Input/Output Devices - IBM Mainframe

Input/output or I/O is the communication between an information processing system (such as a computer) and the outside world, possibly a human or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. The term can also be used as part of an action; to "perform I/O" is to perform an input or output operation. I/O devices are used by a person (or other system) to communicate with a computer. For instance, a keyboard or a mouse may be an input device for a computer, while monitors and printers are considered output devices for a computer. Devices for communication between computers, such as modems and network cards, typically serve for both input and output.

Note that the designation of a device as either input or output depends on the perspective. Mouse and keyboards take as input physical movement that the human user outputs and convert it into signals that a computer can understand. The output from these devices is input for the computer. Similarly, printers and monitors take as input signals that a computer outputs. They then convert these signals into representations that human users can see or read. For a human user the process of reading or seeing these representations is receiving input. These interactions between computers and humans is studied in a field called human–computer interaction.

In computer architecture, the combination of the CPU and main memory (i.e. memory that the CPU can read and write to directly, with individual instructions) is considered the brain of a computer, and from that point of view any transfer of information from or to that combination, for example to or from a disk drive, is considered I/O. The CPU and its supporting circuitry provide memory-mapped I/O that is used in low-level computer programming, such as the implementation of device drivers. An I/O algorithm is one designed to exploit locality and perform efficiently when data reside on secondary storage, such as a disk drive.

Unit Record Devices

Unit record devices include two types of devices: card devices and printers.

The term Unit Record Devices Unit record devices include two types of devices: card devices and printers. The term "unit record device" implies that each record processed by the device is a physical unit. In the case of card devices, each record is a punched card. As for printers, each record is a printed line. Unit record devices usually have built-in control units that attach directly to channels, so separate control units are not required. Card devices, which are not commonly used anymore, come in three types: readers, punches, and reader/punches. A card reader is an input-only device: It can read data from punched cards, but cannot punch data into blank cards. A cardpunch is just the opposite: It can punch data into cards, but cannot read previously punched cards. A reader/punch combines the functions of a reader and a punch, and serves as both an input and an output device.

Unlike card devices, printers are in widespread use today; they provide the primary form of permanent output from the computer. There are a variety of different types of printers, but the most commonly used printers fall into two categories: impact printers and non-impact printers. Impact printers produce printed output by striking an image of characters to be printed against a ribbon, which in turn transfers ink to the paper. The most common type of impact printer uses a train of characters that spins at high speed; when the correct character passes a print position, a hammer strikes the character against a ribbon to produce the printed text. Most impact printers operate in the range of 600 to 2,000 lines per minute.

Non-impact printers use laser technology to print text and graphic images. IBM's 3800 Printing Subsystem can print at rates of up to a remarkable 20,000 lines per minute. The actual speed of the 3800 printer depends on the size of each page and the number of lines per inch, because the 3800 transfers images to the paper an entire page at a time. For standard size paper (11X14) and normal print size (6 lines per inch), the 3800 prints 10,020 lines per minute. At that print rate, the 3800 can process more than a mile and a half of paper each hour.

Magnetic Tape Devices

A tape drive is a data storage device that reads and writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.

A tape drive provides sequential access storage, unlike a disk drive, which provides random access storage. A disk drive can move to any position on the disk in a few milliseconds, but a tape drive must physically wind tape between reels to read any one particular piece of data. As a result, tape drives have very slow average seek times. For sequential access once the tape is positioned, however, tape drives can stream data very fast. For example, as of 2010 Linear Tape-Open (LTO) supported continuous data transfer rates of up to 140 MB/s, comparable to hard disk drives.

Blocking of Records on a Magnetic Tape

Blocking of Records on a Magnetic Tape

How much data a reel or cartridge of tape can contain depends on the length of the tape and the density used to record the data. Density is a measurement of how many bytes are recorded in one inch of tape. Tape densities for standard reel tapes are usually 1600 or 6250 bytes per inch (bpi). Cartridge tape drives can record data using much higher densities. Data records are normally written to tape in groups called blocks. Here, five records are stored together as a single block. As you can see, empty spaces called gaps are required to separate blocks from one another. The larger the block, the less the amount of wasted space on a tape. However, there is an extra cost involved when blocking is used: A buffer is required in main storage to contain the entire block. As a result, the larger the block, the more main storage that is required to contain it.

Tape processing has one serious drawback: It must be sequential. In other words, to read the 50,000th record on a tape, the first 49,999 records must be read first. As a result, tape is not suited for applications that require direct access to stored data. Instead, tape is most often used for off-line storage of large quantities of data, especially data that serves as a backup for online data on DASD devices.

To attach a tape drive to a processor, a control unit is required. For some models, the control unit is inside one of the tape drives. For other models, it is in a separate cabinet. Depending on the model, the controller can attach up to four or eight tape drives.

Direct Access Devices

The official IBM term for a disk drive is direct access storage device, or DASD. Because DASDs allow direct and rapid access to large quantities of data, they have become a key component of mainframe systems. They are used not only to store user programs and data, but also to store programs and data for operating system functions. Disk drives read and write data on a disk puck (sometimes called a volume). A disk pack, shown is a stack of metal platters coated with a metal oxide material. Data is recorded on both sides of the platters.

Most of IBM's older DASDs used removable disk packs, but the newer IBM DASDs use a disk pack that is fixed in a permanent, sealed assembly inside the drive. Non-removable disk packs have two advantages over removable packs: they are faster and they are more reliable. Because speed and reliability are important requirements of online applications, DASDs with non-removable packs are well suited for today's mainframe systems.

Tracks and Cylinders

Data is recorded on the usable surfaces of a disk pack in concentric circles called tracks,. The number of tracks per surface varies with each device type. For example, a standard surface has 808 tracks, numbered from 0 to 807. A disk pack with 19 usable surfaces, each with 808 tracks, has a total of 15,352 tracks.

