# TTL Triggered State Change

Digital input channels generate events whenever their logic level changes — for
example, `TTLIn1_High` when a TTL pulse arrives on TTL input 1, and `TTLIn1_Low`
when the line returns to low. These events can drive state transitions just like any
other event, allowing external hardware to control the state machine.

In this example, `Port1Light` waits for a TTL pulse on TTL input 1. The
`TTLIn1_High` event then advances the state machine to `Port2Light`, which exits
after one second.

![image](state_machines/examples/ttl_triggered_state_change__light.svg)![image](state_machines/examples/ttl_triggered_state_change__dark.svg)

### Python

```python
from bpod_core.fsm import StateMachine

fsm = StateMachine()

fsm.add_state(
    name='Port1Light',
    transitions={'TTLIn1_High': 'Port2Light'},
    actions={'PWM1': 255},
)
fsm.add_state(
    name='Port2Light',
    timer=1,
    transitions={'Tup': '>exit'},
    actions={'PWM2': 255},
)
```

### JSON

```json
{
  "states": {
    "Port1Light": {
      "transitions": {
        "TTLIn1_High": "Port2Light"
      },
      "actions": {
        "PWM1": 255
      }
    },
    "Port2Light": {
      "timer": 1.0,
      "transitions": {
        "Tup": ">exit"
      },
      "actions": {
        "PWM2": 255
      }
    }
  }
}
```

### YAML

```yaml
states:
  Port1Light:
    transitions:
      TTLIn1_High: Port2Light
    actions:
      PWM1: 255
  Port2Light:
    timer: 1.0
    transitions:
      Tup: '>exit'
    actions:
      PWM2: 255
```