# VDIFPacker — Interfaces

## Input Streams

The pipeline consumes one or more SPEAD2 multicast streams in `DigitalDownConverter:1`
format, one per subband channel. The stream descriptions are provided via
`input_data_streams`.

| Field | Value |
|---|---|
| Format | `DigitalDownConverter:1` |
| Bit depth | 32 (float-32) |
| SPEAD2 item: timestamp | id 5632 |
| SPEAD2 item: channel | id 5633 |
| SPEAD2 item: polarization | id 5634 |
| SPEAD2 item: data | id 5635 |

## Output Format

```{include} ../generated/vdif_packer/streams.md
```

### VDIF Frame Layout

VDIF frames are produced according to the VLBI Data Interchange Format standard. The
payload size defaults to 8000 bytes. Each thread ID carries the channels assigned to it
via `thread_ids`.

## UDP Output

When `output_type` includes `"network"`, the pipeline sends VDIF frames via UDP:

| Parameter | Config key | Default |
|---|---|---|
| Destination IP | `udp.dest_ip` | `127.0.0.1` |
| Destination port | `udp.dest_port` | `4991` (jive5ab default) |
| Bind IP | `udp.bind_ip` | `0.0.0.0` |
| Max rate | `udp.max_rate` | `1.1` (relative to nominal) |

## POSIX Message Queue

Recording is gated by a POSIX message queue. The Field System Interface writes
`start`/`stop` messages to this queue during an observation.

| Parameter | Config key | Default |
|---|---|---|
| Queue name | `mq_control` | `/vdif_control_message_server` |

## KATCP Interface

| Request | Description |
|---|---|
| `?configure <json>` | Load a new configuration |
| `?capture-start` | Begin data capture |
| `?capture-stop` | Stop data capture |
| `?deconfigure` | Release resources |
| `?sensor-list` | List available sensors |
| `?sensor-value <name>` | Query a sensor |