this line in a mark condition when idle.
RTS&stands for&Request To Send. This line and the CTS line are used when "hardware flow control" is enabled in both the DTE and DCE devices. The DTE device puts this line in a mark condition to tell the remote device that it is ready and able to receive data. If the DTE device is not able to receive data (typically because its receive buffer is almost full), it will put this line in the space condition as a signal to the DCE to stop sending data. When the DTE device is ready to receive more data (i.e. after data has been removed from its receive buffer), it will place this line back in the mark condition. The complement of the RTS wire is CTS, which stands for Clear To Send. The DCE device puts this line in a mark condition to tell the DTE device that it is ready to receive the data. Likewise, if the DCE device is unable to receive data, it will place this line in the space condition. Together, these two lines make up what is called RTS/CTS or "hardware" flow control. WinWedge supports this type of flow control, as well as Xon/XOff or "software" flow control. Software flow control uses special control characters transmitted from one device to another to tell the other device to stop or start sending data. With software flow control the RTS and CTS lines are not normally used.&
DTR&stands for&Data Terminal Ready. Its intended function is very similar to the RTS line. DSR (Data Set Ready) is the companion to DTR in the same way that CTS is to RTS. Some serial devices use DTR and DSR as signals to simply confirm that a device is connected and is turned on. WinWedge sets DTR to the mark state when the serial port is opened and leaves it in that state until the port is closed. The DTR and DSR lines were originally designed to provide an alternate method of hardware handshaking. It would be pointless to use both RTS/CTS and DTR/DSR for flow control signals at the same time. Because of this, DTR and DSR are rarely used for flow control.
CD&stands for&Carrier Detect. Carrier Detect is used by a modem to signal that it has a made a connection with another modem, or has detected a carrier tone.
The last remaining line is&RI&or&Ring Indicator. A modem toggles the state of this line when an incoming call rings your phone.&
The Carrier Detect (CD) and the Ring Indicator (RI) lines are only available in connections to a modem. Because most modems transmit status information to a PC when either a carrier signal is detected (i.e. when a connection is made to another modem) or when the line is ringing, these two lines are rarely used.&
9 to 25 Pin Adapters
The following table shows the connections inside a standard 9 pin to 25 pin adapter.
9 Pin Connector
25 Pin Connector
Pin 20 DTR
Baud vs. Bits per Second
The baud unit is named after Jean Maurice Emile Baudot, who was an officer in the French Telegraph Service. He is credited with devising the first uniform-length 5-bit code for characters of the alphabet in the late 19th century. What baud really refers to is modulation rate or the number of times per second that a line changes state. This is not always the same as bits per second (BPS). If you connect two serial devices together using direct cables then baud and BPS are in fact the same. Thus, if you are running at 19200 BPS, then the line is also changing states 19200 times per second. But when considering modems, this isn't the case.
Because modems transfer signals over a telephone line, the baud rate is actually limited to a maximum of 2400 baud. This is a physical restriction of the lines provided by the phone company. The increased data throughput achieved with 9600 or higher baud modems is accomplished by using sophisticated phase modulation, and data compression techniques.&
Cables, Null Modems, and Gender Changers
In a perfect world, all serial ports on every computer would be DTE devices with 25-pin male "D" connectors. All other devices to would be DCE devices with 25-pin female connectors. This would allow you to use a cable in which each pin on one end of the cable is connected to the same pin on the other end. Unfortunately, we don't live in a perfect world. Serial ports use both 9 and 25 pins, many devices can be configured as either DTE or DCE, and - as in the case of many data collection devices - may use completely non standard or proprietary pin-outs. Because of this lack of standardization, special cables called null modem cables, gender changers and custom made cables are often required.&
Cables Lengths
The RS-232C standard imposes a cable length limit of 50 feet. You can usually ignore this "standard", since a cable can be as long as 10000 feet at baud rates up to 19200 if you use a high quality, well shielded cable. The external environment has a large effect on lengths for unshielded cables. In electrically noisy environments, even very short cables can pick up stray signals. The following chart offers some reasonable guidelines for 24 gauge wire under typical conditions. You can greatly extend the cable length by using additional devices like optical isolators and signal boosters. Optical isolators use LEDs and Photo Diodes to isolate each line in a serial cable including the signal ground. Any electrical noise affects all lines in the optically isolated cable equally - including the signal ground line. This causes the voltages on the signal lines relative to the signal ground line to reflect the true voltage of the signal and thus canceling out the effect of any noise signals.
