Semaphore arm and industrial robot: what they have in common

1. Step 1. Semaphore arm — is part of — Railway semaphore. The position of the arm is the signal.
2. Step 2. Railway semaphore — evolved into — Traffic light. The first traffic light was a railway-style semaphore.
3. Step 3. Traffic light — has part — Relay. Relay controllers.
4. Step 4. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.
5. Step 5. Programmable logic controller — is used in — Industrial automation. Controllers.
6. Step 6. Industrial automation — uses — Industrial robot. Robots.

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