Upon introduction in May 1960, the Monrobot XI sold for US$24,500. In March 1961, the US Army reported that seven units had been made. In November 1961, the price remained unchanged and leasing ran US$700 monthly. By 1966, there were about 350 machines in the field, but by 2013 no machines were known to remain in existence.
The bare-bones Monrobot XI resembled an ordinary steel desk in length, breadth, and height, surmounted by an ordinary typewriter and a breadbox-like-volume control panel. At a weight of, its purveyors pronounced it portable. It could operate outside of an air-conditioned room, using a conventional mains power line and about half as much electrical power as a toaster.
Architectural philosophy
Unlike virtually all electronic digital computers ever built, as an early machine, the Monrobot XI was one of the small family of computers which totally lacked random-access memory, which alternative would have allowed it to access all memory words equally fast. Even by the time it was introduced, it was not uncommon for electronic digital computers to use magnetic-core memory for RAM, the price of which would fall from over US$1 in the early 1950s to about US$0.20 by the mid-1960s. But to keep the cost of the machine very low, instead the Monorobot XI used a form of memory in which words were cyclically accessible in sequential order, using an electromechanical moving part called magnetic drum memory. Thus, physically, it bore some resemblance to the hypothetical Turing machine of computer science, albeit with the latter's tape being of finite length and joined end-to-end, and then finally replicated many times collaterally. The long latency of memory access, which followed from intensive use of such a macroscopic moving part, made the Monrobot XI operate very slowly, despite the exploitation of non-mechanical electronics for logical functions. The Monorobot XI might best be thought of as a modernized, low cost version of the IBM 650, the world's first mass-produced computer, leased at US$3,250 per month, almost 2,000 of which were made between 1954 and 1962, 800 by 1958. Both the IBM 650 and Monrobot XI used a magnetic drum for primary memory, but the former used vacuum tubes and bi-quinary coding, rather than transistors and binary coding, for its electronics.
Persistent electro-mechanical memory
The Monrobot XI's rewritable, persistent memory consisted of a rotating magnetic drum storing 1,024 words of 32-bits, which could record either a single integer or a pair of zero/single-address instructions. The average access time of 6,000 microseconds implies the drum made a rotation every 12 milliseconds, i.e. rotated at 5,000 RPM. Even the high-speed registers of the central processing unit physically resided on the drum, being replicated 16 times, so that they might be read or written 16 times as fast as the bulk of persistent memory.
Electronics
Save for the neon lamps in the control panel and 10-30 vacuum tubes employed for output devices, the electronics used only discrete solid-state components, including 383 transistors and 2,300 diodes. This astoundingly small active component count - little more than in the Manchester Baby, the world's first stored-program Turing-complete computer, is in stark contrast to the many millions of transistors present in modern microprocessors; this was a key advantage of using its slow, now-primitive, electromechanical memory, which exploited measuring a drum's rotational angle, not adding switches, to multiplex bits. Even Intel's first microprocessor, the four-bit Intel 4004, required about 2,300 transistors in its monolithic design. Construction was via pluggable printed-circuit boards, allowing economical partial replacement of a broken system as the principal means of repair. This continued an electronics tradition pioneered when relatively unreliable short-lived vacuum tubes had been used as active components, prior to the advance to more modern, highly reliable solid-state transistors which the Monorobot XI exploited.
The arithmetic unit performed computations using the binary number system, with machine-language programming using hexadecimal digits, employing the unusual digit nomenclature set of. Statistically, addition of 32-bit integers required 3ms to 9ms, multiplication 28ms to 34ms, and division 500ms. The longer durations reflect the mean latency of accessing a persistent memory location, rather than a register, for the second of the two operands. The computer could be programmed using an assembly language system called QUIKOMP, but its simple machine language instruction set and slow operation speed encouraged many programmers to code directly in machine language.
