Twin

Fraternal twin boys in the tub

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures.

Types of twins

Fraternal twins

Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.

Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum.

Identical twins

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic. Also note that any monochorionic or monoamniotic twins are identical twins. This condition does not occur for fraternal twins.

The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning after 12 days post-fertilization will typically result in conjoined twins.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. About 50% of mono-mono twins die from umbilical cord entanglement.

Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.

While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. The exact cause for the splitting of a zygote or embryo is unknown.

Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Twins are unique individuals that establish their own individual likes and dislikes. There are usually obvious signs of differences when the identical twins are observed separately or together.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).

Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. The incidence of mirror twinning is comparatively rare. They result from a late split of the fertilized egg at around 9-12 days. One mirror may or may not have situs inversus. This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects.

Complications of twin pregnancy

Vanishing twins

Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.

Conjoined twins

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.

Parasitic twins

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.

Sometimes the parasitic twin just becomes an almost indistinguishable part of the other.

A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body.

Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. Similar to vanishing twin.

Human twins

Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.

Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Multiple births

Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

Predisposing factors

The cause of monozygotic twinning is unknown. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

  • She is of African descent
  • Between the age of 30 and 40 years
  • Greater than average height and weight
  • Several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.

Twin studies

Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature.

Unusual Twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Twinning in animals

Multiple births are common in many animal species, such as cats, sheep, and ferrets. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed.


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The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. Alternatively, Microchip offers COM port emulation firmware for their range of USB PIC microcontrollers. Multiple births are common in many animal species, such as cats, sheep, and ferrets. FTDI Chip provides virtual COM drivers with its chips, to make the USB device look to the host software like a COM (RS-232) port. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. If your Operating System and language combination is not supported, another option is a USB to RS-232 bridge. When monozygotic twins are born with different genders it is because of chromosomal birth defects. Communication between software and USB devices depends upon the Operating System (Windows, Macintosh, Linux etc) and the language you choose (Java, C++, Delphi etc).

The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. See http://www.usb.org/developers/wusb/ for more details. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). Wireless USB is well suited to wireless connection of PC centric devices, just as Bluetooth is now widely used for mobile phone centric personal networks (at much lower data rates). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details. Wireless USB is intended as a cable-replacement technology, and will use Ultra wideband wireless technology for data rates of up to 480 Mbit/s. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). The USB Implementers Forum is working on a wireless networking standard based on the USB protocol.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). And [Powered USB] uses standard USB signalling with the addition of extra power lines for Point of sale terminals. There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. (However, Microsoft uses standard USB 2.0 connectivity in its newer Xbox 360.) Similarly IBM UltraPort uses standard USB signalling, but uses a proprietary connection format. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature. Microsoft's Xbox game console uses standard USB 1.1 signalling in its controllers, but features a proprietary connector rather than the standard USB connector. Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. It typically uses USB as the underlying communication layer.

Many fertility treatments have no effect on the likelihood of multiple births. The PictBridge standard allows for interconnecting consumer imaging devices. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Wireless USB uses UWB (Ultra Wide Band) as the radio technology. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Released in May 12, 2005. This can vary depending on what types of fertility treatments are used. IEEE 1394b also provides rates up to approximately 3.2 Gbit/s; however, the higher rates use special physical layers which are incompatible with 1394a devices.

Women undergoing certain fertility treatments may have a greater chance of multiple births. However unlike USB Hi-Speed systems which can change the speeds on each branch a 1394a device on a 1394b system requires all devices to fall to 1394a speeds. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. S800 requires a new physical layer, but S800 nodes can be connected to existing FireWire 1394a ports, just as USB Hi-Speed nodes will operate with older full-speed hosts. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. This provides a new mode called S800, which operates at 786.432 Mbit/s. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). In 2003, FireWire was updated with the IEEE 1394b specification.

