Saturday, March 14, 2009

ssr......underconstruction!!!

SSR

A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 amps, have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.

Operation

Voltage applied to the control line of an SSR causes the LED to shine on the photo-sensitive diode. This produces a voltage between the MOSFET source and its gate, causing the MOSFET to turn on. An SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array works well for DC loads.

There is an inherent substrate diode in all MOSFETs that conducts in the reverse direction. This means that a single MOSFET can't block current in both directions. For AC (bi-directional) operation, two MOSFETs are arranged back to back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes then are alternately reverse biased in order to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.

It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if switching a DC load. There is also commonly some circuitry to discharge the gate when the LED is turned off, speeding the relay's turn-off.

Friday, March 13, 2009

Again... It's Interfacing!

The connection and interaction between hardware, software and the user. Users "talk to" the software. The software "talks to" the hardware and other software. Hardware "talks to" other hardware. All this is interfacing. It has to be designed, developed, tested and redesigned; and with each incarnation, a new specification is born that may become yet one more de facto or regulated standard.

Hardware Interfaces

Hardware interfaces are the plugs, sockets, cables and electrical signals traveling through them. Examples are USB, FireWire, Ethernet, ATA/IDE, SCSI and PCI.

Software/Programming Interfaces

Software interfaces (programming interfaces) are the languages, codes and messages that programs use to communicate with each other and to the hardware. Examples are the Windows, Mac and Linux operating systems, SMTP e-mail, IP network protocols and the software drivers that activate the peripheral devices.

User Interfaces

User interfaces are the keyboards, mice, commands and menus used for communication between you and the computer. Examples are the command lines in DOS and Unix, and the graphical interfaces in Windows, Mac and Linux.

Format & Function

Every interface implies a structure. Electrical signals are made up of voltage levels, frequencies and duration. The data passed from one device or program to another has a precise format (header, body, trailer, etc.).

Every interface implies a function. At the hardware level, electronic signals activate functions; data are read, written, transmitted, received, checked for error, etc. At the software level, instructions activate the hardware (access methods, data link protocols, etc.). At higher levels, the data transferred or transmitted may itself request functions to be performed (client/server, program to program, etc.).

Language & Programming

An interface is activated by programming language commands. The complexity of the functions and the design of the language determine how difficult it is to program.

User Interface, Protocol, API and ABI

The design of the interaction between the user and the computer is called a "user interface." The rules, formats and functions between components in a communications system or network are called "protocols." The language and message formats between routines within a program or between software components is called an "application programming interface" (API). The specification for an operating system working in a specific machine environment has been known as an "application binary interface" (ABI), but this term is not widely used.

All the above interactions are interfaces. Regardless of what they are called, they all create rules that must be precisely followed in a digital world.

A Whole Lot of Talking To

No matter what they're called, interfaces boil down to a format and language that defines the services one system is capable of delivering to another.

Saturday, March 7, 2009

Solid state relay

This week we just studied about the Solid state relay which plays a big role in our project(computer interfacing-"push botton").

What is Solid state relay(SSR)?

Solid state relay(SSR) is an electronic switch, which, unlike an electromechanical relay, contains no moving parts. The types of SSR are photo-coupled SSR, transformer-coupled SSR, and hybrid SSR. A photo-coupled SSR is controlled by a low voltage signal which is isolated optically from the load.

Operation

Voltage applied to the control line of an SSR causes the LED to shine on the photo-sensitive diode. This produces a voltage between the MOSFET source and its gate, causing the MOSFET to turn on. An SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array works well for DC loads.
There is an inherent substrate diode in all MOSFETs that conducts in the reverse direction. This means that a single MOSFET can't block current in both directions. For AC (bi-directional) operation, two MOSFETs are arranged back to back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes then are alternately reverse biased in order to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.
It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if switching a DC load. There is also commonly some circuitry to discharge the gate when the LED is turned off, speeding the relay's turn-off.

Friday, March 6, 2009

wHat do We hAve???

We are currently making our project regarding the topic "computer interfacing using C". In this project,some materials that we used are:::::

Resistors

A resistor is a two-terminal electronic component designed to oppose an electric current by producing a voltage drop between its terminals in proportion to the current.

Resistors are used as part of electrical networks and electronic circuits. They are extremely commonplace in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

PCB

PCB is an interactive printed circuit board editor for the X11 window system. PCB includes a rats nest feature, design rule checking, and can provide industry standard RS-274-X (Gerber), NC drill, and centroid data (X-Y data) output for use in the board fabrication and assembly process. PCB offers high end features such as an autorouter and trace optimizer which can tremendously reduce layout time.

