view to be one of the most basic external connections to a reckoner , theserial porthas been an integral part of most computers for more than 20 years . Although many of the newer systems have done away with the serial port completely in favor ofUSBconnections , mostmodemsstill use the serial port , as do some printers , PDAsanddigital camera . Few computers have more than two serial ports .
basically , serial ports furnish a standard connection and protocol to let you attach devices , such as modems , to your figurer . In this variant ofHow Stuff Works , you will acquire about the difference between a parallel port and a serial embrasure , what each pivot does and what flow controller is .
UART Needed
All computeroperating systemsin utilization today sustain serial ports , because sequent port have been around for decades . Parallel portsare a more recent innovation and are much fast than serial port . USB portsare only a few years previous , and will likely substitute both nonparallel and parallel ports completely over the next several years .
The name " serial " comes from the fact that a nonparallel port wine " serialise " data . That is , it assume abyteof information and convey the 8 scrap in the byte one at a time . The reward is that a serial port needs only one wire to transmit the 8 mo ( while a parallel port needs 8) . The disadvantage is that it take on 8 times longer to transmit the datum than it would if there were 8 wire . Serial port lower transmission line cost and make cable smaller .
Before each byte of datum , a serial port sends a commencement snatch , which is a single scrap with a economic value of 0 . After each byte of data , it sends a layover bit to point that the byte is complete . It may also institutionalize a para bit .
Serial larboard , also calledcommunication ( COM ) port , arebi - directional . Bi - directional communication allow for each machine to have information as well as transmit it . Serial gadget habituate dissimilar pins to get and channel datum – using the same thole would limit communicating tohalf - duplex apartment , meaning that information could only travel in one counselling at a time . Using dissimilar pins allow forfull - duplexcommunication , in which selective information can jaunt in both direction at once .
consecutive porthole rely on a special controller microprocessor chip , theUniversal Asynchronous Receiver / Transmitter ( UART ) , to function in good order . The UART flake takes the parallel outturn of the computer ’s organization jalopy and transforms it into serial form for transmittance through the sequential port . for function quicker , most UART chips have a build up - inbufferof anywhere from 16 to 64 KB . This fender allows the chip tocachedata come in from the organization jitney while it is processing data going out to the serial port . While most standard serial port have a maximal transfer pace of 115 Kbps ( kilobits per mo ) , high upper sequential ports , such asEnhanced Serial Port ( ESP)andSuper Enhanced Serial Port ( Super ESP ) , can accomplish data transfer pace of 460 Kbps .
The Serial Connection
The external connection for a serial port can be either 9 peg or 25 pins . Originally , the primary use of a serial embrasure was to connect a modem to your computing machine . The pin assignments reflect that . have ’s take a close look at what happens at each peg when a modem is connect .
9 - pin connector :
25 - pin connection :
Voltage sent over the pins can be in one of two states , OnorOff . On ( binary value " 1 " ) means that the thole is transmitting a signal between -3 and -25 volt , while Off ( binary value " 0 " ) means that it is transmitting a signal between +3 and +25 volts …
Going With The Flow
An crucial aspect of serial communications is the construct offlow dominance . This is the power of one machine to narrate another machine to stop post data for a while . The commands Request to station ( RTS ) , Clear To post ( CTS ) , Data Terminal Ready ( DTR ) and Data Set Ready ( DSR ) are used to enable current control .
Let ’s look at an example of how stream ascendency work : You have a modem that communicates at 56 Kbps . The serial joining between your computer and your modem transmits at 115 Kbps , which is over twice as tight . This means that the modem is getting more datum coming from the computer than it can transmit over the phone line . Even if the modem has a 128 K buffer to store data point in , it will still quickly lead out of buffer space and be unable to function properly with all that data streaming in .
With flow control , the modem can finish the rate of flow of data from the computer before it overruns the modem ’s buff . The data processor is constantly send off a signal on the postulation to Send pin , and insure for a signal on the Clear to station pin . If there is no Clear to send off response , the figurer stops sending information , waiting for the Clear to Send before it resumes . This set aside the modem to keep the flow of data point running swimmingly .