The cause of monozygotic twinning is unknown. Therefore if high speed transfer is what you need you should match this with a good host controller and operating system. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets. Reducing the maximum transfers from say the theoretical 13 per frame to 10 or 9. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. In addition to this some operating systems take a conservative approach to scheduling transactions and limit the number of transfers per frame. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. It is a testament to the flexibilty of the USB bus that it can handle wide variances in device performances.

Sometimes multiple births may involve more than two fetuses. So the sustained transfer rate is a limitation of the individual device technology not the infrastructure. Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly. Why then can some USB devices only sustain 34 MB/s not 55 MB/s? The main reason is usually that the devices themselves are slow and spend most of the time NAK'ing the host to indicate they are not ready - this is particularly true of memory sticks. Thus, approximately 6% of children born in the US in 2001 were twins. Furthermore, the host-centric nature of USB allows the host to allocate more bandwidth to high priority devices instead of forcing them to compete for bandwidth as in Firewire. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Conversely, for USB the maximum timing model is fixed and is limited only by the host-device branch (not the entire network).

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". The more devices on the bus the lower the peak performance. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The peer to peer nature of Firewire requires devices to arbitrate, which means a FireWire bus must wait until a given signal has propagated to all devices on the bus. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. In a multi device environment Firewire rapidly loses ground to USB: Firewire's mixed speed networks and long connection chains dramatically affect its performance. Similar to vanishing twin. While for USB 2.0 the rate can be higher 55 MB/s (for a single device).

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. A single Firewire device may achieve a transfer rate for Firewire 400 as high as 41 MB/s. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body. USB transfer rates are generally higher than Firewire due to the need for Firewire devices to arbitrate for bus access. A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. USB can require more host resources than Firewire due to the need for the host to provide the arbitration and scheduling of transactions. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. The signalling rate of USB 2.0 Hi-Speed mode is 480 megabits per second, while the signalling rate of FireWire 400 (IEEE 1394a) is 393.216 Mbit/s [4].

One fetus acts as a parasite towards the other. These and other differences reflect the differing design goals of the two busses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. The most significant technical differences between FireWire and USB include the following:. This condition occurs in about 1 in 100,000 pregnancies. FireWire retains its popularity in many professional settings, where it is used for audio and video transfer, and data storage. This occurs where the single zygote of identical twins fails to separate completely. Today, USB Hi-Speed is rapidly replacing FireWire in consumer products.

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. The introduction of USB 2.0 Hi-Speed, with its widely advertised 480 Mbit/s signaling rate, convinced many consumers that FireWire was outdated (although this was not necessarily the case; see "USB 2.0 Hi-Speed vs FireWire" below). Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. However, because FireWire ports were more costly to implement than USB ports, primarily due to their per-port licence fee, they were rarely provided as standard equipment on computers, and peripheral manufacturers offered many more USB devices. Researchers suspect that more pregnancies start out as multiples than come to term that way. USB originally operated at a far lower data rate and used much simpler hardware, and was suitable for small peripherals such as keyboards and mice. Such conditions are usually associated with a higher incidence of other birth defects. USB was originally seen as a complement to FireWire, which was designed as a high-speed serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment.

This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Apple computers have used USB mice and keyboards exclusively since January 1999. One mirror may or may not have situs inversus. Mouses and keyboards are frequently fitted with USB connectors, but supplied with a small USB-to-PS/2 adaptor so that they can be used with either USB or PS/2 ports. They result from a late split of the fertilized egg at around 9-12 days. Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, PC game port, and PS/2 connectors to USB. The incidence of mirror twinning is comparatively rare. Motherboards for non-portable PCs usually have a number of USB 2.0 high-speed ports, some available at the back of the computer case, others requiring USB sockets on the front or rear of the computer to be connected via a cable to a header on the motherboard.

These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. AT keyboard connectors are less frequently found. Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. As of 2006, most PCs and motherboards have at least one USB port, but still retain PS/2 keyboard and mouse connectors. (Fraga, et al., 2005). Additionally, when multiple devices are connected, USB has significant advantages over FireWire,. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, USB ports are more usual than Firewire on consumer-level computers, which enhances the compatibility of a USB drive.