Shielded Cable Length
Unshielded Cable Length
Gender Changers
A problem you may encounter is having two connectors of the same gender that must be connected.& You can purchase gender changers at any computer or office supply store for under $5.
Note:&The parallel port on a PC uses a 25 pin female connector which sometimes causes confusion because it looks just like a serial port except that it has the wrong gender. Both 9 and 25 pin serial ports on a PC will always have a male connector.&
Null Modem Cables and Null Modem Adaptors
If you connect two DTE devices (or two DCE devices) using a straight RS232 cable, then the transmit line on each device will be connected to the transmit line on the other device and the receive lines will likewise be connected to each other. A Null Modem cable or Null Modem adapter simply crosses the receive and transmit lines so that transmit on one end is connected to receive on the other end and vice versa. In addition to transmit and receive, DTR & DSR, as well as RTS & CTS are also crossed in a Null modem connection.
Null modem adapter are available at most computer and office supply stores for under $5.
Synchronous and Asynchronous Communications
There are two basic types of serial communications, synchronous and asynchronous. With Synchronous communications, the two devices initially synchronize themselves to each other, and then continually send characters to stay in sync. Even when data is not really being sent, a constant flow of bits allows each device to know where the other is at any given time. That is, each character that is sent is either actual data or an idle character. Synchronous communications allows faster data transfer rates than asynchronous methods, because additional bits to mark the beginning and end of each data byte are not required. The serial ports on IBM-style PCs are asynchronous devices and therefore only support asynchronous serial communications.
Asynchronous means "no synchronization", and thus does not require sending and receiving idle characters. However, the beginning and end of each byte of data must be identified by start and stop bits. The start bit indicate when the data byte is about to begin and the stop bit signals when it ends. The requirement to send these additional two bits cause asynchronous communications to be slightly slower than synchronous however it has the advantage that the processor does not have to deal with the additional idle characters.
An asynchronous line that is idle is identified with a value of 1, (also called a mark state). By using this value to indicate that no data is currently being sent, the devices are able to distinguish between an idle state and a disconnected line. When a character is about to be transmitted, a start bit is sent. A start bit has a value of 0, (also called a space state). Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a data character is about to come down the line.
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信用卡产品mkaay - Hey! I create this blogg to write in english. Maybe I don't spell so well but I don't give a shit about it! Just for fun and try some new stuff. I hope you like what you read.
Over and out bitches!
Carolyn Thomas
Her boyfriend shot her face at point-blank range, but Carolyn Thomas survived. Follow how doctors restored her obliterated face.Carolyn was born in Midland, Texas. Her granmother and mother have raised her. She grow up in a strong, faith enviroment with strict rules. She loved spending time with friends and her mom. After graduation from Midland High School she moved to Waco(Texas). In 1995 she met the man who would cange her life forever. Eight years long relationship with this man. He kept her inside from the outside world. He had the control and he manipulate her. She was helpless and powerless. Isolation from the world. On December 5, 2003 the impossible moment happend. Her boyfriend shot her with a point-black range at her face. She wasn't expected to live. He killed her mom. She think that God let her survived for her on strength and for her mother. She has done 11 surgeries at Methodist Hospital in Houston(Texas) over just two years.Now she wants to help other women in the same position. Carolyn's miraculous, powerful, and inspirational story has touched so many that various media outlets have been attracted to her and continues to be attracted to her story. Carolyn has change so many lives and provided hope to thousands victims/survivors of domestic violence, which she is not even aware of.
@ 00:43:12
Maria José Cristerna, The Mexican Vampire Woman.
Hard to look at for some and impossible not to stare for others.
Marias family is a deeply Catholic. She went to a Catholic shool, fell in love and got married when she was only seventeen. She wanted to make things right. She says relationship didn't work. Following devastating and systematic episodes of abuse from her ex-husband, she decided to turn her life around, reinvent herself and become the vampire woman.Every piercing and tattoo of her body tells a story of her life. Modification tells a part of her story, of the things she did. She is a human art and her voice want to reach other women with some kind of problem. She said '' I want to reach other mistreated women and empower them to not take abuse fram anyone.'' Maria sees herself as a normal person with four children. “I am a normal stay at home mom, a woman also dedicated to tattooing, body art, I do suspension shows, but at the same time I try to help and empower other women who have been abused, because I come from domestic abuse”
she says.She got her first piercing at age twelve, her first tattoo at fourteen, and hasn't stopped her. She has stopped counting long ago how many tattoos and other body modifications she has had “about 95% of her body, the left over space is for like, fillers, backgrounds, you know? Like a canvas painting...” She has tattoos all over, multiple piercings where even the most daring would only think to put one, her ears are gauged, and she even has titanium implants in her forehead simulating horns. these were done without anesthetics, and she says she wants a couple more, on the back of her head.Maria José Cristerna is a strong woman with a dark bakground. I can understand her at the same time I can't? Why do implants in her forehead look like horns? Why a vampire? How can this help her to be a better person? Perheps this help her mental I don't know. Good for her that she is so strong and can take comments from other people.
