[IBM MODEL 2420 WITH PHASE ENCODING]

[Music] [Music] the magnet tape unit is a key element in electronic data [Music] processing for IBM its design has evolved through several generations of computer technology [Music] a significant product advancement is the 2420 Magnetic Tape unit using phase encoded [Music] recording the IBM 2420 with phase encoding the now generation in Magnetic Tape technology to further its dedication to Excellence in Magnetic Tape products IBM has centralized development and production of the domestic product line at its Boulder Colorado facility here and at locations in France and Japan extensive engineering and Manufacturing knoow complement the most modern of facilities and sophisticated [Music] Technologies the singular mission is to develop and build new and better tape subsystems for IBM computers [Music] [Applause] [Music] tape technology can be better understood by reviewing the recording process the computer under program control generates groups of electrical signals representing the data to be recorded these electrical commands reach the tape through a multi-track read ride head and its related control circuits the same mechanism is used to read data from the tape the tape's recording surface is covered with microscopic iron oxide particles suspended in a binder when magnetized they assume either positive or negative the horizontal lines represent the tracks through which data pulses reach the tape in current technology there are nine tracks within each track a specific number of cells is allocated to each inch of tape the speed at which to each inch of tape the speed at which the tape passes The Head and the spacing of these cells determine the systems recording density measured in bits per inch across three generations of ibbm tape technology recording density has increased Eightfold to its current standard of, 1600 BPI increased bit densities were paralleled by faster tape transport speeds one method of data recording is known as non Return To Zero IBM or nrzi here we see the recording tracks represented for Simplicity we'll illustrate only one of them the ones and zeros appearing in the imaginary head Gap at left are the binary digits by which a computer counts and stores data a combination of ones and zeros as recorded simultaneously by all tracks represents a conventional number letter or other meaningful symbol to record a one bit the computer sends a command to change the direction of magnetization in the desired bit cell this transition can be of either positive or negative polarity the tape is always polarized in one of these states it is never neutral hence the term non Return To Zero if however a zero bit is desired in a given cell its polarity is not changed the tape therefore retains the same state either positive or negative throughout the duration of the cell in other words only one bits are written on tape each multitrack group of bits is known as a data bite ideally these bites would pass the read gaps in perfect alignment causing all bits to be read simultaneously while the bits that make up a bite arrive at their respective gaps at almost the same time there is a slight time lag from the first to last this displacement is known as skew if excessive it causes the leading or trailing bits of a bite to be confused with those of preceding or following bites the problem of skew is compounded if the need for tape interchangeability is considered it is easy to see how skew introduced during recording could be accentuated by the reading drive if its skew characteristics were out of phase with those of the writing drive as shown earlier ones are recorded by changing the magnetic polarity within a bit cell if the system should experience an unwanted signal or noise condition it is likely to appear as a magnetic transition and be incorrectly decoded as a one bit where a zero is intended the opposite of a noise condition is known as Dropout or the loss of a wanted signal Dropout can occur when the tape lifts away from the head when contamination separates it from the head or if the tape is damaged since the lack of a magnetic transition indicates the presence of a zero Dropout can cause one bits to be read incorrectly shortly after the introduction of system 360 IBM directed its engineering efforts toward developing an encoding system to provide greater data throughput higher reliability and expanded error detection and correction the result was phase encoding here there is a transition for every bit cell be it a one or a zero unlike nrzi however the polarity of the transition is also important the polarity always changes from negative to positive when a one is recorded and from positive to negative when a zero is recorded therefore when like bits are to be recorded in adjacent cells the system orders an additional magnetic transition between them to maintain the direction of polarity in Reading phase encoded data recorded at 1600 BPI the system makes use of an electronic self- timing clock which times the data rate for each of the nine tracks independently a burst of 40 all zero bytes followed by an identifyed all one bite precedes every block of data the clocking for a track starts when the zeros are recognized and synchronized the one bit which follows the series of 40 zeros indicates the start of data data blocks and their synchronizing bytes are separated by interblock gaps or ibgs because each track is clocked individually and because every bit cell contains a magnetic transition phase encoding enables the use of an electronic skew buffer which accumulates all nine bits of each bite independent of skew during skew the lagging part of a bite is still being read as the leading parts of