50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. An operating system designed to handle Hi-Speed USB 2.0 optimally is capable of data rates higher than Firewire, but the most commonly found [early 2006] operating systems and drivers are not. The number of differences between identical twins increases with age. Additionally, some operating systems transfer blocks limited to the USB 1.1 size of 64 bytes, without taking advantage of the larger block sizes allowed by USB 2.0. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The main reason for this is that the tests are conducted point to point (only one device) which means the USB system is always waiting for the drive. This is called epigenetic modification. FireWire tends to perform better in speed benchmark tests than even Hi-Speed USB 2.0, although the latter supports a numerically higher bit-rate.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. FireWire technology is also commonly used with portable hard drives; some have both USB and FireWire ports. There are usually obvious signs of differences when the identical twins are observed separately or together. Functionally, the drive appears to the user just like another internal drive.. Twins are unique individuals that establish their own individual likes and dislikes. These external drives usually contain a translating device that interfaces a drive of conventional technology (IDE, ATA, SATA, ATAPI, or even SCSI) to a USB port. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Today, a number of manufacturers offer external, portable USB hard drives, or empty enclosures for drives, that offer performance comparable to internal drives.

They develop their own individual personalities to enable themselves to be identified as individual persons. However, USB has one important advantage in making it possible to install and remove devices without opening the computer case, making it useful for external drives. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). USB is not intended to be a primary bus for a computer's internal storage: buses such as ATA (IDE) and SCSI fulfill that role. The exact cause for the splitting of a zygote or embryo is unknown. This was initially intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices. While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. USB implements connections to storage devices using a set of standards called the USB mass-storage device class.

As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. Those problems with the abuse of the USB power supply have inspired a number of April Fool hoaxes, like the introduction of a USB-powered George Foreman iGrill [2] and a desktop USB Fondue Set [3]. Examination of details such as fingerprints can tell them apart. USB-powered devices attempting to draw large currents without requesting the power will not work with certain USB controllers, and will either disrupt other devices on the bus or fail to work themselves (or both). (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Amongst others, a number of peripherals for IBM laptops (now made by Lenovo) are designed to use dual USB connections. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. For portable devices where external power is not available, but not more than 1 A is required at 5 V, devices may have connectors to allow the use of two USB cables, doubling available power but reducing the number of USB ports available to other devices.

About 50% of mono-mono twins die from umbilical cord entanglement. Such devices can be used with an external power supply of adequate rating; some external hubs may, in practice, supply sufficient power. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. This is a common requirement of external hard and optical disc drives and other devices with motors or lamps. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. Some USB devices draw more power than is permitted by the specification for a single port. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. This can cause problems with some computers—the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. In most cases, these items contain no electronic circuitry, and thus are not proper USB devices at all. Twinning after 12 days post-fertilization will typically result in conjoined twins. The typical example is a USB-powered reading light, but fans, battery chargers (particularly for mobile telephones) and even miniature vacuum cleaners are available. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. A number of devices use this power supply without participating in a proper USB network. Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available (and will generally shut down devices or hubs in order to keep power consumption within the available resource).

Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). When USB devices (including hubs) are first connected they are interrogated by the host controller, which inquires of each their maximum power requirements. The later in pregnancy that twinning occurs, the more structures will be shared. Devices that need more than 500 mA must provide their own power. This condition does not occur for fraternal twins. Many hubs include external power supplies which will power devices connected through them without taking power from the bus. Also note that any monochorionic or monoamniotic twins are identical twins. This disallows connection of a bus-powered hub to another bus-powered hub.

