Strowger switch and electronic calculator: what they have in common

1. Step 1. Strowger switch — is part of — Telephone exchange. Automatic selectors.
2. Step 2. Telephone exchange — has part — Relay. Relay switching.
3. Step 3. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.
4. Step 4. Programmable logic controller — has part — Microprocessor. A processor.
5. Step 5. Microprocessor — evolved from — Electronic calculator. The Intel 4004 was designed for a Busicom calculator.

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