Capacitors

A capacitor or condenser is a passiveelectronic component consisting of a pair of conductors separated by a dielectric. When a voltagepotential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated conductors.

Transistors

A transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals.The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. Some transistors are packaged individually but most are found in integrated circuits.

TRIAC

A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. The formal name for a TRIAC is bidirectional triode thyristor.

Heatsink

A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using thermal contact (either direct or radiant). Heat sinks are used in a wide range of applications wherever efficient heat dissipation is required; major examples include refrigeration, heat engines, cooling electronic devices and lasers.

Drill bits

Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes.

Again??? With Application???

Interface in Computer Science Field

Interface generally refers to an abstraction that an entity provides of itself to the outside. This separates the methods of external communication from internal operation, and allows it to be internally modified without affecting the way outside entities interact with it, as well as provide multiple abstractions of itself. It may also provide a means of translation between entities which do not speak the same language, such as between a human and a computer. Because interfaces are a form of indirection, some additional overhead is incurred versus direct communication.

The interface between a human and a computer is called a user interface. Interfaces between hardware components are physical interfaces. This article deals with software interfaces, which exist between separate software components and provide a programmatic mechanism by which these components can communicate.

Interfaces in practice

A piece of 'software' provides access to computer resources (such as memory, CPU, storage, etc.) by its underlying computer system; the availability of these resources to other software can have major ramifications—sometimes disastrous ones—for its functionality and stability. A key principle of design is to prohibit access to all resources by default, allowing access only through well-defined entry points, i.e. interfaces.^{[citation needed]}

The types of access that interfaces provide between software components can include: constants, data types, types of procedures, exception specifications and method signatures. In some instances, it may be useful to define variables as part of the interface. It often also specifies the functionality of those procedures and methods, either by comments or (in some experimental languages) by formal logical assertions.

The interface of a software module $A$ is deliberately kept separate from the implementation of that module. The latter contains the actual code of the procedures and methods described in the interface, as well as other "private" variables, procedures, etc.. Any other software module $B$ (which can be referred to as a client to A) that interacts with $A$ is forced to do so only through the interface. One practical advantage of this arrangement is that replacing the implementation of $A$ by another one that meets the same specifications of the interface should not cause $B$ to fail—as long as its use of $A$ complies with the specifications of the interface .

Friday, February 27, 2009

computer interfacing☺

Computer Interfacing

This week we tackled about computer interfacing and the parallel printer port.

The Parallel Printer Port

The PC parallel printer port is an interface between the computer and the printer. It is called parallel because the data is sent in parallel or by group of bits at a time unlike serial flow wherein data is sent one bit at a time.

While the parallel port is designed specifically for the printer, it can also be used as an interface to other electronic/electrical devices.

Introduction to Parallel Ports

The Parallel Port is the most commonly used port for interfacing home made projects. This port will allow the input of up to 9 bits or the output of 12 bits at any one given time, thus requiring minimal external circuitry to implement many simpler tasks. The port is composed of 4 control lines, 5 status lines and 8 data lines. It's found commonly on the back of your PC as a D-Type 25 Pin female connector. There may also be a D-Type 25 pin male connector. This will be a serial RS-232 port and thus, is a totally incompatible port.

Newer Parallel Port’s are standardized under the IEEE 1284 standard first released in 1994. This standard defines 5 modes of operation which are as follows,

The aim was to design new drivers and devices which were compatible with each other and also backwards compatible with the Standard Parallel Port (SPP). Compatibility, Nibble & Byte modes use just the standard hardware available on the original Parallel Port cards while EPP & ECP modes require additional hardware which can run at faster speeds, while still being downwards compatible with the Standard Parallel Port.

Compatibility mode or "Centronics Mode" as it is commonly known, can only send data in the forward direction at a typical speed of 50 kbytes per second but can be as high as 150+ kbytes a second. In order to receive data, you must change the mode to either Nibble or Byte mode. Nibble mode can input a nibble (4 bits) in the reverse direction. E.g. from device to computer. Byte mode uses the Parallel's bi-directional feature (found only on some cards) to input a byte (8 bits) of data in the reverse direction.

1. Write the byte to the Data Port.

2. Check to see is the printer is busy. If the printer is busy, it will not accept any data, thus any data which is written will be lost.

