亚洲第一欧美的日产

亚洲第一欧美的日产

Sabtu, 24 Mei 2014

Network Topologies

In computernetworking, topology refers to the layout of connecteddevices. This article introduces the standard topologies of networking.
Think of atopology as a network's virtual shape or structure. This shape does notnecessarily correspond to the actual physical layout of the devices on the network.For example, the computers on a home LAN maybe arranged in a circle in a family room, but it would be highly unlikely tofind a ring topology there.
Networktopologies are categorized into the following basic types:
  • ·        bus
  • ·        ring
  • ·        star
  • ·        tree
  • ·        mesh


Bus Topology
Busnetworks (not to be confused with the system bus of a computer) use a commonbackbone to connect all devices. A single cable, the backbone functions as ashared communication medium that devices attach or tap into with an interfaceconnector. A device wanting to communicate with another device on the networksends a broadcast message onto the wire that all other devices see, but onlythe intended recipient actually accepts and processes the message.
Ethernetbus topologies are relatively easy to installand don't require much cabling compared to the alternatives. 10Base-2("ThinNet") and 10Base-5 ("ThickNet") both were popularEthernet cabling options many years ago for bus topologies. However, busnetworks work best with a limited number of devices. If more than a few dozencomputers are added to a network bus, performance problems will likely result.In addition, if the backbone cable fails, the entire network effectivelybecomes unusable.

Ring Topology
In a ringnetwork, every device has exactly two neighbors for communication purposes. Allmessages travel through a ring in the same direction (either"clockwise" or "counterclockwise"). A failure in any cableor device breaks the loop and can take down the entire network.
To implementa ring network, one typically uses FDDI, SONET,or Token Ring technology.Ring topologies are found in some office buildings or school campuses.


Star Topology
Many home networks usethe star topology. A star network features a central connection point called a"hub node" that may be a network hubswitch or router. Devices typicallyconnect to the hub with Unshielded Twisted Pair (UTP) Ethernet.
Comparedto the bus topology, a star network generally requires more cable, but afailure in any star network cable will only take down one computer's networkaccess and not the entire LAN. (If the hub fails, however, the entire networkalso fails.)


Tree Topology
Treetopologies integrate multiple star topologies together onto a bus. In its simplestform, only hub devices connect directly to the tree bus, and each hub functionsas the root of a tree of devices. This bus/star hybrid approach supports futureexpandability of the network much better than a bus (limited in the number ofdevices due to the broadcast traffic it generates) or a star (limited bythe number of hub connection points) alone.

Mesh Topology
Meshtopologies involve the concept of routes. Unlike each of the previoustopologies, messages sent on a mesh network can take any of several possiblepaths from source to destination. (Recall that even in a ring, although twocable paths exist, messages can only travel in one direction.) Some WANs,most notably the Internet, employ mesh routing.
A meshnetwork in which every device connects to every other is called a full mesh.As shown in the illustration below, partial mesh networks also exist in whichsome devices connect only indirectly to others.



PLC Analog device (iCMOS)

A programmable-logic controller (PLC) is a compactcomputer-based electronic system that uses digital or analog input/outputmodules to control machines, processes, and other control modules. A PLC isable to receive (input) and transmit (output) various types of electricaland electronic signals and use them to control and monitor practically anykind of mechanical and/or electrical system. PLCs are classified by the numberof I/O functions provided. For example, a nano PLC incorporates fewerthan 32 I/Os, amicro PLC has between 32 and 128 I/Os, a small PLChas between 128 and 256 I/Os, and so on.

Many Analog Devices products used in both the input- andoutput sections of PLC designs benefit from iCMOS, anew high-performance fabrication process that combines high-voltage siliconwith submicron CMOS and complementary bipolar technologies.This powerfulcombination allows a single chip design to mix-and-match 5-V CMOScircuits with higher-voltage 16-, 24-, or 30-V CMOS circuitry—with multiple voltage supplies feeding thesame chip. With this flexibility of combining components and operatingvoltages, submicroniCMOS devices can have higher performance, a more integratedfeature set, and lower power consumption—and require significantly smallerboard area than previous generations of high-voltage products. The bipolar technologyprovides accurate references, excellent matching, and high stability for ADCs,DACs, and low-offset amplifiers.

Thin-film resistors, with their 12-bit initial matching,16-bit trimmed matching, and temperature- and voltage coefficients up to 20times better than conventional polysilicon resistors, are ideal forhigh-precision, high-accuracy digital-to-analog converters. On-chip thin-filmfuses allow digital techniques to be used for calibration of integralnonlinearity, offset, and gain in high-precision converters.

Figure for iCmos Invervter



Pressured Control Ventilation

Pressure Control refers to the type of breath delivered, notthe mode of ventilation. Many different modes are pressure controlled.Conventionally, the term “pressure control” refers to an assist control mode(there is also a SIMV pressure control mode on some ventilators). In pressurecontrol, a pressure limited breath is delivered at a set rate. The tidal volumeis determined by the preset pressure limit. This is a peak pressurerather than a plateau pressure limit (easier to measure). The inspiratory timeis also set by the operator. Again this is a trade off between short times withrapid inflow and outflow of gas, and long times with gas trapping. The flowwaveform is always decelerating in pressure control, this relates to themechanics of targeting airway  pressure: flow slows as it reaches thepressure limit.


