The missing rung: fault recovery

The safety template ships with a hole in it: fault the cell and nothing on the panel brings it back. The hole is half right — and finishing it correctly is the most integrator-shaped job in this course. You're going to wire the missing rung yourself.

Why the hole is half right

A fault that clears itself is worthless — it would un-remember the breach the moment conditions look good, and "looks good" is exactly what a welded contact says. So the robot's rule is strict:

The fault clears only while a RST command arrives and the enable is already healthy.

Re-arming the chain isn't enough (you saw that last lesson: CELL SAFE lit, robot still faulted). Pressing a reset with the chain down does nothing either. Recovery is two separate acts: restore the permission, then explicitly clear the memory.

But a cell with no reset path at all isn't safe — it's a service call. Somebody has to be able to finish the story.

Build the rung

You need one new device and one new rung. In the safety-chain circuit:

1. Add a pushbutton in the Devices panel — it lands as PB3. Rename it FAULT RST with the ✎ if you like.
2. Tap + rung on the canvas — an empty rung appears at the bottom.
3. Set the Next picker to PB3, tap the new rung's number, and hit + Contact. Set the picker to CR1 and add it in series the same way.
4. + Output, and point the coil at RB1.RST.

r4  ──[PB3]──[CR1  SAFETY OK]──( RB1.RST )

Why the CR1 contact?

The robot already refuses a reset while the enable is down — so why put SAFETY OK in the rung at all? Two honest reasons:

• The print now says what recovery requires. Anyone reading rung 4 sees the policy: no reset attempt without a healthy chain. Prints are documentation that happens to run.
• It keeps a held button from cheating. Without CR1, an operator holding FAULT RST while pressing RESET would clear the fault at the exact instant the chain seals — one gesture, not two. With the contact, the reset rung is simply dead until the chain is already standing. Deliberate stays deliberate.

Run the whole story

This is the full arc every robot cell lives through — walk it end to end:

1. Gate closed (GS1 and GS2 on), RESET — chain seals, CELL SAFE, robot READY.
2. CYCLE START — BUSY, countdown running.
3. Flip GS1 off mid-cycle — FAULT. The cycle is gone.
4. Close the gate again. Nothing. Press RESET — chain healthy, CELL SAFE lit… robot still FAULT, READY still dark.
5. Press your PB3 — FLT drops, READY returns.
6. CYCLE START. Back to work.

Six steps, three deliberate human decisions, zero self-healing. That's a cell you can stand next to — and you wired its recovery path yourself.