# modbus.rtu_request

## Function

Build a Modbus RTU request frame and automatically append CRC16.

## Syntax

```bt
modbus.rtu_request(config)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
|------|------|------|------|------|
| config | Object | Yes | None | Requests a configuration object. |

## Configuration fields

| Field | Type | Required | Default value | Description |
|------|------|------|------|------|
| unit_id | Int | No | `1` | Slave address, 0 to 255. |
| function_code | Int | No | None | Modbus function code. The `function` string can also be used. |
| function | String | No | None | Function name, for example `read_holding_registers`. |
| address | Int | Use when reading and writing registers/coils | `0` | Starting address. |
| quantity | Int | When reading, use | `1` | to read the quantity. |
| value | Int/Bool | When writing a single register/coil use | `0` | to write a value. |
| values | Array | Use when writing multiple registers/coils | None | Multi-value writing data. |
| pdu | Bytes/Array | No | None | Original PDU; it will no longer be constructed based on the function code after being passed in. |
| data | Bytes/Array/String | No | None | Unsupported custom function code additional data. |

## Return value

| Type | Description |
|------|------|
| Bytes | RTU ADU complete request frame. |

## Code Example

```bt
packet = modbus.rtu_request({
    unit_id: 1,
    function: 'read_holding_registers',
    address: 0,
    quantity: 2
})

// Output: 010300000002c40b
print packet.to_hex()
```

## Notes

- RTU CRC is written to the end of the frame with the low byte first.
- Result length is limited by `BT_BYTES_LIMIT`.