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Unit - V Hardware Layout of IBM PC/XT
Unit I
Unit II
Unit III
Unit IV
Unit V
Q) What is device controller? List main function performed by device controller?
→ Any I/O device is connected to a CPU through a controller is called as device controller. All the operation or function performed by device is controlled by this device controller. Following are functions performed by device controller.
1) Interfacing with computer system.
2) Interfacing with I/O device.
3) It receives command from computer.
4) It decodes the command.
5) Generates the control signal for I/O device.
6) Transfer the data from device to computer.
7) Converts data from one format to another format.
8) Error checking of data.
9) Interrupting the CPU for data transfer.
Q) Draw a block diagram of hard disk controller? Explain the function of sector buffer and ECC logic in hard disk controller (HDC).
The HDC supports two HDDS. There is no uniformity in LSI used in HDC. Most PC use 1010 IC and some custom LSI’s for additional circuits. Conceptually HDC is similar to an identical to FDC. It has certain additional circuit which are not present in FDC, these are
1) Sector buffer
2) ECC logic
3) RETRY logic
4) Diagnostic logic.
The sector buffer is used to store the data bytes of one full sector both during the read operation and write operation. During write operation the data bytes received from memory in DMA mode are stored in a sector buffer. The HDC takes this bytes from sector buffer serializes them into data bits mixes them with clock bits and sends the MFM write data to the hard disk drive. During read operation the MFM read data consists of data bits and clock bits. The HDC separates them, converts the data bits into parallel bytes and stores them into sector buffer from sector buffer this data are transferred in DMA mode. Due to sector buffer the DMA transfer and hard disk data transfer are isolated.
The ECC logic is a general 32 bit ECC pattern for data bits which is also written in the sector along with data byte. During read operation the HDC checks the ECC pattern and verify whether the data bytes are error free or not. If there is any error then HDC corrects. The ECC is used for the data field in each sector where CRCC is used for ID field in each sector. The retry logic is used to retry an operation an encounting an error.
The diagnostic logic performs various tests on HDC circuits to detect malfunction. It also tests the hard disk media.
Q) What is multi serial I/O card?
It is an enhanced version of FDD controller and is provided with 34 pin male type COM B connector for driving to 360 KB FDDS. The card is also provided with two serial ports. It consist of two 10 pin COMB type male connector defined as COM 1 and COM 2 on ANSI 1 or ANSI 2. The 16 pin D type female port is provided or real panel of the card called as game port. Just below this port a 25 pin D type female connection is provided which is normally used for parallel printed called LPT 2.
The hexadecimal address of this port is 3F811 this card contain IC 8250, IC 82C11, IC 8298 and IC 68167 which is CMOS chip for clock and calendar jumpers are provided for setting the port. It also provides an oscillator and blank slots for 8250, 1488, 1489.
For storing clock and calendar information 34 volts NiCad battery is provided.

Q) What is motherboard?
→ Motherboard is the largest circuit board inside the computer. It contains the computer CPU (Central Processing Unit) memory (RAM and ROM) and various support chip for the CPU. Motherboard also contains many expansion slots on which you can connect smaller circuit boards to interface different devices such as monitor, printer, sound card etc with computer.
Mother Board History:-
Initially when IBM decided to make personal computer two different types of technology where being used to make personal computer. They are,
1) Bus Based Computer
2) Single Board Based Computer.
The motherboard is the large circuit board inside your computer's case. It's sometimes called the system board, the logic board, the baseboard, or less commonly, the planar board. Everything connected to your computer system, plugs either directly or indirectly into the motherboard.
I'm sure everyone has heard the CPU, or Central Processing Unit, referred to as the 'brain' of your computer. Well, the CPU sits on the motherboard, and if it can be called the brain of your computer then the motherboard is truly the central nervous system. The motherboard contains the CPU, the BIOS ROM chip (Basic Input/Output System), and the CMOS Setup information. It has expansion slots for installing different adapter cards like your video card, sound card, Network Interface Card, and modem. This circuit board provides a connector for the keyboard as well as housing the keyboard controller chip. It has RAM slots for your system's Random Access Memory (SIMMs or DIMMs), and provides the system's chipset, controllers, and underlying circuitry (bus system) to tie everything together.
The motherboard, more or less, is your computer. It defines your computer type, upgradeability, and expansion capability.
