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# Write your first Arc sequence

> Write a pressurize, hold, vent, and abort sequence for the sample project tank.

In this tutorial, you write your own copy of the Press sequence from the [sample project](https://docs.synnaxlabs.com/reference/get-started/sample-project). You add one stage at a time, and you run the sequence from the Tank schematic after each step. You need no hardware, and the tutorial takes about 15 minutes.

You need [Synnax Desktop](https://docs.synnaxlabs.com/reference/get-started/install-desktop) with the sample project open. To open it, see [explore the sample project](https://docs.synnaxlabs.com/reference/get-started/sample-project#open-the-sample-project). If you use Synnax Enterprise, you can follow this tutorial in Synnax Desktop, which is free.

The tutorial uses these channels from the sample project:

| Channel                | What it holds                                         |
| ---------------------- | ----------------------------------------------------- |
| `sample_press_vlv_cmd` | The press valve command: `1` opens it, `0` closes it. |
| `sample_vent_vlv_cmd`  | The vent valve command.                               |
| `sample_tank_pt`       | The tank pressure in psi, from the Tank simulator.    |
| `sample_start_cmd`     | `1` when you click **Start** on the Tank schematic.   |
| `sample_abort_cmd`     | `1` when you click **Abort** on the Tank schematic.   |

## Step 1Stop the Press sequence

Your sequence writes to the same valves as the Press sequence. Stop the Press sequence so that only your sequence moves the valves.

1. In the left nav, click **Arcs** (), or press **A**.
2. On the Press sequence, click **Stop** ().

Leave the Tank simulator running. It turns the valve commands into a tank pressure.

## Step 2Create an Arc automation

1. In the Arcs toolbar, click **Create Arc automation** ().
2. Type `My press sequence` as the name, and click **Create**.
3. In the controls bar at the bottom of the editor, open the **Driver** list and select **Node 1**. This entry runs the automation inside the Synnax Core.

The status shows “Task not deployed”.

## Step 3Write the first two stages

1. Copy this code into the editor:

   ```arc
   sequence press {
       stage idle {
           0 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
           sample_start_cmd == 1 => pressurize
       }
       stage pressurize {
           1 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
       }
   }

   true => press
   ```

   * `sequence press` holds stages. One stage of the sequence is active at a time.
   * `0 -> sample_press_vlv_cmd` writes `0` to the channel when the stage starts.
   * `sample_start_cmd == 1 => pressurize` moves the sequence to `pressurize` when you click **Start**.
   * `true => press` starts the sequence, at its first stage, when the automation starts.

2. Click **Start** () in the controls bar. The status shows “Started task”.

3. Select the **Tank** tab, and click **Start** on the schematic.

The press valve opens, and the pressure climbs on the Tank pressure line plot. The sequence stays in `pressurize`, because the stage has no way out yet. The control legend on the schematic shows `My press sequence`.

Your automation writes at authority 255, the default. The schematic writes at 1, so a click on a valve has no effect while your automation runs. See [control authority](https://docs.synnaxlabs.com/reference/control/authority).

[](https://synnax.nyc3.cdn.digitaloceanspaces.com/docs/control/arc/get-started/first-stages/e43f1213388e-dark.mp4#t=0.001)

## Step 4Hold and vent

1. Replace the code with this code. It adds `import time`, a condition in `pressurize`, and the `hold` and `vent` stages:

   ```arc
   import time

   sequence press {
       stage idle {
           0 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
           sample_start_cmd == 1 => pressurize
       }
       stage pressurize {
           1 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
           sample_tank_pt > 300 => hold
       }
       stage hold {
           0 -> sample_press_vlv_cmd
           time.wait(duration=10s) => vent
       }
       stage vent {
           1 -> sample_vent_vlv_cmd
           sample_tank_pt < 5 => idle
       }
   }

   true => press
   ```

   * `time.wait(duration=10s) => vent` moves to `vent` 10 seconds after `hold` starts.
   * `sample_tank_pt < 5 => idle` goes back to `idle`, so you can run the test again.

2. Click **Redeploy** in the controls bar. The automation starts again with the new code, in the `idle` stage.

3. On the Tank schematic, click **Start**.

The tank is still near 500 psi from step 3, so the first run goes straight to `hold`, then vents. When the pressure is below 5 psi, click **Start** again. The pressure climbs to 300 psi, holds for 10 seconds, and falls.

To change timing precision and CPU use, see [set the performance level](https://docs.synnaxlabs.com/reference/control/arc/automations#set-the-performance-level).

## Step 5Add an abort

1. Replace the code with this code. It adds an `abort` stage, and conditions that move to it from `pressurize` and `hold`:

   ```arc
   import time

   sequence press {
       stage idle {
           0 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
           sample_start_cmd == 1 => pressurize
       }
       stage pressurize {
           1 -> sample_press_vlv_cmd
           0 -> sample_vent_vlv_cmd
           sample_tank_pt > 450 => abort
           sample_abort_cmd == 1 => abort
           sample_tank_pt > 300 => hold
       }
       stage hold {
           0 -> sample_press_vlv_cmd
           sample_abort_cmd == 1 => abort
           time.wait(duration=10s) => vent
       }
       stage vent {
           1 -> sample_vent_vlv_cmd
           sample_tank_pt < 5 => idle
       }
       stage abort {
           0 -> sample_press_vlv_cmd
           1 -> sample_vent_vlv_cmd
           sample_start_cmd == 1 => pressurize
       }
   }

   true => press
   ```

   When more than one condition in a stage is true, the first one in the code wins. Put the safety checks first.

2. Click **Redeploy**.

3. On the Tank schematic, click **Start**. Before the hold ends, click **Abort**.

The press valve closes, the vent valve opens, and the pressure falls. To run the test again, click **Start**.

[](https://synnax.nyc3.cdn.digitaloceanspaces.com/docs/control/arc/get-started/abort/befb6f831d22-dark.mp4#t=0.001)

## Step 6Put the sample project back

1. In the controls bar of your automation, click **Stop** ().
2. In the Arcs toolbar, click **Start** () on the Press sequence.

The Press sequence starts in its `idle` stage, and the sample project works as before. You can keep your automation, or delete it from the Arcs toolbar.

## Next steps

* [Execution model](https://docs.synnaxlabs.com/reference/control/arc/execution-model) -> Learn exactly when stages start and conditions fire.
* [Write a test sequence](https://docs.synnaxlabs.com/reference/control/arc/test-sequences) -> Add timed holds, more safety checks, and ranges.
* [Control authority](https://docs.synnaxlabs.com/reference/control/authority) -> Let an operator take control from your sequence.
* [Arc automations](https://docs.synnaxlabs.com/reference/control/arc/automations) -> Manage, redeploy, and delete automations.