Uniservo I

Uniservo I

First tape drive

Why it's important
The UNISERVO-I, magnetic tape drive introduced with the UNIVAC-I, was the first commercially available computer magnetic tape drive. The UNISERVO and the UNIVAC-I, which had up to10 tape drives attached, was the first true commercial "data processing system." It was able to search, sort, and merge data records electronically, replacing punch card processing equipment, also known as unit record equipment. For example, General Electric, one early UNIVAC customer, said, "the speed of computing is perhaps of tertiary importance only.", referring to the advantages of tape storage over punched cards.[54Osb] The Census Bureau took title as the first UNIVAC-I customer on March 31, 1951. A single reel of tape could hold about 1.5 million characters, the equivalent about 20,000 punch cards. With off-line storage of tape reels, there was essentially unlimited data capacity.

J. Presper Eckert and John Mauchly, designers of the UNIVAC-I and UNISERVO realized the broad set of problems to which electronic based computers could be applied [46Biz]. They also recognized that existing data storage technologies (punch cards, paper tape) were neither fast enough nor of large enough in capacity to permit electronic speed central processors to be broadly useful. They had experimented with magnetic tapes for their earlier BINAC computer design, but the magnetic coated plastic tape used proved very unreliable. For the UNIVAC, Eckert switched to a phosphor bronze tape approximately 0.003" thick by 0.50" wide that was then plated with a 50/50 nickel-iron magnetic coating. This formulation proved to be very robust and reliable in use, although each reel of tape, combined with the aluminum reel was quite heavy. The tape Ni-Fe film was also quite abrasive to the magnetic heads, so a thin Mylar wear film was interposed between the tape surface and the head. A small clock motor slowly advanced the "scotch tape like" film when the tape was in motion, to distribute the wear over a small reel of film rather than the recording head. The recording head was a single gap and there was no read-after-write capability.

The recording system permitted densities of 20, 100, or 128 characters per inch (cpi), encoded as phase signals (exact type not currently known). The density recorded by the UNISERVO-I was 100 cpi, but any density could be read. With phase encoding, the recording channel was more tolerant to variations in signal amplitudes and tape defects than other NRZ or NRZI systems. All tape records were fixed length of 60 words of 12 digits of 6 bits. Parity was encoded for each digit in a seventh tape channel and clock in another channel for a total of eight channels. Blocks could be read forward or reverse, and digits were arranged automatically in proper order in the Input buffer memory. Blocks could only be written with forward tape motion. If a read error was detected, a supervisory panel switch could enable automatic hardware controlled rereads for a set 5 retries.

The UNISERVO-I included a console Initial Read (bootstrap) switch which enabled reading one block of 60 words (120 instructions) from any of 10 push button selected UNISERVOs.

The UNISERVO-I was a single capstan drive with a fixed leader that was retained when no tape reel was mounted. A simple hook connected the reel to the leader. A system of springs and pulleys of elastic cord provided the fast buffered tape for instantaneous capstan motion. The capstan motor drove the capstan through a clutch system, for faster access times. While the tape buffer was inferior to the later developed IBM vacuum column buffer, it was adequately reliable in a heavy data processing application. However, the single capstan tape drive technique and phase encoding several generations later became the preferred technology to IBM's Model 729 pinch rollers and NRZ encoding technology. The UNISERVO tape start-stop time was 20 milliseconds (programming example, not specified) and the tape speed was 100 inches per second. McIntosh audio amplifiers were used to drive the synchronous capstan motors, one frequency for read/write and a higher frequency for rewind operations. Blocks could be read forward or backward, but only written forward. Only two tape drives could be rewound simultaneously. Reel motion was controlled by direct drive motors controlled by ignitron tubes. Other tape I/O operations could proceed while two drives were rewinding. Computing continued while tape was being read or written since the UNIVAC I Register and the O Register separately buffered one block of data (see First Overlapped I/O).

Defects in a reel of tape could be marked by punching a hole through the tape with a hand punch. This was optically sensed (“bad spot detector”) and caused a skip forward in the tape motion over the defect.

UNIVAC provided stand alone UNISERVO-I compatible subsystems for card-to-tape, tape-to-card, and tape-to-printer. Perhaps the first keyboard to tape device, the UNITYPER, recorded at 20 characters per inch per keystroke. While of use to programmers, the UNITYPER was too expensive and far too slow for data entry or capture. Input data punch card preparation remained the province of scores of predominantly women keypunch operators, which were then converted off-line to tape.

For operator control, the UNIVAC-I had a large console, an oscilloscope that could show any of the 1000 serial mercury delay line memory locations, and a console typewriter that could print a "core dump" or other application output (e.g. breakpoint/savepoint data was common). No peripheral devices other than UNISERVOs were connected to the CPU.

UNIVAC continued to use the name UNISERVO for later models of tape drive (e.g., UNISERVO IIA, UNISERVO IIIC, UNISERVO VIII-C) for later computers in their product line (e.g 1107/08). The UNISERVO IIA could read metal tapes from the UNIVAC I as well as use higher density PET film base/ferric oxide media tapes that became the industry standard. [11WK]

Additional Information
[46Biz] Outline of Plans for Development of Electronic Computers, Eckert-Mauchly Corporation, 1946, CHM Accession # 102660910

[53 Prog] Programming for the UNIVAC FAC-TRONIC System, Jan 1953, Remington Rand, Eckert-Mauchly Division

[54Osb] Osborn, R. (1954). "GE and UNIVAC: Harnessing the High Speed Computer." Harvard Business Review 32(4): 99-107.

[58Parts] PX899 Mechanical Parts of a UNISERVO, Remington Rand, 1958

[58Manual] UNIVAC I Maintenance Manual, Remington Rand, Jan 1958

[99Ph] Phillips, W., "The Uniservo 1 - The First Digital Tape Recorder," in Daniel, E., et. al., Magnetic Recording - The First 100 Years, IEEE Press, ©1999, Chapter 17, pp. 294-5

[11WK] Uniservo, Wikipedia

Provenance Note: This article was authored by G. Saviers and Version 9 was approved by the Computer History Museum's Storage SIG on June 20, 2012.

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