Non-Integrated Motherboards
Non-Integrated Motherboards have assemblies such as the I/O Port connectors (serial and parallel ports), hard drive connectors or paddle boards, floppy controllers and connectors, joystick connections, etc. installed as expansion boards. This takes up one or more of the motherboard's expansion slots and reduces the amount of free space inside your computer's case. Hence, the individual motherboard is relatively cheap to produce but, because of the cost of manufacturing, testing, and installing the expansion boards separately, there's an added cost to the computer system. However, if something should go wrong with the individual assemblies, such as a bent or broken pin in a connector, or a defective controller chip etc., you could repair the problem by replacing the individual expansion card at a relatively minor cost.
Most of the older motherboards were Non-Integrated. Some of the later 486 system boards began to integrate some of these assemblies right onto the circuit board.
Integrated Motherboards
Integrated Motherboards have assemblies that are otherwise installed as expansion boards, integrated or built right onto the board. The serial and parallel ports, the IDE and floppy drive, and joystick all connect directly to the motherboard. This is now standard on any late model 486 and above. It tends to free up some space inside the case and allows for better accessibility and air flow. The systems are cheaper to produce because there's less material involved, less installation, and testing can all be done at the same time. They are more expensive to repair because, if you end up with a controller failure or broken pin, it means a new motherboard (and, of course, because of the added assemblies, the motherboard can be more expensive than its non-integrated counterpart). However, these particular integrated assemblies are generally fairly stable and although problems can occur, they tend to be fairly rare.
All in all, the integrated motherboard tends to be a good thing as opposed to the 'Embedded Motherboard'.
Bus Board Computer:-
In this bus based approach a main board is used as backbone of computer system.
• This main board contains a no. of buses as connecting lines along which electric/electronic signal can be sent from one plane to another.
• It also contains a no. of slots providing connection to these buses.
• Basically the data, address, control and power signals where provided on these buses.
When an expansion board is connected to any of these expansion slots, the board gets connected to the bus and becomes a part of total system. The main advantage of this type of approach is.
• The system can be expanded as and when required.
• Different parts of computer can be purchased as and when the need arises and connected to the bus through the expansion slots.
• This makes the maintenance of system very easy whenever any part falls, the board on which that part is located can be removed and easily replaced with a working board.
As a common bus links the entire component together this system is called bus oriented system.
Single Board Based Computers
The other approach in computer design is a single board based system,
2) On these systems a single board contains all the chips and circuits required to make a complete computer system.
3) This type of computer system are easy to design and manufacturer. It cost less compared to bus type computer.
4) The type of system design was very common among the home computers and video game computer.
Disadvantages of these type of system was
1) Most of this system had no option to connect any additional device other that what is available on board itself.
2) Their was no scope of upgrading or adding any new device introduced in the market into these system.
3) Being a single board system they were difficult to maintain. Even if one component fails the user have to either discard the complete motherboard or spend a lot of amount and time on troubleshooting and maintenance.
Q) Draw and explain the motherboard of an IBM PC system and explain, 1) Expansion slots
2) CPU
3) Co-processor
4) Memory (RAM and ROM)
NOTE: - When IBM decided to make personal computer they decided to use the good feature of both bus based as well as single board based design.
→ The main board of IBM computer follows the approach of a single board based computer. This board contains almost all important parts of computer such as main processor, memory etc.
But at the same time IBM has provided expansion slots on main board to connect any additional device to the system.
This design concept gives best of the both world.
1) Keeping most of the parts on the main board itself keeps the cost down and makes them more reliable.
2) The slots on main board provide facility for future upgrade.
Q) What is Expansion Slots?
Definition: BUS -
A set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer.
Expansion Slot (connector)
Remember that the expansion bus, or external bus, is made up of the electronic pathways that connect the different external devices to the rest of your computer. These external devices (monitor, telephone line, printer, etc.) connect to ports on the back of the computer. Those ports are actually part of a small circuit board or 'card' that fits into a connector on your motherboard inside the case. The connector is called an expansion slot.
Note: Communication ports (com ports), printer ports, hard drive and floppy connectors, etc., are all devices which used to be installed via adapter cards. These connectors are now integrated onto the motherboard, but they are still accessed via the expansion (external) bus and are allocated the same type of resources as required by expansion cards. As a matter of fact (and unfortunately, in my opinion), other devices like modems, video technology, network and sound cards are now being integrated, or embedded, right onto the motherboard.
Expansion slots are easy to recognize on the motherboard. They make up a row of long plastic connectors at the back of your computer with tiny copper 'finger slots' in a narrow channel that grab the metal fingers or connectors on the expansion cards. In other words, the expansion cards plug into them. The slots attach to tiny copper pathways on the motherboard (the expansion bus), which allows the device to communicate with the rest of the computer. Each pathway has a specific function. Some may provide voltages needed by the new device (+5, +12 and ground), and some will transmit data. Other pathways allow the device to be addressed through a set of I/O (input/output) addresses, so that the rest of the computer knows where to send and retrieve information. Still more pathways are needed to provide clock signals for synchronization and other functions like interrupt requests, DMA channels and bus mastering capability.