All monoamniotic twins are monochorionic. Bus-powered hubs can continue to distribute the bus provided power to connected devices but the USB specification only allows for a single level of bus-powered devices from a bus-powered hub. Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). A bus-powered device may use as much of that power as allowed by the port it is plugged into. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). This is often enough to power several devices, although this budget must be shared among all devices downstream of an unpowered hub. The two embryos develop into fetuses sharing the same womb. A given segment of the bus is specified to deliver up to 500 mA.

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. In typical situations the voltage is close to 5 V. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. The compliance spec requires no more than 5.25 V anywhere and no less than 4.375 V at the worst case; a low-power function after a bus-powered hub. Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. In practice, delivered voltage can drop well below 5 V, to only slightly above 4 V. Dizygotic twins may be a different sex or the same sex, just as with any other siblings. The USB connector provides a single nominally 5 volt wire from which connected USB devices may power themselves.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. The maximum length of a USB cable is 5 meters; greater lengths require hubs [1]. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. Wireless USB is a standard being developed to extend the USB standard while maintaining backwards compatibility with USB 1.1 and USB 2.0 on the protocol level. Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. USB On-The-Go has therefore defined two small form factor connectors, the mini-A and mini-B, and a hermaphroditic socket (mini-AB), which should stop the proliferation of proprietary designs. . This facility targets units such as PDAs where the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.

Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). An extension to USB called USB On-The-Go allows a single port to act as either a host or a device - chosen by which end of the cable plugs into the socket on the unit. A fetus alone in the womb is called a singleton. For specification purposes, these devices were treated as having a captive cable. The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. Other manufacturers of small items also developed their own small form factor connector, and a wide variety of these have appeared.

Several previous pregnancies. It uses a different mechanical connector while preserving the USB signaling and protocol. Greater than average height and weight. For example, the IBM UltraPort is a proprietary USB connector located on the top of IBM's laptop LCDs. Between the age of 30 and 40 years. However, the mechanical layer has changed in some examples. She is of African descent. The A-plug is approximately 4x12 mm, the B-plug is approximately 7x8 mm, and the B-mini plug is approximately 3x7 mm.

Thus all compliant USB cables have an A plug on one end, and either a B or Mini-B on the other end. Hosts and devices include connectors (female) while cables contain plugs (male). All connectors are mechanically incompatible, with an A connector always used on the upstream (host) end, and a B connector always used on the downstream (device) end. The USB 2.0 specification also introduces the mini-B connector, for smaller devices such as PDAs, mobile phones or digital cameras.

The USB 1.0, 1.1 and 2.0 specifications define two types of connectors for the attachment of devices to the bus: A, and B. In particular:. The connectors which the USB committee specified were designed to support a number of USB's underlying goals, and to reflect lessons learned from the varied menagerie of connectors then in service. The Mini A also has an additional piece of plastic inside to prevent insertion into slave only device.

This indicates if a device supporting usb on the go (with a mini AB socket) should initially act as host, in the mini B this is open circuit. Pin 4 is called ID and is connected to pin 5 for a mini-A. Most of the pins of a mini USB connector are the same as a standard USB connector, except pin 4. This segregation is for bandwidth only; bus rules about power and hub depth still apply.

The Transaction Translator in a Hi-Speed hub (or possibly each port depending on the electrical design) will function as a completely separate Full Speed bus to Full Speed and Low Speed devices attached to it. Hi-Speed hubs have a special function called the Transaction Translator that segregates Full Speed and Low Speed bus traffic from Hi-Speed traffic. Hi-Speed devices should fall back to the slower data rate of Full Speed when plugged into a Full Speed hub. All devices are tested according to the latest spec, so recently-compliant Low Speed devices are also 2.0.

The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or High-Speed after passing a compliancy test and paying a licensing fee. Though Hi-Speed devices are commonly referred to as "USB 2.0", not all USB 2.0 devices are Hi-Speed. A USB device should specify the speed it will use by correct labeling on the box it came in or sometimes on the device itself. USB supports three data rates. D+ and D− operate together; they are not separate simplex connections.

