********************** *** LASER PRINTERS *** ********************** * First Laser printers were expensive (around $300,000), but were many times faster than the fastest line printers around at thetime. * The high cost of a laser printer was often justified since a single laser printer could replace multiple line printers. For example, At SLAC 5 high speed line printers were replaced by a single laser printer. * Laser printers were incredibly fast. One could empty a boc of paper in 10 minutes. Operator load time became a significant problem since the operators could not load paper in the printer fast enough. *********************** *** RASTER PRINTERS *** *********************** * Pages printed from a raster printer consist of an array of dots called "pixels" (picture elements) Typical pixle density is about 300-1200 pixels per linear inch (about 8 million pixels per page). * Each pixel could be either a black or white dot. Since each dot can be individually controllable, raster printers can be used to print out different fonts and pictures. * Most raster printers use either lasers and xerographic technique (ie. like a Xerox/copy machine), or ink jet. * How a Xerox machine works: Light is reflected on the document being copied, and then reflected back onto a series of selenium-coated drums. These drums pick up a charge at the places where there was an image on the copied document (ie. the black lines/letters/etc.). Toner is then applied to those charged parts of the drums. The paper passes through the drums, taking up the toner as it passes. The toner is then melted perminantly onto the paper (which is why its all warm and toastly when it comes out of the copier). * In a Laser printer, rather than reflect light of an image, a laser is used to directly the onto the drum. Color laser printers go through the drum 4 times, once for each color, resulting in a print rate 1/4 the time of black and white printing. Some expensive laser printers use 4 lasers, one for each color. * Newer color "laser" printers use LED arrays instead of lasers for the print heads. The engine can contain four print elements (one per color) arranged in a line, so that the paper can move along a straight path to pick up all four colors in a single pass. * An Alternate approach is the ink-jet printer, which squirts out small droplets of ink generated either by boining out a small amount of a semi-solid ink block or by putting pressure on the cartrage. *The hardest problem in printer technology is paper mechanics, as can be seen by the high rate of paperjams in printers. * ASSIDE - Will paper printing become obsolete in the future? * Nope. So far no electronic book type technologies have received much success. Paper is just too usefull and inexpensive to be replaced. So unless paper becomes more expensive than an eBook reader, count on paper sticking around * However, some work can on digital paper, which looks and feels like real paper, but uses OLED display technology that allows it to be reusable. **************** *** DISPLAYS *** **************** *Cathode Ray Tube (CRT): * A phosphor-coated screen that glows when an electron beam strikes it. * High voltage generates an electron beam. Electromagnetic field deflects the beam onto specific pixels. * Color is obtained by having different pixels for each color. Beam only writes to appropriate subpixels. * Originally CRTs were used for storage (Williams Tubes). High-persistence phosphor keeps data integrity just long enough for the electron beam to pass over it again and refresh the data there, thus creating a primative form of computer memory. * CRTs are being replaced by various other technologies because they are so large, heavy, and energy inefficeint. * Liquid Crystal Display (LCD): * Each pixel is a liquid crystal which can be oriented by an applied electric field. * 2 standards for selecting if a pixel is turned on/off: * Passive Matrix - Column and row select * Active Matrix - Individual transistor on each pixel * Light for an LCD is provided by a backlight. Light from the backlight passes selectively depending on whether the pixel is on or off. Without the Backlight the LCD image would not be vary visible. Backlight uses more power than any other component in an LCD, between 1.2 to 2.4 watts (total power for LCD is around 2-5 watts). * Colors are provided by having sub-pixels and color filters over each. * Manufacturing is very difficult, Thus they are very expensive. * LCD Manufacturing yeild rate is similar to that of silicon chips * Response time: 150ms for passive, 40ms for active. Lower than that of a CRT. * LCDs often come with a certain number of dead pixels. However, if too many pixels die the display will cease functioning (think Christmas lights). * Other Display Technologies: * Organic Light Emitting Diodes - emit light when current applied. Short lifetime for certain colors. *Plasma - plasma radiates, and causes phosphor to emit light. Short lifetime. Cheaper than LCD, and commonly used in large TVs *Field Emitter Displays - electron gun behind each pixel (also known as flat CRT). * Electroluminescent - semiconductors that emit light when current is passed through them. * Electromechanical - TI Mirror Chip. Manipulate light with mirrors to create an