As with any other part of the computer, technology has evolved in an effort to increase the speed, capability and performance of expansion slots. Now you'll hear about more busses - PCI bus, ISA bus, VESA bus, etc. Not to worry! These are all just types of expansion (external) busses. They just describe the type of connector and the particular technology or architecture being used. Thus, the adapter card being installed must match the architecture or type of slot that it's going into. An ISA card fits into an ISA slot, a PCI adapter card must be installed into a PCI expansion slot, etc.
→ The expansion slots are long thin connections on motherboard near the backside of the computer various expansion card are connected to the motherboard through the data address and control lines/buses on these slots.
Bus is an electronic path on which signal are sent from one part of computer to another.
• One can connect various expansion cards such as display card, hard drive controller, sound card, network card, modem cards etc on these slots.
• When an expansion card is connected to expansion slot it is actually connected to data, address and control bus of motherboard.
These buses are categorized according to no. of binary bits that they can transfer from one place to another at a time. If a data bus is a 8 bit wide then it can transfer 8 bit information at a time and it is called 8 bit data bus.
Another very common term while talking about bus is bandwidth. The bandwidth of bus measure of data that can feed in the bus at a given time depending upon the width and technology the expansion slot bus can be divided into following categories.
a) 8 bit ISA
b) 16 bit ISA
c) MCA
e) VESA local bus
f) PCI local bus
g) PCI express bus
h) AGP bus
8-Bit Expansion Bus
Early IBM PCs and XTs used the 8-bit expansion bus. There were only eight data lines running from the processor to the expansion connectors. It was a single long channel connector with 62 metal "finger connectors" or channels. The bus speed was from 4.77 MHz to 8 MHz. It provided eight interrupts and four DMA channels, all of which were pretty well pre-assigned. It was configured using jumpers and DIP switches.
16-Bit ISA Bus
The PC AT ( I believe it stands for Advanced Technology) hit the markets with a new bus, the Industry Standard Architecture bus (ISA - pronounced "ice-ah"). Also called the AT bus, it had a 16-bit data path for the new 16-bit processors. It provided eight more interrupts (16) and four additional DMA channels (8). Many were pre-assigned, but at least now there were some open for expansion. It had a bus speed of 8 MHz and was capable of using 1 bus-mastering device. The 16-bit ISA slot was characterized by 2 separate channel slots, a shorter one in front of the typical 8-bit slot to create the 16-bit connector. This allowed for backward compatibility with the older technology. A couple of 8-bit slots were often still found on some motherboards. This was perhaps because there were some older cards that had a plastic skirt around the connectors and the second 16-bit slot was in the way.
The 16-bit ISA bus can be found in 286, 386, 486 and Pentium computers. Near the end of the 486's reign, the PCI bus was introduced and adopted. Late 486s and, of course, all Pentiums rely on the PCI bus, but so many legacy ISA cards are out there (and still being made) that most Pentium motherboards still have a couple of 16-bit ISA slots on them for compatibility with older expansion cards.
ISA expansion cards were assigned the proper IRQs and DMA channels through jumpers on the card itself. However, later ISA cards made use of the discovery that you could put jumper settings into an EEPROMM chip, and some devices could be configured using software programs. Some ISA cards being manufactured now are capable of PNP.
ISA architecture also allowed for the fact that faster CPUs were now being developed. The CPU was given an internal clock or multiplier that was dissociated with the bus clock allowing each to run at its rated speed.
Micro-Channel Architecture
IBM came out with a new 32-bit bus architecture that had a speed of 10MHz. The Micro-Channel Architecture (MCA) bus was capable of using multiple bus mastering devices and could be configured using software. A big improvement over jumpers and DIP switches. Actually you needed two disks. One was a "Reference" disk, which allowed access to the configuration program, and the other was an "Options" disk, which provided the options available.
Actually a very good bus, IBM didn't want to share, and the MCA bus was only available on IBM machines. Because of the millions of computers out there with ISA bus architecture, manufacturers probably saw more money in developing a more 'generic' expansion card that would fit in any clone, as opposed to manufacturing a highly proprietary card that would only fit in one type of machine. The number of makes, models, and types of devices for MCA was limited.