The component that reads and writes data on the tracks of a disk pack is called the actuator. The actuator has one read/write head for each recording surface. When the actuator moves, all of its heads move together so they are all positioned at the same track of each recording surface. As a result, the disk drive can access data on all of those tracks without moving the actuator. The tracks that are positioned under the heads of the actuator at one time make up a cylinder. As a result, there are as many tracks in a cylinder as there are usable surfaces on the pack, and there are as many cylinders in a pack as there are tracks on a surface. So a pack that has 19 surfaces, each with 808 tracks, has 808 cylinders, each with 19 tracks.

IBM manufactures two basic types of disk drives that store their data in different formats—count-key-data (CKD) devices and fixed-block-architecture (FBA) devices.. Count-key-data (CKD) devices store data in variable-length blocks. Fixed-block-architecture (FBA) devices store data in fixed-length blocks of 512 bytes each. Since FBA devices are not supported under MVS.

In a CKD device, each data block is preceded by a count area and a key area. (The count area is required; the key area is optional.) Because the disk revolves counterclockwise, the read/write head encounters the count and key areas before the data area. The count area contains the information needed to locate and process the key and data areas. One of the problems with CKD devices is that the data capacity of each track depends on the size of the blocks used to store the data. That is because gaps are required to separate the count, key, and data areas, just as gaps are required on magnetic tape. When smaller areas are used, more blocks of data can be stored on each track. But when more blocks are stored, more gaps are used, so the total capacity of the track is reduced.

The total capacity for each drive is the maximum capacity for the device. That assumes that all of the data in each track is stored in a single block; if more than one block is stored per track (and that is usually the case), the capacity is reduced because of the additional gaps required to separate the blocks.

Each type of DASD device requires two kinds of control units to attach it to a processor channel. The first, called a string controller, attaches a group of DASDs of the same type; the resulting group is called a string. The number of devices that can be connected on one string depends on the device type; for 3390-model disks, up to 32 drives can be connected in one string. The second kind of control unit, called a storage control, connects up to eight strings of DASD units to a channel. The most common type of storage control is the 3990, which attaches two DASD strings. If both strings consist of 3390 drives, up to 64 drives can be connected to a single 3990.

A 3390 Configuration with 2 Strings attached to a 3390 Storage Control

A 3390 Configuration with 2 Strings attached to a 3390 Storage Control

The above figure shows how a 3990 storage control might be used to control two strings of 3390 DASDs, each containing 16 drives. As you can see, the 3990 connect to the processor through channel connections; the 3390 DASDs, in turn, connect to the 3990. The 3990 storage control provides high-speed cache storage that acts as a buffer between the processor and the actual disk units.

Special circuitry keeps track of what disk data is accessed most frequently and tries to keep that data in the cache storage. Then, when that data is referenced, it can be read directly from cache; the DASD unit does not have to be accessed at all. Depending on the 3990 model, the size of the cache can range from 32MB to 1,024MB. Obviously, cache storage in the storage control significantly improves a system's overall performance. In addition, the 3990 storage control can support more than one channel connection to the processor. This enables several simultaneous disk operations to be processed at once. The smallest 3990 models support up to four standard channel connections, and the largest can support 16 standard channel connections or 128 ESCON channel connections.

Data Communications Equipment

Data communications equipment lets an installation create a data communications network (or telecommunications network or just network) that lets users at local terminal (terminals at the computer site) and remote terminals (terminals that are not at the computer site) access a computer system. Now, We will see the components, of a data communications network, with emphasis on the most common type of terminals used on IBM mainframe networks—the 3270 Information Display System. Elements of a data communications network figure shows the basic components that make up a data communications network.

Components of a Data Communications Network

Components of a Data Communications Network

Basically, five elements make up the network:

  1. a host system,
  2. a communications controller,
  3. -modems,
  4. telecommunications lines, and
  5. terminal systems.

At the center of the network is the host system, a System/370 processor. The control unit that attaches to the host system's channels is called a communications controller; it manages the communications functions necessary to connect remote terminal systems via modems and telecommunications lines. A modem is a device that translates digital signals from the computer equipment at the sending end (either the host or remote system) into audio signals that are transmitted over the telecommunications line, which can be a telephone line, a satellite link, or some other type of connection. At the receiving end of the line, another modem converts those audio signals back into digital signals.

Although the terminal systems in the above figure are connected remotely via tele- communications lines and modems, which is not a requirement. If the terminal system is located close enough to the host system, the modems and telecommunications lines can be eliminated. Then, the terminal system is connected directly to the communications controller or one of the host processor's channels. Whether attached locally or remotely, the most commonly used terminal system on IBM mainframes is the 3270 Information Display System.

Because you are likely to use a 3270 terminal as you learn how to use Job Control Language, it is important that you have a basic understanding of its components and how they work together. The 3270 Information Display System The 3270 Information Display System is not a single terminal, but rather a subsystem of terminals, printers, and controllers that attach to a host computer system remotely through a communications controller and telecommunications lines or locally through a communications controller or direct attachment to a channel. A typical 3270 controller (a 3274) controls up to 32 terminals and printers and can be connected to a processor either directly or remotely over a telecommunications network that consists of modems and telephone lines. The two remote 3270 systems in Figure 2.8 each include one controller, four terminals, and one printer.

Because of the enormous popularity of the 3270 system, many manufacturers besides IBM offer compatible terminals, printers, and controllers. And most manufacturers of minicomputers and personal computers offer emulator programs that allow their computers to mimic 3270 devices. Because of cost advantages and additional benefits, it is becoming more, and more common to see terminal emulators in use in 3270 networks.


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