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Univac I

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Title: Univac I  
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Subject: Eckert–Mauchly Computer Corporation, UNIVAC, 1951, J. Presper Eckert, Instructions per second
Collection: 1951 Introductions, Early Computers, Univac Mainframe Computers, Vacuum Tube Computers
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Univac I

UNIVAC I operator's console
UNIVAC I at Franklin Life Insurance Company

The UNIVAC I (UNIVersal Automatic Computer I) was the first commercial computer produced in the United States.[1] It was designed principally by J. Presper Eckert and John Mauchly, the inventors of the ENIAC. Design work was started by their company, Eckert–Mauchly Computer Corporation, and was completed after the company had been acquired by Remington Rand (which later became part of Sperry, now Unisys). In the years before successor models of the UNIVAC I appeared, the machine was simply known as "the UNIVAC".[2]

The first UNIVAC was accepted by the United States Census Bureau on March 31, 1951, and was dedicated on June 14 that year.[3][4] The fifth machine (built for the U.S. Atomic Energy Commission) was used by CBS to predict the result of the 1952 presidential election. With a sample of just 1% of the voting population it famously predicted an Eisenhower landslide while the conventional wisdom favored Stevenson.[5]


  • History 1
    • Market positioning 1.1
    • Installations 1.2
  • Technical description 2
    • Major physical features 2.1
    • Main memory details 2.2
    • Instructions and data 2.3
    • Input/output 2.4
  • See also 3
  • Notes 4
  • External links 5


Market positioning

Remington Rand employees, Harold E. Sweeney (left) and J. Presper Eckert (center) demonstrate the U.S. Census Bureau's UNIVAC for CBS reporter Walter Cronkite (right).
UNIVAC I operator's console closeup

The UNIVAC I was the first American computer designed at the outset for business and administrative use with fast execution of relatively simple arithmetic and data transport operations, as opposed to the complex numerical calculations required of scientific computers. As such the UNIVAC competed directly against punch-card machines, though the UNIVAC originally could neither read nor punch cards. That shortcoming hindered sales to companies concerned about the high cost of manually converting large quantities of existing data stored on cards. This was corrected by adding offline card processing equipment, the UNIVAC Card to Tape converter and the UNIVAC Tape to Card converter, to transfer data between cards and UNIVAC magnetic tapes.[6] However, the early market share of the UNIVAC I was lower than the Remington Rand Company wished. To promote sales, the company joined with CBS to have UNIVAC I predict the result of the 1952 Presidential election. UNIVAC I predicted Eisenhower would have a landslide victory over Adlai Stevenson whom the pollsters favored. The result was a greater public awareness of computing technology.[7]


The first contracts were with government agencies such as the Census Bureau, the U.S. Air Force, and the U.S. Army Map Service.[8] Contracts were also signed by the ACNielsen Company, and the Prudential Insurance Company. Following the sale of Eckert–Mauchly Computer Corporation to Remington Rand, due to the cost overruns on the project, Remington Rand convinced Nielsen and Prudential to cancel their contracts.

The first sale, to the Census Bureau, was marked with a formal ceremony on March 31, 1951, at the Eckert–Mauchly Division's factory at 3747 Ridge Avenue, Philadelphia. The machine was not actually shipped until the following December, because, as the sole fully set-up model, it was needed for demonstration purposes, and the company was apprehensive about the difficulties of dismantling, transporting, and reassembling the delicate machine.[9] As a result, the first installation was with the second computer, delivered to the Pentagon in June 1952.

UNIVAC installations, 1951–1954[10]
Date Customer Comments
1951 U.S. Census Bureau, Suitland, MD Not shipped until 1952
1952 U.S. Air Force Pentagon, Arlington, VA
1952 U.S. Army Map Service Washington, DC. Operated at factory April–September 1952
1953 New York University (for the Atomic Energy Commission) New York, NY
1953 Atomic Energy Commission Livermore, CA
1953 U.S. Navy David W. Taylor Model Basin, Bethesda, MD
1954 Remington Rand Sales office, New York, NY
1954 General Electric Appliance Division, Louisville, KY. First business sale.
1954 Metropolitan Life New York, NY
1954 U.S. Air Force Wright-Patterson AFB, Dayton, OH
1954 U.S. Steel Pittsburgh, PA
1954 Du Pont Wilmington, DE
1954 U.S. Steel Gary, IN
1954 Franklin Life Insurance Springfield, IL
1954 Westinghouse Pittsburgh, PA
1954 Pacific Mutual Life Insurance Los Angeles, CA
1954 Sylvania Electric New York, NY
1954 Consolidated Edison New York, NY

Originally priced at US$159,000, the UNIVAC I rose in price until they were between $1,250,000 and $1,500,000. A total of 46 systems were eventually built and delivered.