Extended ISA Bus (EISA)
As a result of the advanced architecture of the MCA, several different companies put their heads together and came up with their own version, the EISA (pronounced 'ee-sah') bus. It has a 32-bit data path and is capable of using multiple bus mastering devices. The EISA bus has no need for interrupts or DMA channels and is configured with software, using a configuration utility and a device specific program. It still only has an 8 MHz bus speed.
The socket itself is taller than the 16-bit ISA. The EISA expansion cards have two horizontal rows of metal contacts on their edge connector, and there are two corresponding horizontal rows of metal 'fingers' in the socket. If you place an ISA card in an EISA socket, it doesn't fit all the way down, and its contacts only reach the top row of metal 'fingers'. This makes it completely backward compatible with the ISA cards. Although it wasn't completely proprietary, it was found mainly in brand name computers. I can't remember seeing one in a clone, or a custom built computer. As a result, there isn't a large number of EISA devices on the market.
VESA Local Bus (VLB)
The idea of a local bus came from the need for a bus that could keep up with the faster CPUs. A local bus connects directly to the processor and operates at the same speed as the CPU externally (not multiplied). The Video Electronics Standards Association (VESA) developed a local bus that had a 32-bit data path. It was built on the ISA architecture, but was longer, with a third connector that had all its lines running directly to the processor.
It was usually used for video cards, I/O cards and multimedia expansion cards. There couldn't be more than 3 VLB slots on a motherboard because the processors couldn't keep up with the transfer. Often, one of the slots was a shared slot. This meant that if the ISA slot beside it was being used, the VLB slot beside it had to be left empty. If the VESA slot had a card in it, the ISA slot beside it could not be used.
Peripheral Component Interconnect (PCI)
The PCI bus was introduced with the Pentium computer. The 16-bit and 32-bit bus architecture would limit the performance of the 64-bit Pentiums. The PCI bus supports both 32 and 64-bit data paths and uses a chipset that will also support ISA and EISA architectures. This means that the PCI bus can be used for both 486 computers and Pentiums, and motherboards can have a combination of PCI and ISA or EISA slots.
The PCI bus communicates with the processor through a bridge circuit, which acts kind of like an interpreter. This means that it can be processor independent. It can work with CISC or RISC technologies as long as it has the proper bridge circuit to interpret the information.
PCI has a 33 MHz bus speed and can support multiple bus mastering devices. The cards are Plug-and-Play and come in two versions, 5Vdc and 3.3 Vdc. The slots are keyed differently and will not allow the wrong voltage card to be inserted.
USB (Universal Serial Bus)
Universal Serial Bus is a relatively new bus technology. It was designed for low to mid-speed peripherals such as scanners, keyboards, mice, joysticks, printers, modems and some CD-ROMs. USB boasts the ability to daisy-chain up to 127 devices. This means that you could have a joystick plugged in, with a printer plugged into that, and a scanner plugged into the printer, etc...
USB is Plug-and-Play, and is completely "hot-swappable". In other words, devices can be plugged in, and unplugged while the computer is turned on and running
USB was first introduced with new computers around 1997 and the final version of Win95 (SR2) provided very limited support for it. A few problems seemed to develop at first. You had to have a Pentium machine with a BIOS that supported USB, and it had to be enabled in the setup. Your computer had to have USB ports on it, or pins that allowed for the attachment of a USB interface. Aside from that, you could install a USB adapter card in one of your PCI slots.
Some problems did arise at first. One problem was the Operating System. The early versions of Win95 did not support USB. Also, a lot of machines shipped with USB ports or capabilities before the BIOS supported it completely. Updating, or flashing the BIOS could sometimes solve the problem. Despite the growing pains, Windows 98 and computers shipped after 1998 provide excellent support for USB; and the number of devices have increased dramatically.
PCMCIA stands for Personal Computer Memory Card International Association. It's often just called a PC Card bus and is used in laptops and notebook computers to add external devices such as modems, network cards, memory and removable hard drives. The bus has a 16-bit data path and only supports one IRQ. It is software configurable, using what is called Card and Socket Services software.
There are 3 different types of PC Cards and sockets. Type I is 3.3mm thick and has a single row of connectors. It was used mainly for add-on memory cards and isn't found in newer laptops. Type II is thicker (5mm) and has two rows of connectors. This is the most common and is used for modems and network interface cards. Type III cards aren't found much anymore either. They're thicker still, (up to 10.5mm) and have three or four rows of connectors. They're used mainly for adding an external hard drive to a laptop or notebook computer.
Most laptops today have two type II sockets.
The main component of any motherboard is the main component of chip which controls all the inner function of the system. The CPU functions as the brain of every PC.
