Indigo and programmable logic controller: what they have in common

1. Step 1. Indigo — evolved into — Synthetic dye. Synthesizing natural dyes.
2. Step 2. Synthetic dye — is made of — Coal. Aniline from coal tar.
3. Step 3. Coal — is material for — Coal gas. The gas was made by the dry distillation of coal.
4. Step 4. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
5. Step 5. Traffic light — has part — Relay. Relay controllers.
6. Step 6. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.

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