IBM 1750, 2750 and 3750 Switching Systems

In 1969 IBM started marketing in five European countries the IBM 2750 Switching System – worldwide, the first stored-program-controlled PABX (Private Automatic Branch Exchange). Previously only electromechanical Strowger and Crossbar PABXs were available.[1]

The 2750 was sold in France, Germany, Italy, and Belgium. The other systems were sold in these countries and the United Kingdom. The Netherlands installed some in IBM sites.

The family of IBM 1750, 2750 and 3750 Switching Systems was developed from the IBM 1800 by the IBM La Gaude Research Laboratory near Nice, France for European markets only. Each system included twin stored-program controllers (each with 32K main storage, some 600,000 lines of code, and nightly and emergency automatic switchover), twin disks, and solid-state switching. Extension, trunk and tie lines were connected by discrete transistors on plug-in panels.

System Year first marketed Extensions Trunks + tie lines Operator desks
IBM 2750 Switching System 1968 100-500?
IBM 3750 Switching System 1970 248 - 2516 32 - 356 2 - 18
IBM 1750 Switching System 1979 100 - 760 0 - 96 1 - 5

The systems all had both voice and data functions – the marketplace largely bought them for their then-new voice and management functions. Early-1960s computers had hardly any typewriter-like terminals and no screens – the IBM Switching Systems introduced the novelty of simple digital-data capture from every touch-tone telephone extension.

Extension facilities

Facilities for the extensions were revolutionary at the time and particularly valued by organisations in financial and other industries with relatively highly-paid office-based employees:

Management facilities

Most management facilities were new to the PABX market:

Data facilities

The then-novel data facilities included:

IBM’s customers for instance used the data functions for staff to report chargeable activity from their telephones.

Later additional facilities

Over time IBM introduced further functions:

Networking

Multiple interconnected systems with central operating and control functions, automatic recall between systems, tie-line busy back-up or rerouting via the public network, traffic saturation control, tie-line access restriction, network numbering up to 7 digits.


The systems needed considerable space, air conditioning, and secure electricity night and day (from about 2 to 15 kilowatts ).

  1. IBM 1750 Switching System – system overview IBM UK brochure GK10-6264-2 and IBM Germany brochure Vermittlungssystem IBM 1750 Systemüberblick GA12-12-2437-1
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