The CPU is usually inserted into the socket provided for it on motherboard and is not soldered onto the motherboard, this makes its replacement in case of any problem. Another advantage of having a socketed CPU is, one can choose the CPU according to once requirement and budget some of the common CPU chips by INTEL are 8088, 8086, 80286, 80386, 80486, Pentium, Pentium II, Pentium III, Pentium IV.
It is special purpose MP, which is used to speed up main processor job by taking over some of the main processors work. Most common type of co-processor is match co-processor and graphic co-processor chips are used to help the main processor in carrying out its function. The math co-processor helps main processor in performing mathematical calculation. A graphic co-processor helps the main processor is video/graphic related operation. Older system (XT, AT, AT 386) required a co-processor chips to be inserted into special socket on the motherboard but current generation of CPU has math co-processor built inside a main processor.
Memory is the place where computer stores the program and data helps the program in carrying out operation.
For Example: - to print student mark list computer will require mark sheet printing program, student Roll number, name and marks obtained in various subjects and data. This program and data kept in memory there is basically 2 types of memory used in computer.
1) RAM 2) ROM
RAM as Random Access Memory is a read or write type of memory which is used by processor to keep program, data and intermediate result during program execution. It is a volatile type of memory i.e. it loose its content when power supply to it is switched off. The physical installation of RAM memory on motherboard can take place in various ways.
An acronym for Read Only Memory, ROM is computer memory on which data has been pre-recorded. The programming code and/or data on a ROM chip is written to the chip at the factory. It can be read, but it cannot be erased or removed. It's permanent. ROM retains its data or content even when the computer is turned off, unlike a computer's main memory (RAM), which needs a constant charge of electricity to keep its information. For this reason, ROM is considered to be 'non-volatile' and RAM is 'volatile'.
ROM chips are used in all kinds of electronic devices from calculators to video games. Most personal computers have several applications of ROM memory. These chips often store permanent and critical information and programs that don't need to be changed, or don't need to be written to. Most personal computers have a small amount of ROM that stores the code that starts up or boots the computer. Early computers also used ROM to store the BIOS (Basic Input Output System) which acts as a translator between the PC's hardware and the operating system.
The problem with using these ROM chips for BIOS information is that computer manufacturers had to build their systems around the available fabricated ROMs and their coding. Because the information was written to the chip during the fabrication process, changes to the chip would involve developing new assembly lines and purchasing new equipment. It would cost a small fortune if a single computer or motherboard manufacturer had visions of having the coding on these chips changed to accommodate new developments or enhancements they might want to incorporate into their product.
EPROM (Erasable Programmable Read Only Memory)
An EPROM is a special kind of PROM chip that can be reprogrammed. Its information is stored as electrical charges deposited on the chip (1s and 0s). EPROMs are easy to spot on your computer's motherboard. They're still in the form of a DIP chip like ROMs and PROMs, but they'll have a metallic-like label on top of the chip (usually displaying the serial number, version, date and manufacturer's name). This is for good reason. The label is covering a small window.
An EPROM can be erased by removing it from the circuit, and shining an ultraviolet light through the window on top of the chip. It can then be reprogrammed using an EPROM burner. EPROMs are still considered non-volatile, they won't lose their information when the computer is turned off.
(Electronically Erasable Programmable Read Only Memory)
(Pronounced 'double e-prahm')
EEPROM doesn't have to be removed and sent back to the manufacturer to be reprogrammed. It can be electronically reprogrammed while in circuit using a software program designed specifically for that purpose. Also, EPROM has to be erased entirely and then reprogrammed. With EEPROM, a single byte can be erased and re-written. In fact, EEPROM is erased and written one byte at a time, which makes it quite slow as memory goes. Still, it allows manufacturers the ability to put configuration settings on an expansion card's EEPROM chip. By using software that came with a device, DMA channels, IRQs and I/O addresses can be assigned without the use of jumpers and DIP switches. The resource settings for software configurable devices can be changed without even opening the computer's case.
FLASH ROM (Flash Read Only Memory)
FLASH ROM is a type of EEPROM, but its information can be erased and written to in blocks instead of single bytes. This tends to make it faster than regular EEPROM. It also requires less voltage to perform the procedure.
FLASH ROM is now commonly used to store the BIOS information for personal computers. This allows BIOS manufacturers the opportunity to provide updates via the Internet, and it allows users to possibly gain access to new features that weren't originally supported by their computer.
It's important to get the right Flash Program for each particular BIOS. Don't use one you got from a friend or some unknown website. Go to the manufacturer's website and have your BIOS and motherboard's version and model number, serial number and date handy. Print out the instructions and follow them closely. Check out any information on backing up and restoring your previous BIOS if something should go wrong.
