Telephone exchange and truck: what they have in common

1. Step 1. Telephone exchange — has part — Relay. Relay switching.
2. Step 2. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.
3. Step 3. Programmable logic controller — has part — Microprocessor. A processor.
4. Step 4. Microprocessor — is part of — Anti-lock braking system (ABS). A control unit.
5. Step 5. Anti-lock braking system (ABS) — evolved from — Air brake. Anti-skid systems first appeared in aviation and on railways.
6. Step 6. Air brake — is part of — Truck. Air brakes, as on trains.

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