IBM Standard Modular System
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The Standard Modular System (SMS) was a system of standard transistorized circuit boards and mounting racks developed by IBM in the late 1950s, originally for the IBM 7030 STRETCH. They were used throughout IBM's second generation computers and peripherals, notably the 7000 and 1400 lines and the 1620. SMS was superseded by Solid Logic Technology (SLT) introduced with System/360 in 1964, however they remained in use with legacy systems through the 1970s.
SMS cards were constructed of individual discrete components mounted on single sided paper-epoxy printed circuit boards. Single width cards were 2.5 inches wide by 4.5 inches tall by 0.056 inches thick, with a 16 pin gold plated edge connector. Double width cards were 5.375 inches wide by 4.5 inches tall, with two 16 pin gold plated edge connectors. Contacts were labeled A–R (skipping I and O) on the first edge connector, and S–Z, 1–8 on the second.
Some card types could be customized via a "program cap" (a double rail metal jumper bar with 15 connections) that could be cut to change the circuit configuration. Card types with a "program cap" came with it precut for the standard configuration and if a Customer Engineer needed a different configuration in the field he could make additional cuts as needed. This feature was intended to reduce the number of different card types a Customer Engineer had to carry with him to the customer's site.
The card type was a two to four letter code imbossed on the card (e.g., MX, ALQ). If the card had a "program cap" the code was split into a two letter card type code and a two letter "cap connection" code (e.g., AK ZZ).
When SMS was originally developed, IBM anticipated a set of a couple hundred standard card types would be all that would be needed, making design, manufacture and servicing simpler. Unfortunately that proved far too optimistic as the number of different SMS card types soon grew to well over 2500. Part of the reason for the growth was that multiple digital logic families were implemented (RTL, DTL, etc.) as well as analog circuits, to meet the requirements of the many different systems the cards were used in.