Flashing the BIOS should not be done every time an upgrade is available. In my opinion, it should only be done when a required feature is not supported and the only other recourse is a new motherboard, because if things go wrong, that could be what you're replacing. Don't flash it just for the sake of flashing it. If you don't need the upgrade, don't flash your BIOS. Making a mistake in the procedure, losing power, or using the wrong image file could be disastrous
Difference between Assembly Language and Machine Language

Optical Disk:
As compared to magnetic tape and magnetic disk, optical disk is a relatively new secondary storage medium. During the last few years, it has proved to be a promising random access medium for high capacity secondary storage, because it can store extremely large amount of data in a limited space.
An optical disk storage system consists of a rotating disk which is coated with a thin metal or some other material that is highly reflective. Laser beam technology is used for recording/reading of data on the disk. Due to the use of laser beam technology, optical disks are also known as laser disks or optical laser disks.
Storage Organization: - Unlike magnetic disk which has several concentric tracks, an optical disk has one long track which start at the outer edge and spirals inward to the centre. This spiral track is ideal for reading large blocks of sequential data such as music.
The track of an optical disk is split up into sectors, but with optical disks each sector has the same length regardless of whether it is located near the disks centre or away from the centre. This type of data organization allows data to be packed at maximum density over the entire disk. However it also requires a more complicated drive mechanism because the rotation speed of the disk must vary inversely with the radius. The drive must slow down the disk’s rotation speed to read sectors towards the outside of the disk and speed it up to read sectors towards the centre of the disk.
The cost per bit of storage is very low for optical disks, because of their low cost and enormous storage density. They come in various sizes ranging from 12 inch to 4.7 inch diameter. The most popular one is of 5.25 inch diameter whose capacity is around 650 megabytes. This storage capacity is equivalent to about 2, 50,000 pages of printed text, or total capacity of 550 double sided, high density floppy disk of the same size.
How the data can be written on optical disk data storage?
Most current read or write optical disk systems use desks coated with an exotic metal alloy which has the required magnetic properties. The read /write head in this type of system has a laser diode a coil of wire. A current is passed through the coil to produce a magnetic field perpendicular to the disk. At room temperature the applied magnetic field is not strong enough to change the horizontal magnetization present on the disk. To record a one at a spot in a data track, a pulse of light from the laser diode is used to heat up that spot. Hitting the spot makes it possible for the applied magnetic field to flip the magnetic domain around at that spot and create a tiny vertical magnet. This is called magneto-optical or MO reading.

The internal Block Diagram of 8272 is as shown in figure.
1) Data transfer between MP/Memory and FDC is performed in parallel part using non DMA/DMA data transfer. But data transfer between 8272 and floppy disk read write head is performed in serial form.
2) Input Control Pins:-
ii) RDY: RDY=1 indicates disk drive door is closed as floppy disk drive is ready for operation.
iii) WR Protected: - In read write mode (RW¯/Seek=0) FDD gives write protected = 1 to indicate that write protected notch is closed i.e. FD is write protected. In seek mode (RW¯/Seek = 1), FDD gives 2 side=1 to indicate that head drive is double sided.
iv) Index: - FDD gives index=1 to indicate that index is passed between LED and photo diode transistors i.e. it indicates start of track in section 0.
v) Fault/Track 0 :- In read write mode if there is any fault in FDD then fault flip flop is set, so 8272 receives fault=1 which indicate fault is head drive.
In seek mode FDD gives track 0=1 to indicate that read
write head is over track number 0.
3) Output Control Pins :-
i) Drive Select (0 to 3):- There are two bits externally decoded to select 1 out of 4 floppy disk drive.
ii) MFM Mode (Modified Frequency Modulation):-
MFM Mode = I/O indicates that FDD should store data on disk in MFM (double density)/FM (single density) mode.
iii) RW¯/Seek: - When 8272 operates in read write/seek mode than it gives RW¯/Seek 0/1 respectively to inform FDD.
iv) Head Load: - When Read write head is over required track than 8272 gives head load=1, so FDD will put read write head in virtual contact with the disk to start reading writing operation.
vi) Head Select: - If head select=0/1 than FDD will select head number 0/1 to perform read write operation.
4) Data Writing Pins:-
1) WR Clock: - Writing frequency clock signal is applied on this pin.
2) WR Data: - The serial output data bits for storing on disk are transferred through this pin.
3) WR Enable: - It is used to enable write head control logic.
4) Preshift O/I: - It is used to inform FDD to shift bits either forward or backward when they are stored on disk.
