Standard Modular System
| Company type | Technology/Hardware system |
|---|---|
| Industry | Computer hardware |
| Founded | 1950s |
| Founder | IBM |
| Products | Transistor-based circuit boards and mounting frames |
| Parent | IBM |
The Standard Modular System (SMS) represents a collection of uniform transistor-based circuit boards and mounting frames created by IBM in the late 1950s, initially intended for the IBM 7030 Stretch. This system found extensive application throughout IBM's second-generation computing machines, peripheral equipment, the 7000 series, 1400 series, and IBM 1620. Although SMS was superseded by Solid Logic Technology (SLT) with the introduction of the 1964 System/360, it continued to be utilized in legacy systems well into the 1970s.
Overview
Certain peripherals for the System/360, derived from earlier second-generation models, retained SMS circuitry rather than transitioning to SLT. Examples of these devices include the 240x-series tape drives, 2540 card reader/punch, 1403N1 printer, and the 2821 Integrated Control Unit that supported both the 1403 and 2540. Within some of these peripherals, SMS cards incorporated hybrid integrated circuits resembling SLT technology.
SMS cards are built from discrete components affixed to single-sided paper-epoxy printed circuit boards. The dimensions for single-width cards are 2.5 inches in width, 4.5 inches in height, and a thickness of 0.056 inches, featuring a gold-plated edge connector with 16 pins. Double-width cards measure 5.375 inches across and have two such connectors. The contacts on the first connector are labeled from A through R, omitting I and O, while the second connector’s contacts range from S to Z and then 1 through 8.
These cards insert into a card-cage backplane where wire-wrap connections establish links to pins; all connections except those for power distribution employ wire-wrap technology. While most wire-wrap terminations were installed at the factory using automated machinery, this method also allowed field engineers to perform modifications efficiently on-site.
Some SMS cards include a "program cap," which is a double-rail metal jumper bar with fifteen connection points that can be selectively severed to modify circuit behavior. These caps were supplied precut for standard configurations but could be customized further during servicing to minimize the number of different card variants required.
Identification of card types is done through two- to four-letter codes embossed directly on each card (for instance, MX, ALQ). For cards equipped with program caps, the marking is divided into a two-letter card type code plus a two-letter code indicating the cap’s connection pattern (e.g., AK ZZ).
IBM originally anticipated only several hundred standardized card types to streamline both design and maintenance processes. However, this expectation was overly optimistic as more than 2,500 distinct SMS card types were eventually produced. This diversity arose due to multiple digital logic families—such as ECL, RTL, DTL—and various analog circuits necessary across different systems.