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.

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:

• Touch-tone telephones (then practically new to Europe) for both voice and data entry
• Call rerouting from multiple extensions to answer points
• Camp on to a busy extension or external circuit with automatic call back
• Short-code dialling to national and international numbers
• Temporary call barring
• Distinctive ring cadences (different cadences for internal and incoming external calls)
• Dialled paging
• Group answering
• Authorised user intrusion
• Add-on third party
• Call pick-up
• Non-dialled connection (an off-hook extension automatically dials an extension).

Management facilities

Most management facilities were new to the PABX market:

• IBM 3755 Operator Desk: optionally with braille-coded keys and audible alarms
• Extension number and facility changes made from a central keyboard
• Call cost recording
• Classes of service for extensions
• Traffic analysis
• Night service: pre-night, standard, special-type-1, and special-type-2 night services.

Data facilities

The then-novel data facilities included:

• Data collection available from every touch-tone extension to the switching system or through it to an IBM computer
• IBM 3221 Numeric Multi-function Device: an extension-connected desktop terminal with a numeric keyboard and badge/card reader
• IBM 3223 Entry/Exit Reporter: an extension-connected wall-mounted magnetic-card-reader terminal for attendance recording and controlled access to secure areas
• Contact monitoring and operation.

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:

• Satellite operation (remote operator call handling)
• The IBM Audio Distribution System (IBM 7770) (the then-novel centralised message recording for 1000 users)
• Partitioning: multiple organisations using one system.

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 ).

References