image. * Retinal scanning lasers - write the image on your retina. ************ *** TAPE *** ************ *Reel to Reel Tape: * 9 tracks, 1/2" wide by 2400 feet long. Also comes in 600' and 1200' reels. * Old tapes were 7 track. No longer used *From 800 to 6250 bytes per inch (bpi). * Old tapes were also 200bpi and 556 bpi. At that size bits could actually be read of of the tape using a magnifying glass. * Max capacity of 6250 tapes is about 180 Mbytes. *Technology is basically the same as a cassete tape. Tape base is coated with thin magnetic coating. Write head polorizes small bits on tape surface. Read head reads the polorized bits. _______________________________ | | | __ __ | | / \ <-reels->/ \ | | |\__/ \__/| | | | | | | | head | | * Loops of extra tape left to improve start and | | | | | time for tape acess. It takes about a second or | | V | | two for the reels to start rolling, but the head | | __ | | can start moving in a matter of milliseconds. So | | / \ | | its quicker to start the head moving across the | | /\__/\ | | tape loop, and then let the reels catch up once | | | | | | they start moovint. | | | | | | | \ / \ / | | tape->\____/ \____/ | | | | | |_______________________________| * Wrote Variable length records on some machines (about 1-32000 bytes). * Inter-record gaps of about .6 inches are created between tape writes during the time it takes. for the reels to wind down and stop moving. Inter-record gaps waste tape space so it was best to write large blocks of data at a time. +-----Inter-Record Gaps-----+ x=data | | | | _________|______|________|___________|____ | | | | XXXXXXX V XX V XXXX V XXXXXXX V XX __________________________________________ *Tape moves at 20-200 inches per second. Max data rate is about 1.25 Mbytes/sec. High speed drives use air columns to avoid tape reel inertia. *Can read or write, but cannot write in middle or overwrite blocks, since you can accidentally overwrite a block you didn't to overwrite. Can skip records (fast-forward). * The life span of a tape is relatively short. Tapes do not hold their magnetic charge long. Eventually it wares off, resulting in bad data. Also, since the layers of tape are pressed close togeather on the reel, the magnetism of one layer can the magnetism of it adjacent layers (called "print through"). * Tape is normally read only one direction. Some machines (e.g. IBM 370), can read tapes backwards, but can only do this using complicated syscalls if assembler *High performance tape drives are expensive - e.g. $30000, plus the cost of a controller. * IBM tapes have some standard formats. Can be labeled (ie.every file has a standard header) to minimizes the probability of overwriting the wrong tape. * Disks and tapes read and write blocks of information rather than single bytes * Storage efficiency: * For tapes written at 1600bpi, 80-byte records use .05 inch, gaps use .6 inch, only 1/12 of tape is used effectivly * for 8000-byte records use 5 inches so gaps are only about 11% of the tape. * Obviously, writting larger blocks is better * Example: HP 1/2" tape (rack mount), 1993: *6250bpi, 140MB total, mtbf 22,400hrs, 125 inch/per sec, 781KB/sec, power 170-120 watts, rewind time 90 sec (2400'). * Newer tapes (IBM 3480 type) are cartridges (about 6" x 6"). They hold about the same capacity as a 2400' tape. 18 track. 220MBytes. Cost (end user, 1990, IBM - $95,000) * Example: 1992, Cranel: * 18 tracks, 3480 compatible, 540 foot tape, .5 inch wide, cartridge is 1" x 4.3" x 5", MTBF 15000 hrs, tape speed - 1 meter/sec, transfer: 3MB/sec, 18 tracks, cartridge capacity 200MB * Used in automated tape libraries. *DECtape: * "randomly" readable and writeable. Used on DEC minicomputers, as cheap addressable storage. Now obsolete. 1960-70s. * DAT tape - 4mm tape in DAT cartridge, (smaller than audio casette). Capacity about 1-2GB. * Uses 3 level error correction - errors are 1 in 10**15. * Uses embedded subcodes to find files and tracks. Tape is blocked in 512KB blocks. Can fast forward to appropriate block. Data is also organized into `groups' of 126632 bytes. Each group contains 22 logical data frames of fixed capacity. * There is a mode which permits random read/write. Must preformat the tape into frames. Called `update in place.' * Head/tape speed of 123 inches/second. * Track angle: 6 degrees. * Uses 2 read heads and 2 write heads. Write herringbone pattern of tracks, which can overlap without interference. Regular tracks Herringbone Tracks ________________________ ____________________________ ------------------------ / / / / / / / / / / / / / / ------------------------ / / / / / / / / / / / / / / ------------------------ / / / / / / / / / / / / / / ________________________ ____________________________ * Example, 1993: Hewlett-Packard: *2GB, 183KB/sec, ave seek 30 sec, 3.9watts, mtbf: 50,000hrs. One version available with data compression, capacity factor 2-4x. * Example 1992: Cranel: *capacity: 1.3Gbytes, 60meters long, 183KByte/sec max sustained, 1.5Mbytes/sec burst, MTBF: 40000hrs., search at 200 times faster than normal. Errors: 1/10**15. *Exabyte tape: * 8mm tape in cartridge. Holds 2.5 - 5.0 GB * Helican scan device - writes from high speed rotating drum to slow moving tape. * 1995 model: * 5 gigabytes, 500 Kbytes/second, 9 tracks. tape is 8mm. Up to 75Mbytes/square inch. tape is .5 inches/second, (150 inches/second relative to head), 246Kb sustained data rate. * Higher speeds could be obtained by faster drum rotation and higher linear bit densities. 