page_title: IBM 1401
section_title: Hardware implementation
hierarchical_section_title: IBM 1401 / Hardware implementation
caption_attribution_description: w:IBM Standard Modular System card.
context_page_description: The IBM 1401 is a variable-wordlength decimal computer that was announced by IBM on October 5, 1959. The first member of the highly successful IBM 1400 series, it was aimed at replacing unit record equipment for processing data stored on punched cards and at providing peripheral services for larger computers. The 1401 is considered to be the Model-T Ford of the computer industry, because it was mass-produced and because of its sales volume. Over 12,000 units were produced and many were leased or resold after they were replaced with newer technology. The 1401 was withdrawn on February 8, 1971.
context_section_description: Most of the logic circuitry of the 1401 was a type of diode–transistor logic (DTL), that IBM referred to as CTDL (Complemented Transistor Diode Logic). Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the germanium-alloy transistors used), CTRL (Complemented Transistor Resistor Logic, a type of resistor–transistor logic (RTL)). Later upgrades (e.g., the TAU-9 tape interface) used a faster type of DTL using "drift" transistors (a type of transistor invented by Herbert Kroemer in 1953) for their speed, that IBM referred to as SDTDL (Saturated Drift Transistor Diode Logic). Typical logic levels of these circuits were (S & U Level) high: 0 V to -0.5V, low: -6 V to -12 V; (T Level) high: 6 V to 1 V, low: -5.5 V to -6 V.
  These circuits were constructed of discrete components (resistors, capacitors, transistors) mounted on single-sided paper-epoxy printed circuit boards either 2.5 by 4.5 inches (64 by 114 mm) with a 16-pin gold-plated edge connector (single wide) or 5.375 by 4.5 inches (136.5 by 114.3 mm) with two 16-pin gold-plated edge connectors (double wide), that IBM referred to as SMS cards (Standard Modular System). The amount of logic on one card was similar to that in one 7400 series SSI or simpler MSI package (e.g., three to five logic gates or a couple of flip-flops on a single-wide card up to about twenty logic gates or four flip-flops on a double-wide card).
  The SMS cards were inserted in sockets on hinged swing-out racks, that IBM referred to as gates.
  The modules used were fairly delicate, compared to previous unit-record equipment, so IBM shipped them enclosed in a newly invented packing material, bubble wrap.  This was one of the first widespread uses of this packing; it greatly impressed recipients, and brought great publicity to the material.
  Like most machines of the day, the 1401 used magnetic-core memory. The cores were about 1 mm in diameter and used a four-wire arrangement (x, y, sense, and inhibit).
  The memory was arranged in planes of 4000 
  cores each, each core storing one bit. A stack of eight such planes stored the six data bits, word mark bit, and parity bit for 4000 memory locations. Together with eight additional planes with fewer cores on them for additional storage functions, this made up a 4000-character  memory module. One such module could be housed within the 1401's primary enclosure. Systems were commonly available with two, three, or four such modules. The additional modules were contained in an add-on box, the 1406 Core Memory Unit, which was about two feet square and three feet high.
  Operands in memory were accessed serially, one memory location at a time, and the 1401 could read or write one memory location within its basic cycle time of 11.5 microseconds.
  All instruction timings were cited in multiples of this cycle time.
caption: A single-width SMS card of the type used in the 1401
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