5) Data Read Pins:-
i) Read data: - Serial data is input from disk through this pin.
ii) UCO Sync: - UCO sync = 1 is used to enable external phase locked loop (PLL) CIRCUIT.
iii) Read Window: - The speed of data transfer from outer track should be less and from inner track should be less. Whenever PLL gives logic 1 pulse on read window than 8272 will read serial data from read data pins.
By 1935, message routing was the last great barrier to full automation. Large telegraphy providers began to develop systems that used telephone-like rotary dialing to connect teletypes. These machines were called "Telex" (TELegraph EXchange). Telex machines first performed rotary-telephone-style pulse dialing for circuit switching, and then sent data by Baudette code. This "type A" Telex routing functionally automated message routing.
The first wide-coverage Telex network was implemented in Germany during the 1930s as a network used to communicate within the government.
At the rate of 45.45 (±0.5%) baud — considered speedy at the time — up to 25 telex channels could share a single long-distance telephone channel by using voice frequency telegraphy multiplexing, making telex the least expensive method of reliable long-distance communication.
Canada-wide automatic teleprinter exchange service was introduced by the CPR Telegraph Company and CN Telegraph in July 1957 (the two companies, operated by rivals Canadian National Railway and Canadian Pacific Railway, would join to form CNCP Telecommunications in 1967). This service supplemented the existing international Telex service that was put in place in November 1956. Canadian Telex customers could connect with nineteen European countries in addition to eighteen Latin American, African, and trans-Pacific countries.The major exchanges were located in Montreal (01), Toronto (02), and Winnipeg (03).
In 1958, Western Union started to build a Telex network in the United States. This Telex network started as a satellite exchange located in New York City and expanded to a nationwide network. Western Union chose Siemens & Halske AG, now Siemens AG, and ITT to supply the exchange equipment, provisioned the exchange trunks via the Western Union national microwave system and leased the exchange to customer site facilities from the local telephone company. Teleprinter equipment was originally provided by Siemens & Halske AG and later by Teletype Corporation. Initial direct International Telex service was offered by Western Union, via W.U. International, in the summer of 1960 with limited service to London and Paris.
In 1962, the major exchanges were located in New York City (1), Chicago (2), San Francisco (3), Kansas City (4) and Atlanta (5).The Telex network expanded by adding the final parent exchanges cities of Los Angeles (6), Dallas (7), Philadelphia (8) and Boston (9) starting in 1966.
The Telex numbering plan, usually a six-digit number in the United States, was based on the major exchange where the customer's Telex machine terminated.For example, all Telex customers that terminated in the New York City exchange were assigned a Telex number that started with a first digit "1". Further, all Chicago based customers had Telex numbers that started with a first digit of "2". This numbering plan was maintained by Western Union as the Telex exchanges proliferated to smaller cities in the United States. The Western Union Telex network was built on three levels of exchanges.The highest level was made up of the nine exchange cities previously mentioned. Each of these cities had the dual capability of terminating both Telex customer lines and setting up trunk connections to multiple distant Telex exchanges. The second level of exchanges, located in large cities such as Buffalo, Cleveland, Miami, Newark, Pittsburgh and Seattle, were similar to the highest level of exchanges in capability of terminating Telex customer lines and setting up trunk connections. However, these second level exchanges had a smaller customer line capacity and only had trunk circuits to regional cities. The third level of exchanges, located in small to medium sized cities, could terminate Telex customer lines and had a single trunk group running to its parent exchange. Loop signaling was offered in two different configurations for Western Union Telex in the United States. The first option, sometimes called local or loop service, provided a 60 milliampere loop circuit from the exchange to the customer teleprinter. The second option, sometimes called long distance or polar was used when a 60 milliampere connection could not be achieved, provided a ground return polar circuit using 35 milliamperes on separate send and receive wires. By the 1970s, and under pressure from the Bell operating companies wanting to modernize their cable plant and lower the adjacent circuit noise that these Telex circuits sometimes caused, Western Union migrated customers to a third option called F1F2. This F1F2 option replaced the DC voltage of the local and long distance options with modems at the exchange and subscriber ends of the Telex circuit. Western Union offered connections from Telex to the AT&T Teletypewriter eXchange (TWX) system in May 1966 via its New York Information Services Computer Center.[ These connections were limited to those TWX machines that were equipped with automatic answerback capability per CCITT standard.
In 1970, Cuba and Pakistan were still running 45.5 baud type A Telex.Telex is still widely used in some developing countries' bureaucracies, probably because of its reliability and low cost. The UN asserted at one time that more political entities were reliably available by Telex than by any other single method. Around 1960[?], some nations began to use the "figures" Baudot codes to perform "Type B" Telex routing.
