Sabtu, 24 Mei 2014

Network Topologies

In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.
Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there.
Network topologies are categorized into the following basic types:
  • ·         bus
  • ·         ring
  • ·         star
  • ·         tree
  • ·         mesh

Bus Topology
Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.
Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

Ring Topology
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.
To implement a 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 use the star topology. A star network features a central connection point called a "hub node" that may be a network hubswitch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.
Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

Tree Topology
Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the root of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

Mesh Topology
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.
A mesh network 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 which some devices connect only indirectly to others.

PLC Analog device (iCMOS)

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

Many Analog Devices products used in both the input- and output sections of PLC designs benefit from iCMOS, a new high-performance fabrication process that combines high-voltage silicon with submicron CMOS and complementary bipolar technologies.This powerful combination allows a single chip design to mix-and-match 5-V CMOS circuits with higher-voltage 16-, 24-, or 30-V CMOS circuitry—with multiple voltage supplies feeding the same chip. With this flexibility of combining components and operating voltages, submicroniCMOS devices can have higher performance, a more integrated feature set, and lower power consumption—and require significantly smaller board area than previous generations of high-voltage products. The bipolar technology provides 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 20 times better than conventional polysilicon resistors, are ideal for high-precision, high-accuracy digital-to-analog converters. On-chip thin-film fuses allow digital techniques to be used for calibration of integral nonlinearity, offset, and gain in high-precision converters.

Figure for iCmos Invervter

Pressured Control Ventilation

Pressure Control refers to the type of breath delivered, not the 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 pressure control, a pressure limited breath is delivered at a set rate. The tidal volume is determined by the preset pressure limit. This is a peak pressure rather than a plateau pressure limit (easier to measure). The inspiratory time is also set by the operator. Again this is a trade off between short times with rapid inflow and outflow of gas, and long times with gas trapping. The flow waveform is always decelerating in pressure control, this relates to the mechanics of targeting airway  pressure: flow slows as it reaches the pressure limit.

Gas flows into the chest along the pressure gradient. As the airway pressure rises with increasing alveolar volume the rate of flow drops off (as the pressure gradient narrows) until a point is reached when the delivered pressure equals the airway pressure: flow stops. The pressure is maintained for the duration of inspiration. Obviously, longer inspiratory times lead to higher mean airway pressures (the “i” time (Ti) is a pressure holding time after flow has stopped). The combination of decelerating flow and maintenance of airway pressure over time means that stiff, noncompliant lung units (long time constants) which are difficult to aerate are more likely to be inflated. Gas distribution in pressure control is like dropping a glass of water on the floor: the water trickles into every nook and cranny. (3).

Brushed DC Motor Controller

A DC motor is an electric motor that runs on DC current. They are commonly found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, disk drives, and many more. Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats. Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available 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 motor types (i.e., brushless DC,AC induction), BDC motors do not require a controllerto switch current in the motor windings. Instead, thecommutation of the windings of 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 slide over the commutator, coming in contact with different segments of the commutator. The segments are attached to different rotor windings, therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor. It is important to note that the brushes and commutator are the parts of a BDC motor that are most prone to wear because they are sliding past each other.

Arc Suppression

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

example of arc suppressions

Arc suppression techniques can produce a number of benefits:
1.   Minimised contact damage from arcing and therefore reduced maintenance, repair and replacement frequency.
2.   Increased Contact reliability.
3.   Reduced heat generation resulting in less heat management measures such as venting and fans.
4.   Reduced Ozone and pollutant emissions.
5.   Reduced Electromagnetic Interference (EMI) from arcs - a common source of radiated EMI.

Latching Relays

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

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

 This is video for make you understand about Latching Relays:

AC Generators

A generator consists of some magnets and a wire (usually a very long one that's wrapped to form several coils and known as an armature). A steam engine or some other outside source of motion moves the wire or armature through the magnetic field created by themagnets. a loop of wire is spinning within a magnetic field. Because it is always moving through the field, a current is sustained. A.C. generators or alternators (as they 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 magnetic field or by rotating a magnetic field within a stationary coil.  But, because the loop is spinning, it's moving across the field first in one direction and then in the other, which means that the flow of electrons keeps changing. Because the electrons flow first in one direction and in the other, the generator produces 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.