IBM 1750, 2750 and 3750 Switching Systems
| Company type | Product line |
|---|---|
| Industry | Telecommunications equipment |
| Predecessor | Electromechanical PABXs (Strowger, Crossbar) |
| Founded | 1968 |
| Founder | IBM engineers at IBM La Gaude Research Laboratory |
| Fate | Discontinued; maintenance continued by third parties into 2000s |
| Successor | IBM 8750 (planned replacement, not fully adopted) |
| Headquarters | Montpellier , France |
Area served | Europe (France, Germany, Italy, Belgium, United Kingdom, Netherlands) |
Key people | IBM La Gaude engineers |
| Products | Stored-program-controlled Private Automatic Branch Exchanges (PABX) |
| Owner | IBM |
| Parent | IBM |
The IBM 1750, 2750, and 3750 Switching Systems represent a series of telephone exchange solutions developed by IBM spanning from the 1960s through the 1990s.
Conceived by IBM engineers at the IBM La Gaude Research Laboratory near Nice, France, these systems were pioneering stored-program-controlled Private Automatic Branch Exchanges (PABX) introduced to the global market. This product line accommodated up to 2,516 extensions and 356 trunk lines, maintaining operational status across five European nations for approximately four decades following the 1969 introduction of the IBM 2750 Switching System.
Prior to 1969, only electromechanical PABXs such as Strowger and Crossbar types were available.
The IBM 1750, 2750, and 3750 models originated from the IBM 1800 platform. Each system incorporated dual stored-program controllers equipped with either 32K or 64K of main memory, running nearly 600,000 lines of code. These systems featured automatic nightly and emergency failover capabilities, paired disk drives (except in the 2750), and solid-state switching technology. Connections for extensions, trunk lines, and tie lines utilized discrete transistors mounted on plug-in panels. All units were manufactured in IBM’s Montpellier facility for deployment in markets including France, Germany, Italy, and the United Kingdom.
Early development
During the late 1960s, engineers at IBM La Gaude created a prototype based on the IBM 1130 designed to control roughly 500 extensions alongside about 20 trunk lines linked to local exchanges.
This effort culminated in the commercial release of the IBM 2750 Switching System (internally known as Carnation), which centered on two fault-tolerant IBM 1800 computers—each initially outfitted with 32K magnetic-core memory and later upgraded to 64K—and utilized IBM Solid Logic Technology (SLT). Disk storage was omitted due to its prohibitive cost during that period. Each processor incorporated a transistorized switching network featuring crosspoint switches to boost system dependability.
The architecture complied with European national telephone authorities’ mandates requiring isolating-line transformers to safeguard their technicians. The 2750 employed sizable plug-in extension modules managing four telephones apiece, compatible with standard PTT 50-volt handsets. The assembler-coded software was loaded into each CPU via punched paper tape, taking approximately ninety minutes per unit. Smaller diagnostic programs operated within unprotected memory areas. Troubleshooting relied on compact routines necessitating an oscilloscope; however, overall system robustness supported uninterrupted operation.
An attached IBM Selectric typewriter enabled users to adjust system parameters such as extension assignments for relocated personnel. The transistorized design contributed to quieter operation compared to widely used electromechanical Strowger PABX systems across Europe.
Countries, costs, installations, 1968 to 2010
Introduced in 1968, the IBM 2750 was marketed in France, Germany, Italy, and Belgium. Subsequent releases of the 3750 in 1970 and the 1750 in 1979 extended availability to these countries plus the United Kingdom. Within the UK, installations included a 2750 at IBM’s London office on Wigmore Street and another at their Havant manufacturing site for demonstration purposes and employee use. Additionally, IBM facilities in the Netherlands deployed units. Pricing ranged between $150,000 and $600,000, with specialized cabling requirements for the 3750 necessitating up to $60,000 in additional costs for dedicated room accommodations. By the time the 1750 was discontinued in 1988, roughly three hundred systems had been installed throughout Europe. The longevity of the 1750 and 3750 models extended into at least 2010, marking them among IBM’s most enduring product lines.
Innovations introduced to markets
During the 1970s, making long-distance and international calls was expensive. Although the IBM 3750 initially carried a price tag nearly double that of similar non-programmable devices, it significantly reduced the need for telephone operators by half and enabled organizations to monitor call activity by department or individual users. This capability facilitated accurate distribution of telephone expenses, supported by detailed call logs that recorded numbers dialed and call durations. The systems gained traction largely due to features such as touch-tone telephones, call forwarding, and abbreviated dialing—capabilities that were groundbreaking at the time but are now commonplace.
Sales in the United Kingdom
Among the first UK adopters of the 3750 system were British Caledonian Airways, Bland Payne Insurance Brokers, Derbyshire County Council, and Geest Bananas. Following these initial installations, numerous major enterprises implemented 3750 systems, including Great Universal Stores, Grosvenor House Hotel, Rowntree Mackintosh, Selfridges, Kwik Save, American Express, Mobil (with networks extending to North Sea oil rigs), Unigate (dairies), Royal Bank of Scotland, Reckitt & Colman, Tesco, CWS (Co-operative Wholesale Society), and Greater Manchester Police (GMP). GMP installed a 3750 at their Chester House headquarters and later expanded with a network of 1750 systems throughout Greater Manchester.