These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. USB signals are transmitted on a twisted pair of data cables, labelled D+ and D−. The most used device classes (grouped by assigned class ID) are:. These can be used as the main device classes are continuously revised.

Each class also optionally supports a SubClass and Protocol subdefinition. If bDeviceClass is set to 0x00, the operating system will look at bInterfaceClass of each interface to determine the device class. Both of these are a single byte each, so a maximum of 253 different device classes are possible (values 0x00 and 0xFF are reserved). If the class is to be set for the entire device, the number is assigned to the bDeviceClass field of the device descriptor, and if it is to be set for a single interface on a device, it is assigned to the bInterfaceClass field of the interface descriptor.

Device classes are decided upon by the Device Working Group of the USB Implementers Forum. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. These classes define an expected behavior in terms of device and interface descriptors so that the same device driver may be used for any device that claims to be a member of a certain class. Devices that attach to the bus can be full-custom devices requiring a full-custom device driver to be used, or may belong to a device class.

On BSD systems, dmesg will show the detailed information hierarchy. Most Linux systems also provide the lsusb command which provides USB-specific details about ports and controllers. On Microsoft Windows platforms, one can tell whether a USB controller is version 2.0 by opening the Device Manager and checking for the word "Enhanced" in its description; only USB 2.0 drivers will contain the word "Enhanced." On Linux systems, the lspci -v command will list all PCI devices, and controllers will be named OHCI, UHCI or EHCI respectively, which is also the case in the Mac OS X system profiler. All other vendors use virtual OHCI controllers.

The virtual HCD on Intel and Via EHCI controllers are UHCI. Each EHCI controller contains four virtual HCD implementations to support Full Speed and Low Speed devices. Only EHCI can support high-speed transfers. The USB 2.0 HCD implementation is called the Extended Host Controller Interface (EHCI).

During the design phase of USB 2.0 the USB-IF insisted on only one implementation. The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations which increased cost. The main difference between OHCI and UHCI is the fact that UHCI is more software-driven than OHCI is, making UHCI slightly more processor-intensive but cheaper to implement (excluding the license fees). VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI.

However, Intel subsequently created a specification they called the Universal Host Controller Interface (UHCI) and insisted other implementers pay to license and implement UHCI. Compaq's Open Host Controller Interface (OHCI) was adopted as the standard by the USB-IF. At version 1.0 and 1.1 there were two competing HCD implementations. In practice, these are hardware registers (ports) in the computer.

The hardware that contains the host controller and the root hub has an interface toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer. An endpoint may however be reused among several interfaces and alternate interface settings. These interface descriptors in turn have one default interface setting and possibly more alternate interface settings which in turn have endpoint descriptors, as outlined above. Each configuration descriptor in turn has one or more interface descriptors, which describe certain aspects of the device, so that it may be used for different purposes: for example, a camera may have both audio and video interfaces.

low power mode. active vs. These configurations often correspond to states, e.g. The device connected to the bus has one (and only one) device descriptor which in turn has one or more configuration descriptors.

To access an endpoint, a hierarchical configuration must be obtained. The interrupt transfers on corresponding endpoints does not actually interrupt any traffic on the bus, they are just scheduled to be queried more often and in between any other large transfers, thus "interrupt traffic" on a USB bus is really only high-priority traffic. The host controller then polls the bus for traffic, usually in a round-robin fashion, so no device can transfer any data on the bus without explicit request from the host controller. When a device (function) or hub is attached to the host controller through any hub on the bus, it is given a unique 7 bit address on the bus by the host controller.

The pipes are also divided into four different categories by way of their transfer type:. There is always an inward and an outward pipe numbered 0 on each device. All USB devices have at least two such pipes/endpoints: namely endpoint 0 which is used to control the device on the bus. Each endpoint can transfer data in one direction only, either into or out of the device/function, so each pipe is uni-directional.