@ 23:48:07
What would I do if I don't have a choice? What would YOU do if you don't have a choice? What to do when you can't help someone? Were should I put my feelings if I can't control them? Why is life so unfair? Why can't anybody answer my questions?
@ 02:24:25
body piercing
Piercings actually arise from religious people from Africa and south america. strain lip is for woman becuase thay don't have beard. They used wood, walrusbone, stone and mire etc to strain they faces.. An african headman(for his tribe) said: ''It is the most beautiful thing a woman can have. Men have beard and woman have nothing. What would the human being be without Pelele? She wouldn't be something..'' (Pelele is strain lip.) Naturepeople belived that piercings is for something good in our lives.
3000 years before Christ was it a man called the ''iceman'' from Austria that he was the first man on earth who had piercing in his ear but he had strain his ear lobe about 4-7 millimetre's. He had piercing because magic reasons. He and other people thought that demons gets ''our body's'' throught ears and take over our body. But if you had metal in your ear piercing the demons would turn away. Sailors belived that ear piercing could improve your eyesight. How stupid is that? Hahaha..Why do people think that piercings are so cool to have on our body? To hurt our self with needle's and some people like it? Uuuh.. We don't belive that it will do something good to our live? Or religious reasons? We just do it for fun and for our looks. We know that it can be risky for our own body, paralyzed and nervedamage.Why piercings?
@ 02:59:15
tyra banks
Tyra Banks is a strong, intelligent, independent and beautiful woman. I do really look up to Tyra because she is so amazing. When she was younger she get teased and drop of high school. She attempt to be a model.
Tyra gets a contract with Elite Model Management and moved to Paris. She really succeed with her job as a model and get to work for Victoria's Secret! Thats awesome!In 2003 she started a own program called America's Next Top Model and she have done 12 season after that. In 2005 she also started a own talk show called The Tyra Banks Show. In this show she talks about everything. Sex, teenages, drogs, alcohol, model, school/education, parenting, jobs, food, games etc! She has take parts of some movies and series. That woman has done it all! She have ALSO write a book, Tyra's Beauty Inside And Out. Music is something she have done to.. She has price for be one of the most beautiful woman in the world, top model for the year and woman of the year.I can't imagine that a Swedish girl/boy give up their school time for trying to get a job they really want. Here we really need a good education to get just a dirty/lazy/bad job..If I should drop of school everyone would think that I am an asshole to do something like that. That I don't have a future. That I would end with a child when I'm only 18 and live for my parents money. (Maybe something like that?) But I think if someone drop of school to live thiers own dreams, maybe it would end fine. I don't belive that everyone have a dream. I go to school but I think it is so hard.. I don't know what I want to do when school is over?? I think we are to young to decide our future now. We need to live! We only live once..Song by Tyra.Brittany Brower from season four America's Next Top Model.(SO FUN GIRL, LOOK LIKE JANICE DICKINSON!)
Kahlen Rondot and Brittany Bower from antm.&3
@ 19:34:05
Jo?o de Deus
Jo?o de Deus known also as ''John of God''. He is a self described medium and psychic surgeon from Brazil. He looks at him self as a ''spirit doctor'' and he says that God heals trought him, that he is a merely an instrument in God's divine hands. Jo?o has success in helping people to heal from cancer, AIDS, blindness, mental desease etc. How can he do someting like that? That is impossible! He has up to 3000 people per day stand and wait to see him healing people with ''the power'' he has. He is just a simple old Brazilian man with barely an education and now living to heal people with mediumship? Is he an angel? He use his body to perform seemingly miraculous healing, including both physical and psychic surgery.(ABC) news did a report about Jo?o de Deus live program. The program was about people with various medical conditions, including cronic fatigue syndrome etc, and an inoperable brain tumor. Each patient actually saw Jo?o and ABC claimed that in three of the cases there had been an improvement. According to other people Matthew Ireland is now free of his brain tumor, which is physiologically possible? Do you belive in miracles?