Gas flows into the chest along the pressure gradient. As theairway pressure rises with increasing alveolar volume the rate of flow dropsoff (as the pressure gradient narrows) until a point is reached when thedelivered pressure equals the airway pressure: flow stops. The pressure ismaintained for the duration of inspiration. Obviously, longerinspiratory times lead to higher mean airway pressures (the “i” time (Ti) is apressure holding time after flow has stopped). The combination of deceleratingflow and maintenance of airway pressure over time means that stiff, noncompliantlung units (long time constants) which are difficult to aerate are more likelyto be inflated. Gas distribution in pressure control is like droppinga glass of water on the floor: the water trickles into every nookand cranny. (3).


Brushed DC Motor Controller

A DC motor is an electricmotor that runs on DC current. They are commonly found in applications asdiverse as industrial fans, blowers and pumps, machine tools, householdappliances, power tools, disk drives, and many more. Brushed DC motors arewidely used in applications ranging from toys to push-button adjustable carseats. Brushed DC (BDC) motors are inexpensive, easy to drive, and are readilyavailable in all sizes and shapes All BDC motors are made of the same basic components:a stator, rotor, brushes and a commutator.



Unlike other electric motortypes (i.e., brushless DC,AC induction), BDC motors do not require a controllertoswitch current in the motor windings. Instead, thecommutation of the windingsof a BDC motor is done mechanically. A segmented copper sleeve, called a commutator,resides on the axle of a BDC motor. As the motor turns, carbon brushes slideover the commutator, coming in contact with different segments of the commutator.The segments are attached to different rotor windings, therefore, a dynamicmagnetic field is generated inside the motor when a voltage is applied acrossthe brushes of the motor. It is important to note that the brushes andcommutator are the parts of a BDC motor that are most prone to wear because theyare sliding past each other.

Arc Suppression

Any method used for extinguishing electrical arcs betweencontacts. Arc suppression is necessary to ensure worker safety and prolongcontact life. The energy contained in the resulting electrical arc is very high(tens of thousands of degrees Fahrenheit), causing the metal on the contactsurfaces to melt, pool and migrate with the current. The extremely Any method used for extinguishing electrical arcs betweencontacts. Arc suppression is necessary to ensure worker safety and prolongcontact life. The energy contained in the resulting electrical arc is very high(tens of thousands of degrees Fahrenheit), causing the metal on the contactsurfaces to melt, pool and migrate with the current. The extremely hightemperature of the arc cracks the surrounding gas molecules creating ozone,carbon monoxide, and other compounds. The arc energy slowly destroys thecontact metal, causing some material to escape into the air as fineparticulate matter. This very activity causes the material in the contacts todegrade quickly, resulting in device failure


example of arc suppressions

Arc suppression techniques can produce a number of benefits:
1.  Minimised contactdamage from arcing and therefore reduced maintenance, repair and replacementfrequency.
2.  Increased Contactreliability.
3.  Reduced heatgeneration resulting in less heat management measures such as venting and fans.
4.  Reduced Ozone andpollutant emissions.
5.  Reduced Electromagnetic Interference (EMI) from arcs - a common source ofradiated EMI.


Latching Relays

A latching relay is a two-position electrically-actuatedswitch. It is controlled by two momentary-acting switches or sensors, one that'sets' the relay, and the other 'resets' the relay. The latchingrelay maintains its position after the actuating switch has been released, soit performs a basic memory function.
The latching relay is similar to a two-position ('doublethrow') toggle switch. The handle of a toggle switch is physically pushedto one position, and it stays in that position until pushed to the oppositeposition. A latching relay is electrically 'set' to one position, and itremains 'latched' in that position until it is electrically 'reset' to theopposite position.

There are two kinds of latching relays:
  • ·        An electrically latched relay is a standard relay withone of its own contacts wired into its coil circuit. An external switchinitially turns the relay on, then it is kept on by its own contact. Anexternal reset switch interrupts power to the relay, which turns it off.
  • ·        bistable, or mechanicallylatched relay typically has two internal coils and an internal latchmechanism. Energizing one coil 'sets' the contacts in oneposition, and thecontacts stay in that position until the 'reset' coil is energized.



 This is video for make you understand about Latching Relays:

http://www.youtube.com/watch?v=n7SuHDmuVUk

AC Generators

A generator consists of some magnets and a wire (usually avery long one that's wrapped to form several coils and known as an armature). Asteam engine or some other outside source of motion moves the wire or armaturethrough the magnetic field created by themagnets. a loop of wire isspinning within a magnetic field. Because it is always moving through thefield, a current is sustained. A.C. generators or alternators (asthey are usually called) operate on the same fundamental principles of electromagnetic induction as D.C. generators.

Alternating voltage may be generated by rotating a coil in the magneticfield or by rotating a magnetic field within a stationary coil.  But, because the loop is spinning, it'smoving across the field first in one direction and then in the other,which means that the flow of electrons keeps changing. Because theelectrons flow first in one direction and in the other, the generatorproduces an alternating current.

Advantages of AC Generator

  • A D/C generator must produce voltage at the level at which it will be used. A/C has the advantage of allowing you to convert the current to a different voltage using a transformer. Transformers work with A/C only, not D/C
  • A/C has the advantage of traveling over long distances with less loss of power than D/C. Transforming the current to a high voltage reduces the current, which in turn reduces power loss. This can be seen from the formula P=R*I-squared, where P is power loss due to resistance, R is resistance, and I is current. Because I is squared in the power formula, decreasing I a little (by increasing the voltage) decreases the power loss a lot.
  • A safety-related disadvantage of A/C is the increased danger of electric shock due to the use of higher voltage for long-distance transmission. This is why long-distance transmissions use power lines kept high up off the ground.