SP-1 was the name of a computerized telephone exchangemanufactured by Northern Electric in Canada. It was introduced in 1971 As indicated by the name "Stored Program", the SP-1 introduced computer control to the telephone switching market. A central computer controlled the operation of the switch. Switch behavior was determined by the operation of a computer program. With its use of computer hardware and software control, the SP-1 marked an evolutionary step in telephony design. It was an intermediate form between the previous generation of electromechanical systems and the next generation of fully digital systems. Like the previous generation of systems, the SP-1 was an analog switch that used a special form of mechanical relay to provide the voice connections. The voicesignal remained in the analog domain throughout the exchange, with the crossbar switches providing a metallic path to connect the end points of a call together. However the SP1 replaced the previous generation's complex relay-based controllers with a modern Harvard architectureelectronic computer, with separate data and program memories. SSI DTL logic was used throughout. Two types of memory was used - ferrite sheet memory for the CPU and Piggyback Twistor memory for the storage of program and routing information. A number of variations on the base switch were available. A "two wire" version for local service, a four wire version for toll service. Two/four wire versions were also available as was a TOPS variant that was used to provide Operator services. The first four wire switch was placed in service in Thunder Bay circa 1972. The operator service consoles used an Intel 4004 processor. The next generation of systems evolved from the SP1 design to replace the analog switch with a digital technology. SP-1 proved the feasibility of software-controlled systems for telephony. It set the stage for the introduction of fully digital systems with the development of the DMS by Nortel in the 1970s.
