Bellows and programmable logic controller: what they have in common

1. Step 1. Bellows — is part of — Barrel organ. Bellows feed air into the pipes.
2. Step 2. Barrel organ — has part — Pinned cylinder. A pinned cylinder sets the tune.
3. Step 3. Pinned cylinder — works like — Punched card. Pins on a cylinder and holes in a card are two ways of recording a sequence for a machine.
4. Step 4. Punched card — is used in — Harvard Mark I. The program was read from punched tape and the data from punched cards.
5. Step 5. Harvard Mark I — has part — Relay. The Mark I was electromechanical, built from relays.
6. Step 6. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.

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