# Control

> Learn what control is in Synnax, what it is for, and which way to use.

To control hardware in Synnax, you write values to a [command channel](https://docs.synnaxlabs.com/reference/hardware/tasks#command-channels), such as `press_vlv_cmd`. A [write task](https://docs.synnaxlabs.com/reference/hardware/tasks#write-tasks) listens on that channel and sends each value to the device: it opens a valve, starts a pump, or changes a setpoint. Most write tasks also write what the device did to a [state channel](https://docs.synnaxlabs.com/reference/hardware/tasks#state-channels), such as `press_vlv_state`. [Modbus](https://docs.synnaxlabs.com/reference/hardware/modbus/write-task), [OPC UA](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task), and [HTTP](https://docs.synnaxlabs.com/reference/hardware/http/write-task) write tasks have [no state channels](https://docs.synnaxlabs.com/reference/hardware/tasks#outputs-with-no-state-channel), so a [read task](https://docs.synnaxlabs.com/reference/hardware/tasks#read-tasks) on the same device reads the state back. For how commands and state work, see [write tasks](https://docs.synnaxlabs.com/reference/hardware/tasks#write-tasks).

An operator writes commands by hand from a schematic. An Arc automation or a Python script writes them for you. All three write to channels, not to hardware, so you can change a device or a task without a change to the schematic or the code.

## What control is for

### Open a valve by hand

An operator clicks a valve on a schematic, and the schematic writes to the command channel of the valve. See [control hardware from a schematic](https://docs.synnaxlabs.com/reference/control/manual).

### Start a test from a button

A button on a schematic writes to a channel, and an Arc sequence starts when the channel becomes true. See [start a sequence from a schematic](https://docs.synnaxlabs.com/reference/control/arc/start-from-schematic).

### Run a test sequence with aborts

An Arc sequence moves through stages, such as pressurize, hold, and vent. In each stage, it watches for unsafe values and goes to an abort stage when it finds one. See [write a test sequence](https://docs.synnaxlabs.com/reference/control/arc/test-sequences).

### Hold a value in a band

An Arc automation turns a heater or a valve on below a low limit and off above a high limit. See [regulate a value with bang-bang control](https://docs.synnaxlabs.com/reference/control/arc/bang-bang-control).

### Hand control between an operator and an automation

An automation controls the valves at a low authority. An operator takes control at a higher authority, and the automation gets control back when the operator releases it. See [control authority](https://docs.synnaxlabs.com/reference/control/authority).

### Build an automation without code

In the graph editor, you connect blocks for channels, comparisons, and statuses instead of writing Arc code. See [Arc](https://docs.synnaxlabs.com/reference/control/arc#text-and-graph-editors).

Enterprise

### Script a sequence in Python

A Python script takes control of channels, sets values, and waits for conditions. See [write a control sequence in Python](https://docs.synnaxlabs.com/reference/control/python/write-sequences).

## Ways to control

| Way                                                                                              | Who acts      | Where it runs         | Timing                   | Use it to                                                                                                                                                                                                          |
| ------------------------------------------------------------------------------------------------ | ------------- | --------------------- | ------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| [Schematic](https://docs.synnaxlabs.com/reference/control/manual)                                | An operator   | The Console           | When the operator clicks | [Command hardware by hand](https://docs.synnaxlabs.com/reference/control#open-a-valve-by-hand), and [take over](https://docs.synnaxlabs.com/reference/control#hand-control-between-an-operator-and-an-automation). |
| [Arc automation](https://docs.synnaxlabs.com/reference/control/arc)                              | Your Arc code | A Driver, or the Core | Steady, next to the data | [Run sequences](https://docs.synnaxlabs.com/reference/control#run-a-test-sequence-with-aborts), [control loops](https://docs.synnaxlabs.com/reference/control#hold-a-value-in-a-band), and checks.                 |
| [Python script](https://docs.synnaxlabs.com/reference/control/python/write-sequences) Enterprise | Your script   | Your computer         | Depends on the network   | [Use Python libraries](https://docs.synnaxlabs.com/reference/control#script-a-sequence-in-python), or connect to other software.                                                                                   |

A Python script sends each command over the network to the Synnax Core, so network delays change its timing. An Arc automation runs where the data arrives, so its timing does not depend on your computer.

## Control authority

More than one writer can write to the same command channel, but only one writer at a time controls it. Each writer has an authority from 0 to 255, and the writer with the highest authority gets control. See [control authority](https://docs.synnaxlabs.com/reference/control/authority).