The UNIVAC I was too expensive for most universities, and Sperry Rand, unlike companies such as IBM, was not strong enough financially to afford to give many away. However Sperry Rand donated UNIVAC I systems to Harvard University (1956), the University of Pennsylvania (1957), and Case Institute of Technology in Cleveland, Ohio (1957); the UNIVAC I at Case was still operable in 1965 but had been supplanted by a UNIVAC 1107.

A few UNIVAC I systems stayed in service long after they were made obsolete by advancing technology. The Census Bureau used its two systems until 1963, amounting to 12 and nine years of service, respectively. Sperry Rand itself used two systems in Buffalo, New York until 1968. The insurance company Life and Casualty of Tennessee used its system until 1970, totaling over 13 years of service.

Technical description

Mercury delay line memory of UNIVAC I

Major physical features

UNIVAC I used 5,200 vacuum tubes,[11] weighed 29,000 pounds (13 metric tons), consumed 125 kW, and could perform about 1,905 operations per second running on a 2.25 MHz clock. The Central Complex alone (i.e. the processor and memory unit) was 4.3 m by 2.4 m by 2.6 m high. The complete system occupied more than 35.5 m² (382 ft²) of floor space.

Main memory details

The main memory consisted of 1000 words of 12 characters. When representing numbers, they were written as 11 decimal digits plus sign. The 1000 words of memory consisted of 100 channels of 10-word mercury delay line registers. The input/output buffers were 60 words each, consisting of 12 channels of 10-word mercury delay line registers. There are six channels of 10-word mercury delay line registers as spares. With modified circuitry, seven more channels control the temperature of the seven mercury tanks, and one more channel is used for the 10 word "Y" register. The total of 126 mercury channels is contained in the seven mercury tanks mounted on the backs of sections MT, MV, MX, NT, NV, NX, and GV. Each mercury tank is divided into 18 mercury channels.

Each 10-word mercury delay line channel is made up of three sections:

  1. A channel in a column of mercury, with receiving and transmitting quartz piezo-electric crystals mounted at opposite ends.
  2. An intermediate frequency chassis, connected to the receiving crystal, containing amplifiers, detector, and compensating delay, mounted on the shell of the mercury tank.
  3. A recirculation chassis, containing cathode follower, pulse former and retimer, modulator, which drives the transmitting crystal, and input, clear, and memory-switch gates, mounted in the sections adjacent to the mercury tanks.

Instructions and data

Instructions were six alphanumeric characters, packed two instructions per word. The addition time was 525 microseconds and the multiplication time was 2150 microseconds. A non-standard modification called "Overdrive" did exist, that allowed for three four-character instructions per word under some circumstances. (Ingerman's simulator for the UNIVAC, referenced below, also makes this modification available.)

Digits were represented internally using excess-3 ("XS3") binary coded decimal (BCD) arithmetic with six bits per digit using the same value as the digits of the alphanumeric character set (and one parity bit per digit for error checking), allowing 11-digit signed magnitude numbers. But with the exception of one or two machine instructions, UNIVAC was considered by programmers to be a decimal machine, not a binary machine, and the binary representation of the characters was irrelevant. If a non-digit character was encountered in a position during an arithmetic operation the machine passed it unchanged to the output, and any carry into the non-digit was lost. (Note, however, that a peculiarity of UNIVAC I's addition/subtraction circuitry was that the "ignore", space, and minus characters were occasionally treated as numeric, with values of –3, –2, and –1 respectively, and the apostrophe, ampersand, and left parenthesis were occasionally treated as numeric, with values 10, 11, and 12.)


Besides the operator's console, the only I/O devices connected to the UNIVAC I were up to 10 UNISERVO tape drives, a Remington Standard electric typewriter and a Tektronix oscilloscope. The UNISERVO was the first commercial computer tape drive commercially sold. It used data density 128 bits per inch (with real transfer rate 7,200 characters per second) on magnetically plated phosphor bronze tapes. The UNISERVO could also read and write UNITYPER created tapes at 20 bits per inch. The UNITYPER was an offline typewriter to tape device, used by programmers and for minor data editing. Backward and forward tape read and write operations were possible on the UNIVAC and were fully overlapped with instruction execution, permitting high system throughput in typical sort/merge data processing applications. Large volumes of data could be inputted via magnetic tapes created on offline card to tape system and outputted via a separate offline tape to printer system. The operators console had three columns of decimal coded switches that allowed any of the 1000 memory locations to be displayed on the oscilloscope. Since the mercury delay line memory stored bits in a serial format, a programmer or operator could monitor any memory location continuously and with sufficient patience, decode its contents as displayed on the scope. The on-line typewriter was typically used for announcing program breakpoints, checkpoints, and for memory dumps.

A typical UNIVAC I installation had several ancillary devices. There were, typically: a printer that read a magnetic tape and printed output on continuous-form paper; a card-to-tape converter, that read punched cards and recorded their images on magnetic tape; and a tape-to-card converter, that read a magnetic tape and produced punched cards.

