# modbus.tcp_request

## Function

Construct Modbus TCP request frame, including MBAP header and PDU.

## Syntax

```bt
modbus.tcp_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 |
|------|------|------|------|------|
| transaction_id | Int | No | `1` | TCP transaction number. |
| protocol_id | Int | No | `0` | Modbus TCP protocol number, usually 0. |
| unit_id | Int | No | `1` | Unit ID. |
| function_code/function/address/quantity/value/values/pdu/data | Same as rtu_request | Determined by function | None | PDU construction field. |

## Return value

| Type | Description |
|------|------|
| Bytes | Modbus TCP ADU complete request frame. |

## Code Example

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

// Output: 000100000006010300000002
print packet.to_hex()
```

## Notes

- MBAP length field is automatically written with `unit_id + pdu` length.