<sod>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 [[electromechanics|electromechanical system]]s 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 (Minibar [[crossbar switch]]) to provide the voice connections. The voice signal 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 ([[Marker (telecommunications)|markers]]) with a modern [[Harvard architecture]] electronic computer, with separate data and program memories. The next generation of systems evolved from the SP1 design to replace the analog switch with a digital technology.<eod>
<sod>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 [[electromechanics|electromechanical system]]s 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 (Minibar [[crossbar switch]]) to provide the voice connections. The voice signal 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 ([[Marker (telecommunications)|markers]]) with a modern [[Harvard architecture]] electronic 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 Twister 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 next generation of systems evolved from the SP1 design to replace the analog switch with a digital technology.<eod>