The tape-to-card converter had a feature that was not at all publicized. It consisted of a card punch that was connected to two bays of equipment. The right-hand bay contained the tape drive and all of the necessary electronics, while the left-hand bay was the air-conditioning unit. Opening the left-hand bay revealed, at the bottom, an air-conditioning unit that pumped cooled air up modest (perhaps 6" square) duct. When the duct reached about half way up, it fanned out to occupy the entire horizontal area of the bay opening to the front, and was covered with a sturdy horizontal grill. About a foot above this grill, the duct returned to square shape that continued to the top of the bay and thence to the right-hand bay where the air-conditioning was needed. When the left bay door was closed, it sealed this opening so that the cooled air did not leak. This feature was designed into the tape-to-card converter at the request of one of the early UNIVAC I programmers so that he would have a place to keep a case of beer cold when he worked overnight.

See also


  1. ^ The first commercial computer in the world was the BINAC built by the Eckert–Mauchly Computer Corporation and delivered to Northrop Aircraft Company in 1949.
  2. ^ Johnson, L.R., "Coming to grips with Univac," Annals of the History of Computing, IEEE , vol.28, no.2, pp.32,42, April–June 2006. doi: 10.1109/MAHC.2006.27
  3. ^ Reference: CNN's feature on the 50th anniversary of the UNIVAC.
  4. ^
  5. ^
  6. ^ Univac i. (2003). In Encyclopedia of computer science. Retrieved from
  7. ^ Brinkley, Alan. American History: A Survey. 12th Ed.
  8. ^ Johnson, L.R., "Coming to grips with Univac," Annals of the History of Computing, IEEE , vol.28, no.2, pp.32,42, April–June 2006. doi: 10.1109/MAHC.2006.27
  9. ^ UNIVAC conference, Charles Babbage Institute, University of Minnesota. 171-page transcript of oral history with computer pioneers involved with the Univac computer, held on 17–18 May 1990, Washington DC. The meeting involved 25 engineers, programmers, marketing representatives, and salesmen who were involved with the UNIVAC, as well as representatives from users such as General Electric, Arthur Andersen, and the U.S. Census.
  10. ^ Ceruzzi, Paul E. A history of modern computing, MIT, 1998. The source notes that the list is compiled from a number of sources and does not include UNIVACs that were completed not delivered in the period 1951–54. In some cases the dates are approximate. Depending on the definition of "installed" the order may be slightly different.
  11. ^ The vacuum tubes used in the UNIVAC I were mostly of type 25L6, but the machine also used tubes of type 6AK5, 7AK7, 6AU6, 6BE6, 6SN7, 6X5, 28D7, 807, 829B, 2050, 5545, 5651, 5687, 6AL5, 6AN5, 6AH6, 5V4, 5R4, 4D32, 3C23, and 8008.

External links

  • UNIVAC Conference Oral history on 17–18 May 1990. Charles Babbage Institute, University of Minnesota, Minneapolis. 171-page transcript of oral history with computer pioneers, including Jean Bartik, involved with the Univac computer, held on 17–18 May 1990. The meeting involved 25 engineers, programmers, marketing representatives, and salesmen who were involved with the UNIVAC, as well as representatives from users such as General Electric, Arthur Andersen, and the U.S. Census.
  • Margaret R. Fox Papers, 1935–1976, Charles Babbage Institute, University of Minnesota. collection contains reports, including the original report on the ENIAC, UNIVAC, and many early in-house National Bureau of Standards (NBS) activity reports; memoranda on and histories of SEAC, SWAC, and DYSEAC; programming instructions for the UNIVAC, LARC, and MIDAC; patent evaluations and disclosures relevant to computers; system descriptions; speeches and articles written by Margaret Fox's colleagues; and correspondence of Samuel Alexander, Margaret Fox, and Samuel Williams.
  • UNIVAC I documentation – From computer documentation repository
  • Unisys History Newsletter, Volume 5, Number 1 – From Randy Carpenter's home page at Georgia Tech
  • The UNIVAC and the Legacy of the ENIAC – From the University of Pennsylvania Library (PENN UNIVERSITY/exhibitions)
  • UNIVAC 1 Computer System – By Allan G. Reiter, formerly of the ERA division of Remington Rand
  • UNIVAC I & II Simulator – By Peter Zilahy Ingerman; Shareware simulator of the UNIVAC I and II
  • Core memory slide show – This slide show contains a photo of a 1951 core memory module for a UNIVAC I
  • Remington-Rand Presents UNIVAC – Promotional film from the collection of the Computer History Museum, Mountain View, California
  • Popular Science"Want To Buy A Brain", May 1949, early illustrated article on the UNIVAC for the general public
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