3Mbyte/second feasible. * Example: Cranel: * transfer: 1.5Mbyte/sec peak, 246Kbyte/sec sustained, 34200 bits/inch, 819 tracks/inch, 35 million bits/sq in., .429 inches per second of tape,speed, rotor 1800 rpm (effective tape speed 150in/sec), rewind: 75 times normal, file search speed: 10 times normal, MTBF: 20000 hrs, error rate: 1/10**13. * Digital Linear Tape - DLT (quantum 2004): * Records data in serpentine pattern - along length of tape, then reverse and back. Serpentine Tracks ___________________________ --------------------------+ | +-------------------------+ | +-------------------------- ___________________________ *Tape cartridge is about 4.1"x4.1"x1" * Tape length is about 2000' * Tape width is about .5" * Purported tape life is >30 years. Claim of "less than 10% loss in demagnetization at 20C and 40% non-condensing humidity". Professor points out the falseness of that claim. * Media durability: 1M passes. * Claimed reliability: 250,000 hours MTBF * Capacity is up to 300GB/tape (before compression). Compression yields 2X. * Maximum tape transfer rate (depending on drive) is up to 36MB/sec. Burst transfer rate up to 200MB/sec. * Typical track density is around 1490 tpi (640 tracks serial serpentine) * Purported error rate - uncorrected 1/10**17, undetected 1/20**27. * Recording density - 233Kbits/inch. * Media durability- 1*10**6 passes * Power (according to quantum) - 32 watts * Average file access time (according to quantum): 79sec. * Low end version: capacity 40GB (uncompressed), 3MB/sec, 168 tracks, 123KBits/inch, 336 tracks/inch, MTBF 200,000 hours, 15 watts) * Other Tapes: * Variety of other tape formats available. * Two general formats: *Linear tracks - along length of tape. *Helical scan- tracks are diagonal along tape. * Variety of sizes of tape, lengths of tape. * Variety of tape reel sizes; some are 2 reel cartridges. * Problems with Tapes: * Tapes deteriorate over time. * Variety of formats - mutually incompatible. Too many competeing standards. * Tapes are slow (usually 1-3MB/sec, except for very high end). * Formats become obsolete; may not be able to read in future. Professor says its almost impossible to replace an old obsolete tape reader if yours breaks, so must keep readers in good working order * High end tape drives are expensive. * Tapes are cheapest way to save massive amounts of data. ************ *** DISK *** ************ * Hard Disk: * Technology similar to tape. Heads float over disk surface, at a distance about 1 micron ( thats less than the width of human hair). * If disk becomes contaminated, head can crash. * The heads literally crash onto the disk platter * Number of platters (2 surfaces per platter, sometimes except top and bottom), cylinders, tracks, sectors, bytes per track, etc. are all variable. * Terms: * Read/Write Arm - Head is attached to the end of arm. The arm array seeks out the correct cylinder to read from on a a platter and performs reads and writes when the correct sector(s) come under the head. There is One arm for each platter. All arms move at the same time. * Platter - A magnetic disk used to hold data. Can write to one or both sides * Track Platter is made of concentric circles called tracks. * Cylinder - Term for all the platters underneath a single track. Basically, all tracks that can be read now without moving the arm. * Sector - The disk platter is divided into wedges by radial lines. A sector is the portion of one of these wedges within a particular track. It is the fixed size of a read/write block. +----------+ <- arm | V <- read/write head | ---------------|-|--- <- platter +----------+ | | | V | | <- cylinder | ---------------|-|--- sector | | ___|______ / V | \ <-Platter / \ ___|__ \ / \ | \ \ | / \ _|_ \ | | | \ | \ | | | | | + | | <---track | | \___/ | | | \ / | \ \______/ / \ / \__________/ *Data can be written in variable size or fixed size blocks (sectors). IBM style high end disks permit variable size blocks (called "count key data (CKD) disks"). Most other machines use fixed sectors. (almost always 512 bytes) * Material written looks like tape block: inter-block gap, key field, control info (physical block address, record number, error correction, key length, byte count), plus actual data. Overhead for variable blocks is about 50-100 bytes, plus IRG. Smaller overhead for fixed blocks. * Most disks used to be removable. Now no hard disks are removable (except ZIP (100, 200, 750MB), JAZZ (1-2GB) disk- IOMEGA, Syquest, etc.). (last "major" removable regular disk was IBM 3330.) * The problem with removable hard disk is that the arms were part of the drive and not disk itself. No garuntee that the arms would position themselves in the same position as they did in the drive that wrote the disk. Therefore, no garuntee of the compatability of the disk on different machines (even if they use the same type of reader). * Not very usefull if you can only read your disk in the drive that wrote it. Especially if the drive brakes making it impossible to read your data. *Floppy Disk - 1.44MB. 60KB/sec.