Telex grew around the world very rapidly. Long before automatic telephony was available, most countries, even in central Africa and Asia, had at least a few high-frequency Telex links. Often these radio links were first established by government postal and telegraph services (PTTs). The most common radio standard, R.44 had error-corrected retransmitting time-division multiplexing of radio channels. Most impoverished PTTs operated their Telex-on-radio (TOR) channels non-stop, to get the maximum value from them. The cost of TOR equipment has continued to fall. Although initially specialised equipment was required, many amateur radio operators now operate TOR with special software and inexpensive hardware to adapt computer sound cards to short-wave radios. Modern "cablegrams" or "telegrams" actually operate over dedicated Telex networks, using TOR whenever required.
Operation and applications
Telex messages are routed by addressing them to a Telex address, e.g., "14910 ERIC S", where 14910 is the subscriber number, ERIC is an abbreviation for the subscriber's name (in this case Telefonaktiebolaget L.M. Ericsson in Sweden) and S is the country code. Solutions also exist for the automatic routing of messages to different Telex terminals within a subscriber organization, by using different terminal identities, e.g., "+T148".
A major advantage of Telex is that the receipt of the message by the recipient could be confirmed with a high degree of certainty by the "answerback". At the beginning of the message, the sender would transmit a WRU (Who aRe yoU) code, and the recipient machine would automatically initiate a response which was usually encoded in a rotating drum with pegs, much like a music box. The position of the pegs sent an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient. The WRU code would also be sent at the end of the message, so a correct response would confirm that the connection had remained unbroken during the message transmission. This gave Telex a major advantage over less verifiable forms of communications such as telephone and fax. The usual method of operation was that the message would be prepared off-line, using paper tape. All common Telex machines incorporated a 5-hole paper-tape punch and reader. Once the paper tape had been prepared, the message could be transmitted in minimum time. Telex billing was always by connected duration, so minimizing the connected time saved money. However, it was also possible to connect in "real time", where the sender and the recipient could both type on the keyboard and these characters would be immediately printed on the distant machine.
Telex could also be used as a rudimentary but functional carrier of information from one IT system to another, in effect a primitive forerunner of Electronic Data Interchange. The sending IT system would create an output (e.g., an inventory list) on paper tape using a mutually agreed format. The tape would be sent by Telex and collected on a corresponding paper tape by the receiver and this tape could then be read into the receiving IT system.
One use of Telex circuits, in use until the widescale adoption of x.400 and Internet email, was to facilitate a message handling system, allowing local email systems to exchange messages with other email and Telex systems via a central routing operation, or switch. One of the largest such switches was operated by Royal Dutch Shell as recently as 1994, permitting the exchange of messages between a number of IBM Officevision, Digital Equipment Corporation All-In-One and Microsoft Mail systems. In addition to permitting email to be sent to Telex addresses, formal coding conventions adopted in the composition of Telex messages enabled automatic routing of Telexes to email recipients.
Teletypewriter eXchange
The Teletypewriter eXchange (TWX) was developed by the Bell System in the United States and originally ran at 45.45 baud or 60 words per minute, using five level Baudot code. Bell later developed a second generation of TWX called "four row" that ran at 110 baud, using eight level ASCII code. The Bell System offered both "3-row" Baudot and "4-row" ASCII TWX service up to the late 1970s.
TWX used the public switched telephone network. In addition to having separate Area Codes (510, 610, 710 and 810) for the TWX service, the TWX lines were also set up with a special Class of Service to prevent connections to and from POTS to TWX and vice versa.
The code/speed conversion between "3-row" Baudot and "4-row" ASCII TWX service was accomplished using a special Bell "10A/B board" via a live operator. A TWX customer would place a call to the 10A/B board operator for Baudot – ASCII calls, ASCII – Baudot calls and also TWX Conference calls. The code / speed conversion was done by a Western Electric unit that provided this capability. There were multiple code / speed conversion units at each operator position.
Western Union purchased the TWX system from AT&T in January 1969.[ The TWX system and the special area codes (510, 610, 710 and 810) continued right up to 1981 when Western Union completed the conversion to the Western Union Telex II system. Any remaining "3-row" Baudot customers were converted to Western Union Telex service during the period 1979 to 1981.
The modem for this service was the Bell 101 dataset, which is the direct ancestor of the Bell 103 modem that launched computer time-sharing. The 101 was revolutionary, because it ran on ordinary unconditioned telephone subscriber lines, allowing the Bell System to run TWX along with POTS on a single public switched telephone network.
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