@ 00:52:01
across the universe
I want to honour John Lennon, Paul McCartney, George Harrison and Ringo Starr who create The Beatles. I really look up to them. They believed that every human can succeed/arrived in their lives. Don't ever give up. Their love songs are so good.. I really belive in the songs.. I know that they did believe in them to. I have read books about The Beatles and about John, Paul, George and Ringo and their family. Every book who is written about their lives is so good. I recommend to read them. Cynthia Powell wrote about her and Johns relationship and when they got Julian. She wrote about how John was to them. How bad he treat Cynthia and Julian.. So sad storie but true..Across the universe is a movie about music of the Beatles and the Vietnam war form the backdrop for the romance between an upper-class American girl and a poor Liverpudlian artist. I like this movie because the music, storie and how they show so much love...
The Beatles
Lennon/McCartney Yesterday Yesterday, all my troubles seemed so far away.Now it looks as though they're here to stay.Oh, I believe in yesterday.Suddenly, I'm not half the man I used to be,There's a shadow hanging over me,Oh, yesterday came suddenly.Why she had to go I don't know she wouldn't say. I said something wrong,Now I long for yesterday.Yesterday, love was such an easy game to play.Now I need a place to hide away.Oh, I believe in yesterday.Why she had to go I don't know she wouldn't say. I said something wrong,Now I long for yesterday.Yesterday, love was such an easy game to play.Now I need a place to hide away.Oh, I believe in yesterday.Mm mm mm mm mm mm mm.
Lennon/McCartney All you need is loveLove, love, love, love, love, love, love, love, love.
There's nothing you can do that can't be done.
Nothing you can sing that can't be sung.
Nothing you can say but you can learn how to play the game
It's easy.
There's nothing you can make that can't be made.
No one you can save that can't be saved.
Nothing you can do but you can learn how to be you
in time - It's easy.
All you need is love, all you need is love,
All you need is love, love, love is all you need.
Love, love, love, love, love, love, love, love, love.
All you need is love, all you need is love,
All you need is love, love, love is all you need.
There's nothing you can know that isn't known.
Nothing you can see that isn't shown.
Nowhere you can be that isn't where you're meant to be.
It's easy.
All you need is love, all you need is love,
All you need is love, love, love is all you need.
All you need is love (all together now)
All you need is love (everybody)
All you need is love, love, love is all you need.
@ 14:13:22
Thoughts about Transformers
Yesterday I saw Transformers 3 and it was so good. I can't imagine how they create this movie. The story in this movie is so amazing, well done! All about the robots and how they transform to vehicles, so fascinated and so many detail! And their planet Cybertron and how the robot break to two parts, the good once Autobots and the evil once Decepticons. How Optimus Prime lead the team and help the people to fight and survive Megatrons plans for our world.. But I was shocked when Sentinel Prime abandoned the Autobots and take Megatrons side. WTF?! I really love Transformers and I am a big fan!goddammit!
Peter Cullen as Optimus Prime
Leonard Nimoy as Sentinel Prime
Hugo Weaving as Megatron
Tom Kenny as Wheelie
Reno Wilson as Brains
Charlie Adler as Starscream
Jess Harnell as Ironhide
Robert Foxworth as Ratchet
Frank Welker as Shockwave & Soundwave
James Remar as Sideswipe
Francesco Quinn as Dino/Mirage
George Coe as Que/Wheeljack
John DiMaggio as Leadfoot
Ron Bottitta as Amp/Roadbuster
Keith Szarabajka as Laserbeak
Greg Berg as Igor
I think Peter Cullen IS the movie. Whitout him would't be Transformers. Shia LaBeouf play Sam Witwicky so fucking good. I was disappointed that Megan Fox wasn't in this movie. I didn't think we get a end from her. But the new girl Rosie Huntington-Whiteley as Carly Spencer, she was good to. Transformers 3 get☆☆☆☆ four star off five.''I have witnessed their capacity for courage, and though we are worlds apart, like us, there's more to them than meets the eye. I am Optimus Prime, and I send this message to any surviving Autobots taking refuge among the stars: We are here. We are waiting. ?''This is the best part of all the three movies.
@ 17:22:07
thought about music
Earlier today/night I saw MTW WORLD STAGE and I was thinking how can music touch a persons hearts so badly? How can one group or solo person do so much for people and for the world? We don't even know them. Sometimes we don't understand their songs or why they write them? Do every songs mean something for them who write it? We just love it and are desperate to hear the next one. And I wonder what they do with all the fucking money? We know about the party/drinking but I mean some groups/solo make so much money they can't even get rid of it. Why dont give it to charity who really need money. Not ALL the money. You know what I mean..When it came to music I enjoy almost everything. Rock, country, blues what ever, everything is unique.
@ 23:04:18}