succeeding bites reach the head nonetheless when all nine bits are properly assembled in a buffer the bite is transferred to the computer thus phase en coding overcomes the problem of skew when decoding phase encoded data the detected signal from each cell is compared to a computer generated reference signal if this comparison shows the two to be in Phase during at least half of the cell's duration the detected signal is read as a one bit if however the signals are out of phase for half or more of the cell the bit is registered as a zero phase encoding thereby minimizes noise related transitions which cause zeros to be read incorrectly as ones in Phase encoding data are recorded in eight tracks numbered 0 through 7 a parody bit is recorded in the 9th as each bite is accumulated its parody is checked by an error correction register an odd number of ones must be present if the characters to be read correctly should signal loss cause a parody error in one track for example track four the system automatically inserts information in the track and maintains parity for the remainder of the data block this happens on the fly without slowing data transfer to the computer thus phase encoding provides greater reliability based on the number of permanent read errors carefully control laboratory studies using 3200 FCI tested tape show that 1600 BPI phase encoding improves reliability by 2:1 to as much as 10 to1 these higher data densities and reduced error recovery times improve system throughput significantly paralleling the development the phase encoding IBM Engineers developed a magnetic tape unit which improves tape handling data throughput reliability performance and maintainability the result is the 2420 series the models five and seven the most advanced tape units in the marketplace let's look at the operational features which enable these improvements automatic threading cartridge loading linear rewind improved access time inverted tape path air bearings low inertia cap standand motor and functional circuit packaging automatic threading enables the tape to be loaded with minimum effort to load the tape the operator mounts any standard size reel in the file position which on the 2420 is on the right positions the end of the tape in the threading shoot and presses the load button the 2420 also accommodates cartridge loading using any standard 10 and 1/2 in reel the cartridge open s automatically and guides the tape into the threading Channel automatic threading and cartridge loading result in significant Savings in machine setup time when compared to manual tape loading in addition to obvious operator conveniences the self-contained cartridge lessens the chance for tape contamination since the tape is rewound completely onto the Reel and the cartridge is closed automatically there is no need to remove the Reel from or return it to a separate container unlike more conventional tape drives the 2420 file reel is placed on the right a design feature which improves system reliability since this clockwise threading path inverts the tape its oxide recording side contacts only the tape cleaner erase head and read right head reliability of the tape media is further enhanced by a series of air bearings which cushions the tape's non-recording side and minimizes its contact with the surface of the columns the separ ation helps to eliminate friction and heat another unique feature of the 2420 is its low inera direct current capstan motor the capstan drives the tape on the non-recording side at a maximum operating speed of 200 in per second on the model 7 and 100 IPS on the model 5 the cap standand and the motor's lightweight printed circuit Armature are formed as one unit this reduces inertia during rapid starts and greatly improves data access time the capstan achieves such rapid acceleration because the tape is wrapped around 180° of its surface to accomplish this the tape is decoupled from the vertical vacuum columns into two short tapered columns below the read right station since the length of tape in the tapered columns is nearly constant decoupling helps to provide rapid acceleration regardless of the amount of tape in the vertical vacuum columns this rapid acceleration shortens access time another advantage of soft tape handling on the 2420 is the capacity to rewind tape in columns at a linear speed of approximately 500 in pers second the 2420 requires approximately the same amount of time to rewind a full reel as does the 2400 series linear rewind on the 2420 however reduces the time significantly for less than a full reel for example the 2401 model 6 requires 50 seconds to rewind 600 ft of tape whereas the time is reduced to 22 seconds by the 2420 Model 5 and 17 seconds by the model 7 since most rewinds involve only partial reels linear speed results in significant Savings in machine time the 2420 is data handling performance is matched by improved reliability and maintainability its simplified design reduces the number of mechanical moving parts to a minimum snap out printed circuit boards control the power for the cap standand motor Real Motors and other control circuitry ease of maintenance is further enhanced by functional packaging of the logic circuits a greatly simplified maintenance and logic manual calls out the circuit pins which must be tested for each failure mode should a failure occur functional circuit packaging enables the customer engineer to isolate the problem to a specific area of the machine and correct it with a minimum of downtime in summary the IBM 2420 using 1600 BPI phase encoding provides maximum performance and reliability the 2420 and phase encoding significant advancements in the data processing [Music] Marketplace for