Each pipe has a maximum packet length, typically 2n bytes, so a USB packet will often contain something on the order of 8, 16, 32, 64, 128, 256, 512 or 1024 bytes. In these pipes, data is transferred in packets of varying length. (The OUT direction shall be interpreted out of the host controller and the IN direction is into the host controller.) Endpoint 0 is however reserved for the bus management in both directions and thus takes up two of the 32 endpoints. These endpoints (and their respective pipes) are numbered 0-15 in each direction, so a device/function can have up to 32 active pipes, 16 inward and 16 outward.

The pipes are synonymous to byte streams such as in the pipelines of Unix, however in USB lingo the term endpoint is (sloppily) used as a synonym for the entire pipe, even in the standard documentation. These devices/functions (and hubs) have associated pipes (logical channels) which are connections from the host controller to a logical entity on the device named an endpoint. There always exists one hub known as the root hub, which is attached directly to the host controller. The hubs are special purpose devices that are not officially considered functions.

In USB terminology devices are referred to as functions, because in theory what we know as a device may actually host several functions, such as a router that is a Secure Digital Card reader at the same time. USB connects several devices to a host controller through a chain of hubs. The specification is at revision 1.0a (Jan 2006). Smaller USB plugs and receptacles, called Mini-A and Mini-B, are also available, as specified by the On-The-Go Supplement to the USB 2.0 Specification.

Equipment conforming with any version of the standard will also work with devices designed to any of the previous specifications (backwards compatibility). Previous notable releases of the specification were 0.9, 1.0, and 1.1. The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Hewlett-Packard, Intel, Lucent, Microsoft, NEC, and Philips jointly led the initiative to develop a higher data transfer rate than the 1.1 specification.

The USB specification is at version 2.0 (with revisions) as of February 2006. Notable members have included Apple Computer, Hewlett-Packard, NEC, Microsoft, Intel, and Agere. The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. As of 2005, the only large classes of peripherals that cannot use USB, because they need a higher data rate than USB can provide, are displays and monitors, and high-quality digital video components.

As of 2004 there were about 1 billion USB devices in the world. USB is also used extensively to connect non-networked printers, replacing the parallel ports which were widely used; USB simplifies connecting several printers to one computer. For many devices such as scanners and digital cameras, USB has become the standard connection method. USB can connect peripherals such as mice, keyboards, gamepads and joysticks, scanners, digital cameras, printers, external storage, networking components, etc.

When a device is first connected, the host enumerates and recognises it, and loads the device driver it needs. USB was designed to allow peripherals to be connected without the need to plug expansion cards into the computer's ISA, EISA, or PCI bus, and to improve plug-and-play capabilities by allowing devices to be hot-swapped (connected or disconnected without powering down or rebooting the computer). USB cables do not need to be terminated. Modern computers often have several host controllers, allowing a very large number of USB devices to be connected.

Not more than 127 devices, including the bus devices, may be connected to a single host controller. Additional USB hubs may be included in the chain, allowing branching into a tree structure, subject to a limit of 5 levels of branching per controller. A USB system has an asymmetric design, consisting of a host controller and multiple daisy-chained devices. .

Universal Serial Bus (USB) provides a serial bus standard for connecting devices, usually to computers such as PCs and the Apple Macintosh, but is also becoming commonplace on video game consoles such as Sony's PlayStation 2, Microsoft's Xbox 360, Nintendo's Revolution, and PDAs, and even devices like televisions and home stereo equipment. It appears that no work has been done on this package since 2003 so it may be abandoned. The usb.windows package has a partial Windows implementation of a usb.core.Host object, bootstrapping support, and other classes leveraging Windows USB support. Java - The Mike Stahl started work on this combination in 2003.

Java development is possible via JNI. Their COM interface allows for Delphi, C# and VB development. General - USBIO has C++ drivers for USB communication on windows from C & C++. Java - No info is available on this combination.

General - Apple has this page on General Mac USB Development. This API is unfortunately limited to Linux. Java - The jUSB project provides a Free Software (and Open Source) Java API for USB, supporting applications using Java host-side software to drive USB devices. General - http://www.linux-usb.org/.