3. Take the Strobe (Pin 1) low. This tells the printer that there is the correct data on the data lines. (Pins 2-9)

4. Put the strobe high again after waiting approximately 5 microseconds after putting the strobe low. (Step 3)

This limits the speed at which the port can run at. The EPP & ECP ports get around this by letting the hardware check to see if the printer is busy and generate a strobe and /or appropriate handshaking. This means only one I/O instruction need to be performed, thus increasing the speed. These ports can output at around 1-2 megabytes per second. The ECP port also has the advantage of using DMA channels and FIFO buffers, thus data can be shifted around without using I/O instructions.

Hardware Properties

Below is a table of the "Pin Outs" of the D-Type 25 Pin connector and the Centronics 34 Pin connector. The D-Type 25 pin connector is the most common connector found on the Parallel Port of the computer, while the Centronics Connector is commonly found on printers. The IEEE 1284 standard however specifies 3 different connectors for use with the Parallel Port. The first one, 1284 Type A is the D-Type 25 connector found on the back of most computers. The 2nd is the 1284 Type B which is the 36 pin Centronics Connector found on most printers.

IEEE 1284 Type C however, is a 36 conductor connector like the Centronics, but smaller. This connector is claimed to have a better clip latch, better electrical properties and is easier to assemble. It also contains two more pins for signals which can be used to see whether the other device connected, has power. 1284 Type C connectors are recommended for new designs, so we can look forward on seeing these new connectors in the near future.

Pin No (D-Type 25)	Pin No (Centronics)	SPP Signal	Direction In/out	Register	Hardware Inverted
1	1	nStrobe	In/Out	Control	Yes
2	2	Data 0	Out	Data
3	3	Data 1	Out	Data
4	4	Data 2	Out	Data
5	5	Data 3	Out	Data
6	6	Data 4	Out	Data
7	7	Data 5	Out	Data
8	8	Data 6	Out	Data
9	9	Data 7	Out	Data
10	10	nAck	In	Status
11	11	Busy	In	Status	Yes
12	12	Paper-Out / Paper-End	In	Status
13	13	Select	In	Status
14	14	nAuto-Linefeed	In/Out	Control	Yes
15	32	nError / nFault	In	Status
16	31	nInitialize	In/Out	Control
17	36	nSelect-Printer / nSelect-In	In/Out	Control	Yes
18 - 25	19-30	Ground	Gnd

Table 1. Pin Assignments of the D-Type 25 pin Parallel Port Connector.

The above table uses "n" in front of the signal name to denote that the signal is active low. e.g. nError. If the printer has occurred an error then this line is low. This line normally is high, should the printer be functioning correctly. The "Hardware Inverted" means the signal is inverted by the Parallel card's hardware. Such an example is the Busy line. If +5v (Logic 1) was applied to this pin and the status register read, it would return back a 0 in Bit 7 of the Status Register.

The output of the Parallel Port is normally TTL logic levels. The voltage levels are the easy part. The current you can sink and source varies from port to port. Most Parallel Ports implemented in ASIC, can sink and source around 12mA. However these are just some of the figures taken from Data sheets, Sink/Source 6mA, Source 12mA/Sink 20mA, Sink 16mA/Source 4mA, Sink/Source 12mA. As you can see they vary quite a bit. The best bet is to use a buffer, so the least current is drawn from the Parallel Port.

wHats nEwWw????..hhhHHHmmmMMM

Computer Interfacing

The art of connecting computers and peripherals. In a lot of circumstances, it looks more like magic than art. It is not uncommon that you end up removing all unnecessary hardware from your computer to get that communication device to work. Despite all plug-and-play internal hardware solutions for the PC, connecting a number of external devices still requires some amount of technical knowledge and experience.

Cable information

In computer interfacing it is often difficult to find the right cable for a specific purpose. Although the USB interface tries to solve this problem, there are many situations where you need to search for the right cable. This can be the case when you need a RS-232 or parallel cable to connect a device to your computer. There is also information about modular cables and cables for connecting PLC's if you happen to work in the industrial automation business.

General interfacing information

Interfacing is not just about cables. In a lot of circumstances you need also background information, to select the right null modem cable for example. These pages contain detailed—yet readable—information about RS-232, RS-422, RS-423, RS-485, USB and how interfacing in the PC is organized using I/O ports and interrupts and UART's. Also ASCII is covered. After all this standard made computer interfacing possible in the first place.