Product lines
| System | Marketing period | Installation period | Number of extensions | Trunks and tie lines | Operator consoles | Primary memory per controller |
|---|---|---|---|---|---|---|
| IBM 2750 Switching System | 1968–1970 | 1968–late 1970s (approximate) | Approximately 100–500 | Up to 20 | Up to 4 | Initially 32K, later upgraded to 64K |
| IBM 3750 Switching System | 1970–1990s | 1970–circa 2010 | Between 248 and 2,516 | Ranging from 32 to 356 | Between 2 and 18 | 64K |
| IBM 1750 Switching System | 1979–1990s | 1979–circa 2010 | From 100 to 760 | Between 0 and 96 | Between 1 and 5 | 64K |
These platforms integrated voice communication with data processing functions, pioneering digital data capture via touch-tone telephone extensions. Their sophisticated voice management features were highly valued by clients. The systems demanded a reliable and uninterrupted power source, consuming between two and fifteen kilowatts.
Extension capabilities
Several advanced extension features were particularly attractive to financial institutions and office environments:
- Touch-tone telephones supporting both voice and data input, a novel concept in Europe at that time
- Call forwarding from various extensions to designated answering points
- Camp-on functionality with automatic callback for busy lines
- Short-code dialing enabling quick access to domestic and international numbers
- Temporary restrictions on outgoing calls
- Unique ring patterns distinguishing internal from external calls
- Dial-activated paging services
- Group answering with authorized user intrusion capabilities
- Integration of third-party conference calling options
- Call pickup combined with automatic dialing upon going off-hook
Management capabilities
Significant management advancements included:
- The IBM 3755 Operator Desk featuring optional braille keys alongside audible alert systems
- Centralized modification of extension numbers and system facilities through a keyboard interface
- Recording of call costs coupled with traffic analysis tools
- Implementation of service classes assigned to extensions
- Multiple night service configurations encompassing pre-night, standard night, special-type-1, and special-type-2 modes
Data capabilities
The data functionalities were notably sophisticated for their era:
- Gathering data from all touch-tone extensions or directly through the switching system connected to an IBM computer
- IBM 3221 Numeric Multi-function Device: a desktop terminal equipped with a numeric keypad and badge/card reader
- IBM 3223 Entry/Exit Reporter: a wall-mounted magnetic card reader designed for attendance tracking and secure access management
- Monitoring and managing contacts
IBM customers utilized these data tools for purposes such as reporting chargeable employee activities conducted via telephone.
Additional features introduced later
IBM broadened its portfolio by adding capabilities like satellite operation, which allowed remote operator call handling. The IBM Audio Distribution System (IBM 7770), built on an IBM Series/1 computer platform, provided centralized voice message recording for up to 3,000 users with 100 hours of storage capacity and optional computer-based notifications. Other enhancements included hospitality services such as automated wake-up calls and system partitioning to facilitate multiple organizations operating on a single system.
Networking
IBM focused on the expanding demand for interconnected systems with centralized control, including tie-line busy backup or rerouting through public networks, traffic saturation management, tie-line access limitations, extended network numbering up to seven digits, remote paging capabilities, camping on remote extensions, and integration of third-party call additions.
Subsequent product developments
By 1984, IBM foresaw a decline in sales for the aging 1750 and 3750 systems and encountered difficulties when its French development center in La Gaude failed to deliver a successor. As a result, IBM acquired the American firm ROLM, which had installed over 20,000 PABXs in the United States. IBM planned to introduce ROLM’s system in Europe under the name IBM 8750 as a replacement for the 1750 and 3750 models. However, challenges in adapting ROLM products for European markets hindered this transition. IBM also considered marketing Siemens PABXs but ultimately withdrew from selling all PABX systems. Maintenance services for existing 1750 and 3750 installations persisted into the 2000s through third-party agreements, representing some of IBM’s longest-supported products.
IBM 8750
IBM purchased land in Wootton Bassett, near Swindon, England, intending to establish a ROLM manufacturing facility for Europe; however, this site was never developed. In 1987, IBM introduced the ROLM-based IBM 8750 Business Communications System across Austria, Belgium, France, Germany, Italy, Luxembourg, and the United Kingdom. Although several units were installed at IBM locations such as IBM Havant and IBM Portsmouth primarily for certification purposes, no deployments occurred at customer sites.
The IBM 8750 accommodated between 91 and 3,000 extensions with support for up to 1,000 concurrent calls. It incorporated a Motorola 68020 processor-based computer, supported up to sixteen IBM 8755 Operator Consoles, included a fixed disk of 30MB capacity, and was compatible with main/satellite configurations involving the 3750 and 1750 systems. Digital trunking was available in Belgium, Italy, and the UK, utilizing ISDN and Systems Network Architecture (SNA) communications networks.