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Micron Blog

Part 1: Tape Drives

The 726 and UNISERVO Tape Drives: The Beginning of Digital Storage

  1. Tape Drives
  2. Magnetic Drum Memory
  3. The Birth of the Hard Drive
  4. The 5.25-inch Hard Drive
  5. Limitations of the HDD
  6. The RAM SSD & NAND
  7. NAND in SSDs

Engineers first used magnetic tape to record audio signals prior to World War II [4], but it would take nearly two decades of refinement and development before magnetic tape drives were capable enough for data storage and commercialization.

In 1951, the newly formed UNIVAC company produced the first magnetic tape drive for computer data storage. The UNISERVO used a one-half–inch-wide metal tape made from phosphor bronze coated in nickel plating. This metal tape was a reliable recording media. And the UNISERVO could record "128 characters [or bits] per inch on eight tracks at a linear speed of 100 in/s," for a total data rate of 12,800 bits per second. Six of the UNISERVO's eight tracks were devoted to data, another track was used for parity, and the eighth track was a timing track or clock[5].

Most of the development work for the UNISERVO actually started in the 1940s, at what was then the Eckert-Mauchly division of the Remington Rand Corporation (which became UNIVAC in 1950)[6].  The process of recording data on tape was extremely difficult. Engineers of that era had to develop complicated systems not only to record data, but to manage the spinning reels, heads, capstans, and other mechanical and pneumatic systems associated with early tape drives. The relatively heavy metal tape made this task more difficult, particularly during drive acceleration.

In 1952 IBM introduced its first tape drive, the 726 Magnetic Tape Reader/Recorder [7].  Unlike the UNISERVO and its rugged metal tape, the 726 used a cellulose acetate-based plastic tape coated in iron oxide, similar to what was used in the audio recording industry in the late 1940s. This plastic tape was more prone to break or be damaged than metal tape, but it was much lighter so it required only a fraction of the mechanical inertia needed to spin the reels of a metal tape data storage system like the UNISERVO [8].   It is important to note that this is a relative comparison. Plastic tape drives were less complicated than metal tape drives, but the 726 and other early tape drives were still complex machines.

Figure 2: Operating Characterictics of the Early IBM Half-Inch Tape Drives

By the mid-1970s, reel-to-reel tape drives had become a standard for archival data storage, achieving access speeds of just 1ms. And these devices were still quite complex. To operate, a 200-in/s, half-inch tape drive required two reels, a powerful motor, a read/write head, a cleaning head, and various other mechanical and pneumatic subsystems. In this typical, 1970s-era tape drive, the tape reel on the right side of the drive contained the source data. This reel had to be manually mounted or removed. The operator would place the reel over a hub. The hub automatically expanded to grip the reel and initiate the loading process. Next, the right-hand source data reel would rotate clockwise so that the tape was generally moving toward the reel on the left side of the drive. Jets of air—representing the first pneumatic subsystem—gently supported the tape inside the right-hand threading channel. Next, the tape moved across the read/write head and advanced toward the left-hand threading channel where another set of air jets waited to lift the tape toward the second reel—on the left side of the drive. The hub in the left reel used a vacuum to attract the tape and hold it in place for the first few revolutions. After enough of the tape had been positioned on the left-hand reel sensors, the hub shut down the vacuum and reversed the left reel so that it began to turn counterclockwise, slackening the tape. At this point, two vacuum columns drew the tape down and into position.  Finally, the capstan positioned a special beginning-of-tape marker and the machine was ready for use [9].

Figure 3: Typical Tape Path for a Magnetic Tape Unit, circa 1975

Improving Access Speed in Reel-to-Reel Tape Drives

Early magnetic tape storage units commonly employed two capstans with constant-speed, synchronous motors that worked in tandem with an air and vacuum system that attracted or repelled the tape as the head searched the tape for data. These systems could take several milliseconds to start before the tape even began to move, and it might take an additional 2ms for the drive to reach operating speeds [10].   And getting to the right data could take much, much longer—tens of seconds or more.

Beginning in the 1960s, synchronous motors were being replaced with a new kind of DC motor that used a rotating cylindrical shell to encase the armature conductors and a concentric iron core that did not rotate. This design was an important improvement over the standard armature made of wires and embedded in a solid iron cylinder. These newer motors had a much better torque-to-inertia ratio and they boosted access time. They also enabled designers to use only one capstan instead of two [11].

Enclosed Tape Drives

Competition from semiconductor technology and hard disk drives and the advent of the personal computer forced tape to move to new form factors. Although tape cartridges had been around for years, they were unable to gain popularity in the 1980s because size and cost were the dominant market forces [12].

Tape Drive Technology Lags Behind Computer Advances

Magnetic tape storage has played an important part in the evolution of digital storage and is still a good, low-cost storage media for some applications. And while engineers at drive and tape manufacturers have developed new techniques to improve density and access speed, in general, tape storage has not kept pace with the storage capacity or performance of other new—more evolved—technologies [13].

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Notes [4]  Sandy Stewart and Alan King, "Quarter-Inch Tape Drives: Leading the Pack for Secondary Storage," Storage and Recording Systems, Conference Publication No. 402 (April 1994): page 108. [5] "Magnetic Tape Data Storage," downloaded from [6] Eric D. Daniel, C. Denis Mee, and Mark H. Clark, Magnetic Record, IEEE Press, Piscataway, N.J.: page 253. [7]  "IBM 726 Magnetic Tape Reader/Recorder," IDM, Armonk, N.Y. downloaded from [8]  Eric D. Daniel et al, page 256. [9] Juan A. Rodriguez, "An Analysis of Tape Drive Technology," Proceedings of the IEEE, Volume 63, No. 8 (August 1975): page 1153. [10]  Rodriguez, page 1155. [11]  Rodriguez. page 1155. [12]  Eric D. Daniel et al, page 261. [13]  Atsushi Sawai, Mitsumasa Oshiki, and Gen-ichi Ishida, "The 18 Track Thin Film Magnetic Head for Half-Inch Magnetic Tape Drive," IEEE Transactions on Magnetics, Volume MAG-22, No. 5 (September 1986): page 686.

About Our Blogger

Dean Klein

Dean Klein is Vice President of Memory System Development at Micron Technology. Mr. Klein joined Micron in January 1999, after having held several leadership positions at Micron Electronics, Inc., including Executive Vice President of Product Development and Chief Technical Officer. He also co-founded and served as President of PC Tech, Inc., previously a wholly-owned subsidiary of Micron Electronics, Inc., from its inception in 1984. Mr. Klein’s current responsibilities as Vice President of Memory System Development focus on developing memory technologies and capabilities.

Mr. Klein earned a Bachelor of Science degree in electrical engineering and a Master of Electrical Engineering from the University of Minnesota, and he holds over 220 patents in the areas of computer architecture and electrical engineering. He has a passion for math and science education and is a mentor to the FIRST Robotics team ( in the Meridian, Idaho school district.

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