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IBM's 604 and the Lessons of War

Dr. Dobb's Journal

History of Computing #8

Tom Watson Jr. returned home from World War II impressed by the impact that electronic computing made on winning the war. But convincing IBM's old guard that an "electronic brain" had commercial potential was a battle in itself.

IBM's first mass-produced electronic computing machine was the Model 604, "Electronic Calculating Punch." This calculator could perform the four basic arithmetic functions (as well as square rooting) briskly—in fact, hundreds of times faster than IBM's electromechanical alternatives. Significantly, the 604 also used a new manufacturing technology—the "pluggable unit." Figure 1 shows one of these units. This modular approach consisted of a single vacuum tube mounted in a small vertical package and included a small number of passive components (resistors and capacitors) as well as a handle to allow easy insertion and removal. The modular architecture made for an attractive and successful product. Figure 2 show a rack in the 604 containing pluggable units.

The 604 was announced in June 1948, with first deliveries taking place in early 1949, and, while not a stored-program machine allowing self-modification of its code, the 604 had a plugboard architecture of forty (and later sixty) steps. Over the next ten years, around 5,600 604s were built—an astounding number for IBM's first foray into electronics. The 604 was only the second IBM machine to perform division (the less-versatile Model 602 was the first) and reflected the slowly changing management culture within the world's largest producer of business machines. This change was slow in coming, and even slower in execution, with the "old guard," symbolized by President and Founder Thomas Watson Senior and his lieutenants, squaring off against the younger Thomas Watson Junior (known as "Tom"). While "old man Watson" and the established forces of IBM (many of whom had been with IBM since its founding in 1924) saw little or no business opportunity in the newfangled world of electronics, Tom returned from WWII with a clear sense of how the same technology that had won the war for the Allied powers could also be deployed in commercial products. At issue, however, was the simple business fact of a large company changing direction into uncharted waters.

Inertia came from several sources: first and foremost was IBM's enormous customer base of punch card users. With its superb sales force, IBM knew that its customers did not immediately ask for anything but improved version of the unit record equipment they were already using. Senior IBM management, which listened very closely to its customers (particularly its large insurance and government accounts) thus saw only risk; whereas Watson Jr. and an increasing number of returning war veterans who had been trained in electronics, saw opportunity. It is significant to note that even in 1960, 20 percent of IBM's revenue came from the sale of the blank punch cards alone. Punch card equipment was thus an enormous cash cow for the company in the postwar boom years as well.

It wasn't that IBM had no experience with electronics: it had produced at least two electronic (relay and vacuum-tube based) machines in the early 1940s—the ASCC for Harvard and the SSEC for its own head office in New York City. But these machines were akin to a cyclotron today: gargantuan, finicky, and power-hungry behemoths that required specialized staffs and were highly unreliable (MTBFs in the 10 hour range). This unflattering view an "electronic brain" strongly colored senior management's view of mass market viability and is the true context behind Watson Sr.'s remark that "there is a need in the world for perhaps 5 or 6 of these machines," a quote invariably cited out of context by people today. The truth is that Watson was correct—to compare the hundreds of millions of computers today and make reference to this prognostication is unfair to Watson and as manifestly misleading as comparing apples to oranges.


Machine Quick Facts:


Architecture: Plugboard-based sequence controlled

Instruction Set: approx. 16 instructions

Word Length: 32 decimal digits

Memory: 40 program steps (60 with optional Model 941 relay memory)

I/O: Model 521 card reader/punch

Performance: approx. 1 KIP

Basic machine cycle: 50kHz

Technology: Vacuum tube (1,400)

Size: 52 1/2" x 31" x 62" (HWD)

Weight: approx. 1,300 lbs

Number produced: 5,600

Cost: $645/month, lease-only (1949 dollars)

Power consumption: 7.59kW [230VAC @ 33A]


The 604 itself formed part of a system: a card reader/punch attached for I/O made it a usable machine. Aside from its own virtues, the 604 was critical in spurring one of IBM's biggest customers, Northrop Aircraft, into an interesting little heresy that dramatically altered IBM's destiny in computing machines. Two Northrop engineers, Greg Toben and William Woodbury (Woodbury still lives in San Francisco), connected a 603 to a 405 accounting machine and modified some of the internal wiring. It should be noted that opening the cabinet of IBM equipment was strictly forbidden under the terms of IBM's customer leasing agreement (until 1956 all IBM machines could only be leased). While this was ostensibly to protect the customer from the lethal voltages inside, there obviously was a desire to minimize "cloning" of IBM equipment by its competitors (chiefly Remington-Rand). Once IBM field engineers and head office management got over their initial dismay, the merits of this combination could be seen (it could store a program for example). The new machine, which Toben and Woodbury termed the "poor man's ENIAC," was redesigned around the 604 and a 402 accounting machine and dubbed the IBM Card Programmed Calculator (CPC), an extremely successful mass-produced IBM product, with over 2,500 units produced. Figure 3 shows a similar system.

The 604 was significant for IBM since it forced the company to think in terms of technologies other than the electromechanical devices upon which its success to date had been forged. IBM was just beginning to consider building large electronic scientific machines (IBM viewed the world in terms of two types of customer: business and scientific users). The 604 and CPC allowed it address a much larger market and to develop the manufacturing technologies it would later use when designing and building its "giant brains" of the early 1950s. Vacuum tubes of the type commonly in use at the time (chiefly for radio work) were entirely unsuitable to computing machinery, so IBM spent a great deal of time in perfecting these in concert with the major tube manufacturers.

Interestingly, the knowledge gained by returning war veterans probably did more to advance IBM's efforts in electronic computation that did anything else. Since Watson Sr. had insisted that all IBM employees were guaranteed a job on their return from the war, this ensured that a workforce trained in the latest electronic techniques became valuable IBM assets. A similar propagation of advanced knowledge occurred in England, particularly at the University of Manchester, where computing legends Freddie Williams, Tom Kilburn, and Alan Turing drew extensively on the techniques (if not the actual applications) of their war work. For a time, the U.K. became a world leader in computing machinery, until it was eclipsed by American companies in the early 1960s.

See an IBM Model 604 on display at The Computer Museum History Center.

For further reading:

Basche, C.J., Johnson, L. R., Palmer, J. H., Pugh, E.W., IBM's Early Computers, Cambridge: MIT Press, 1986.

H.H. Goldstine: The computer from Pascal to Von Neumann. Princeton University Press 1972,1993.

P.T. Nims: "The IBM type 604 electronic calculating punch as a miniature card-programmed electronic calculator," Proc Computation Seminar, August 195, IBM, 1951, pp 37-47.

J.W. Sheldon and L. Tatum: "The IBM card- programmed electronic calculator". Review of Electronic Digital Computers, Joint AIEE-IRE Computer Conf. 1952, pp 30-36.

IBM 604 Electronic Calculating Punch, IBM Corporation, 1948.

Ed Thelen's 604 Facts:

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