The chain that arms the cell

The enable is the cell's life switch — so the real question of robot integration is: what is allowed to feed it? The answer is a circuit you already know from motor work, doing the most important job in the building.

The print

r0  ──┬──[PB1 RESET]──┬──[/ES1]──[GS1]──[GS2]──( CR1  SAFETY OK )
      └────[CR1]──────┘
r1  ──[CR1]──────────( RB1.EN ) ∥ ( RB1.P1 ) ∥ ( PL1  CELL SAFE )
r2  ──[PB2]──[RB1.RDY]──────────( RB1.START )
r3  ──[LC1]─────────────────────( RB1.HOLD )

Rung 0 is a seal-in — the same start/stop pattern as a motor, except what it starts is permission. CR1 is the safety relay: it pulls in when someone presses RESET with the whole chain healthy, and it holds itself in through its own contact. Everything the robot is allowed to do (rung 1) hangs off CR1.

Why a seal, and not a straight wire

Imagine rung 1 fed directly by the chain: [/ES1]──[GS1]──[GS2]──(EN). Now close the gate after lunch — and the cell re-arms itself, because a door shut. Whoever is still inside gets an enabled robot for company.

The seal makes re-arming a decision: the chain healthy and a human pressing RESET, on purpose, from outside. That's a bedrock rule of machine safeguarding — restoring a guard must never restart the machine. The door closing takes the cell from "forbidden" to "possible." Only the button takes it to "armed."

Two channels for one gate

GS1 and GS2 are the same physical gate — two independent switches in series. One switch can lie: a welded contact, a crushed wire shorted just right, and a single channel reads "closed" forever, gate wide open. Two channels have to lie at the same time, the same way for the chain to be fooled.

Real safety relays go further than our series pair — they watch for the channels disagreeing, and they verify the contactor actually dropped before allowing a reset (cross-monitoring and EDM, if you want the datasheet words). The series idea is the part the print shows; the relay's extra paranoia is why it costs what it costs.

The e-stop rides the same chain

/ES1 sits in series with everything: press it anywhere, any time, and CR1 drops — which drops EN — which stops the cell and remembers that it stopped. What "remembers" means is the next lesson.

Run it

Open the safety-chain circuit:

1. Close the gate: flip GS1 and GS2 on. Try leaving one off first — one channel is never enough.
2. Chain healthy, e-stop released — and the cell is still dead. Closed contacts don't arm a cell.
3. Press RESET: CELL SAFE lights, the robot goes READY.
4. Press CYCLE START — and while it's running, flip GS1 off (one channel of the gate opens). Watch the whole tower come down. Getting back from that is lesson six.