# Start a sequence from a schematic

> Add a schematic button that starts an Arc sequence, and show the active stage.

An operator starts a sequence with a button on a schematic. The button writes `1` to a channel, and the sequence starts when the channel gets the value. A state indicator on the same schematic shows which stage runs.

Before you start, you need:

* An Arc automation with a sequence, such as the one in [write a test sequence](https://docs.synnaxlabs.com/reference/control/arc/test-sequences).
* A schematic with the valves that the sequence controls. See [control hardware from a schematic](https://docs.synnaxlabs.com/reference/control/manual).

## Create the channels

[Create](https://docs.synnaxlabs.com/reference/channels/create) two channels:

* **`start_cmd`** -> A virtual `uint8` channel. The button writes to it. Only the live value matters, so Synnax does not need to store it.
* **`press_stage`** -> A `uint8` channel with an index. The sequence writes a number for each stage to it. Because Synnax stores it, the schematic shows the last stage when you open it.

## Add a start button

1. Turn on editing for the schematic, and add a **Button** symbol.
2. In the **Style** tab of the symbol, set **Label** to `Start`.
3. In the **Control** tab, under **Command**, set **Channel** to `start_cmd`. Leave **Mode** at **Fire**, which writes `1` each time you click.

## Start the sequence and write the stage

1. Open the Arc automation, and change the code so that each stage writes its number to `press_stage`, and `start_cmd` starts the sequence:

   ```arc
   import time

   sequence press {
       stage pressurize {
           1 -> press_stage
           1 -> press_vlv_cmd
           tank_pt > 300 => next
       }
       2 -> press_stage
       0 -> press_vlv_cmd
       time.wait{duration=10s}
       stage vent {
           3 -> press_stage
           1 -> vent_vlv_cmd
           tank_pt < 5 => next
       }
       0 -> vent_vlv_cmd
       0 -> press_stage
   }

   start_cmd == 1 => press
   ```

2. Click **Start** in the controls bar. If the automation already runs, click **Redeploy**.

`start_cmd == 1 => press` starts the sequence on each new `1`. A click while the sequence runs does nothing. After the last step, the sequence stops, and the next click starts it again. See the [execution model](https://docs.synnaxlabs.com/reference/control/arc/execution-model).

If your sequence has an abort stage, write a number for it too, such as `4 -> press_stage`.

## Show the stage on the schematic

1. Add a **State indicator** symbol to the schematic.

2. In the **Telemetry** tab, under **State**, set **Channel** to `press_stage`. Under **Staleness**, set **Timeout** to a value longer than your longest stage, such as `60` seconds. The indicator changes the color of its label when no new value arrives for that time.

3. In the **Options** tab, add an option for each stage, with a name and a value:

   | Name         | Value |
   | ------------ | ----- |
   | Idle         | 0     |
   | Pressurizing | 1     |
   | Holding      | 2     |
   | Venting      | 3     |

The indicator shows the name of the option whose value equals the channel value. It shows **Unknown** for a value with no option.

## Start the test

Turn off editing, then click **Start** on the schematic. The schematic takes control, then writes to `start_cmd`. The indicator shows **Pressurizing**, and the press valve opens.

[](https://synnax.nyc3.cdn.digitaloceanspaces.com/docs/control/arc/start-from-schematic/start-button-dark.mp4#t=0.001)

The schematic writes at authority 1 by default, and the automation at 255, so the automation keeps control of the valves. If you give the automation a lower authority than the schematic, a click on the button also takes control of the valves. The indicator then shows **Pressurizing**, but Synnax drops the commands from the sequence, and the valves do not move. See [control authority](https://docs.synnaxlabs.com/reference/control/authority).

To add an abort button, repeat these steps with an `abort_cmd` channel. See [write a test sequence](https://docs.synnaxlabs.com/reference/control/arc/test-sequences).