This is the current revision. USB On-The-Go Supplement 1.0a: Released in June 2003. USB On-The-Go Supplement 1.0: Released in December 2001. As an example, a computer's port could be incapable of USB 2.0's hi-speed fast transfer rates, but still claim USB 2.0 compliance (since it supports some of USB 2.0).

This makes the backwards compatibility explicit, but it becomes more difficult to determine a device's throughput without seeing the symbol. Added three speed distinction to this standard, allowing all devices to be USB 2.0 compliant even if they were previously considered only 1.1 or 1.0 compliant. USB 2.0: Revised in December 2002. This is the current revision.

The major feature of this standard was the addition of high-speed mode. USB 2.0: Released in April 2000. USB 1.1: Released in September 1998. USB 1.0: Released in January 1996.

USB 1.0 FDR: Released in November 1995, the same year that Apple adopted the IEEE 1394 standard known as FireWire. In a FireWire network, any capable node can control the network. A USB network relies on a single host at the top of the tree to control the network. A FireWire device can communicate with any other node at any time, subject to network conditions.

USB uses a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. USB networks use a tiered-star topology, while FireWire networks use a repeater-based topology. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does. This was done to avoid circumstances where a device complied with the connector specification but its large size blocked adjacent ports.

Unlike most other connector standards, the USB spec also defines limits to the size of a connecting device in the area around its plug. The USB standard specifies relatively low tolerances for compliant USB connectors, intending to minimize incompatibilities in connectors produced by different vendors (a goal that has been very successfully achieved). This type of enclosure also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel). This sheath is typically connected to the system ground, allowing otherwise damaging static charges to be safely discharged by this route (rather than via delicate electronic components).

The connector construction always ensures that the external sheath on the plug contacts with its counterpart in the receptacle before the four connectors within are connected. The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances or by those with motor disabilities. USB cables and small USB devices are held in place by the gripping force from the receptacle (without the need for the screws, clips, or thumbturns other connectors require). A moderate insertion/removal force is specified.

RJ-45 cabling) gender-changers are never used, making it difficult to create a cyclic USB network. Unlike other communications systems (e.g. USB does not support cyclical networks, so the connectors from incompatible USB devices are themselves incompatible. The connectors enforce the directed topology of a USB network.

The connectors are particularly cheap to manufacture. However, it is not obvious at a glance to the inexperienced user which way round a connector goes, so it is often necessary to try both ways. Connectors cannot be plugged-in upside down, and it is clear from the appearance and kinesthetic sensation of making a connection when the plug and socket are correctly mated. It is difficult to incorrectly attach a USB connector.

The encasing sheath and the tough moulded plug body mean that a connector can be dropped, stepped upon, even crushed or struck, all without damage; a considerable degree of force is needed to significantly damage a USB connector. As a result USB connectors can safely be handled, inserted, and removed, even by a small child. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is further protected by an enclosing metal sheath. Many previous connector designs were fragile, with pins or other delicate components prone to bending or breaking, even with the application of only very modest force.

The connectors are designed to be robust. A Hi-Speed rate of 480 Mbit/s (57 MiB/s). All USB Hubs support Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices.

Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. A Full Speed rate of 12 Mbit/s (1.4 MiB/s). A Low Speed rate of 1.5 Mbit/s (183 KiB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice and joysticks. file transfers.

bulk transfers - large sporadic transfers using all remaining available bandwidth (but with no guarantees on bandwidth or latency), e.g. pointing devices and keyboards. interrupt transfers - devices that need guaranteed quick responses (bounded latency), e.g. realtime audio or video.

isochronous transfers - at some guaranteed speed (often but not necessarily as fast as possible) but with possible data loss, e.g. by the bus control pipe number 0. control transfers - typically used for short, simple commands to the device, and a status response, used e.g.

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