OE stops everything

A pin that overrides the bus

OE is the fifth pin on the control header, and the underside prints HIGH - DISABLE beside it. It goes straight to the chip's output-enable pin, which is active LOW: LOW means outputs on.

A 10 kΩ resistor on the board pulls OE to GND. Leave the pin unconnected and the outputs are always on, which is how every build in this book is wired. Pull it HIGH and all sixteen outputs are driven LOW at once: no pulses leave the board.

What it does and does not stop

It stops the pulses. The chip keeps running, keeps answering on I²C, and keeps every channel's setting; drop OE LOW again and the same pulses come back.

It does not cut servo power. V+ stays on the red row. What a servo does with no pulse depends on the servo: many simply stop driving and can be turned by hand, but do not count on a particular one to go limp.

OE is one of the six chained pins, so in a plugged-together chain one OE wire stops every board.

Wiring a stop

Any free GPIO to OE:

const int KILL = 4;           // any free GPIO, wired to OE

void setup() {
  pinMode(KILL, OUTPUT);
  digitalWrite(KILL, LOW);    // outputs on
}

void stopAll()  { digitalWrite(KILL, HIGH); }
void startAll() { digitalWrite(KILL, LOW); }

Because it is one wire and one GPIO, it works when a stuck I²C bus would not let a command through. Until the sketch sets the pin as an output, the board's resistor holds OE LOW and the outputs stay live, unless the microcontroller itself pulls that GPIO up while it boots.