# PLC Relay

PLC Relay is an edge container service that provides an HTTP API for reading and writing PLC tags. You configure it through the Deployment Manager UI by selecting a protocol, entering connection details, and defining tags.

{% hint style="info" %}
PLC Relay is available exclusively for Enterprise customers. [Contact the Roboflow sales team](https://roboflow.com/sales) to learn more.
{% endhint %}

## Supported Protocols

When adding or editing a PLC Relay service, you select one of three protocols. Each protocol has its own connection settings and tag format.

| Protocol                    | PLCs                                 | Default Port |
| --------------------------- | ------------------------------------ | ------------ |
| Allen-Bradley (EtherNet/IP) | CompactLogix, ControlLogix, Micro800 | 44818        |
| Modbus TCP                  | Any Modbus TCP device                | 502          |
| Siemens S7                  | S7-300, S7-400, S7-1200, S7-1500     | 102          |

{% hint style="warning" %}
Switching protocols clears all configured tags because tag address formats are not interchangeable between protocols. The UI will prompt for confirmation before applying the change.
{% endhint %}

## Connection Settings

### PLC Address

The address format depends on the selected protocol:

* **Allen-Bradley:** IP or hostname, optionally followed by `/slot` (e.g. `192.168.1.100/0`) or a full CIP routing path.
* **Modbus TCP:** IP or hostname, with optional `:port` (e.g. `192.168.1.100:502`). Also requires a Unit ID (0-255) and Word Order (big or little) for 32-bit values.
* **Siemens S7:** IP or hostname, with optional `:port` (e.g. `192.168.1.100:102`). Also requires Rack (0-7) and Slot (0-31).

### Simulation Mode

When enabled, PLC Relay uses an in-memory simulator instead of connecting to a real PLC. All API operations work normally, but values are stored in memory. This is useful for testing without hardware.

## Tag Configuration

Tags define the PLC data points accessible through the API. Each tag has a name, data type, writable flag, and optional description.

### Data Types

| Type   | Description           | Range                           |
| ------ | --------------------- | ------------------------------- |
| `BOOL` | Boolean               | `true` / `false`                |
| `INT`  | 16-bit signed integer | -32,768 to 32,767               |
| `DINT` | 32-bit signed integer | -2,147,483,648 to 2,147,483,647 |
| `REAL` | 32-bit floating point | IEEE 754                        |

### Tag Name Formats

{% tabs %}
{% tab title="Allen-Bradley" %}
Tag names match the PLC program and are case-sensitive.

| Style          | Example                       |
| -------------- | ----------------------------- |
| Simple         | `TagName`                     |
| Program-scoped | `Program:MainProgram.TagName` |
| Array element  | `TagName[0]`                  |
| UDT member     | `MyUDT.Member`                |
| {% endtab %}   |                               |

{% tab title="Modbus TCP" %}
Format: `{area}:{address}` where address is a non-negative integer.

| Area         | Type(s)         | Access    | Example       |
| ------------ | --------------- | --------- | ------------- |
| `coil`       | BOOL            | Writable  | `coil:0`      |
| `discrete`   | BOOL            | Read-only | `discrete:5`  |
| `holding`    | INT, DINT, REAL | Writable  | `holding:100` |
| `input`      | INT, DINT, REAL | Read-only | `input:200`   |
| {% endtab %} |                 |           |               |

{% tab title="Siemens S7" %}
Data Block format: `DB{n}.DB[XWD]{byte}[.{bit}]`

Area format: `[MIQEA][WD]?{byte}[.{bit}]`

| Address         | Type               | Description                     |
| --------------- | ------------------ | ------------------------------- |
| `DB1.DBX0.0`    | BOOL               | Bit 0 of byte 0 in Data Block 1 |
| `DB1.DBW0`      | INT                | 16-bit word in DB1              |
| `DB1.DBD0`      | DINT or REAL       | 32-bit double-word in DB1       |
| `M0.0`          | BOOL               | Merker bit                      |
| `I0.0` / `Q0.0` | BOOL               | Process input/output bit        |
| `MW0` / `MD0`   | INT / DINT or REAL | Merker word / double-word       |

For S7-1200/1500: enable PUT/GET in TIA Portal and disable optimized block access on accessed DBs.
{% endtab %}
{% endtabs %}

## Web Dashboard and REST API

PLC Relay includes a built-in web dashboard for monitoring tag values in real time. Once the service is running, access it at `http://<device-ip>:8007`.

The dashboard also hosts interactive Swagger documentation at `/docs` and a visual config builder at `/static/config-builder.html`.

### API Endpoints

| Method | Path             | Description                                        |
| ------ | ---------------- | -------------------------------------------------- |
| GET    | `/healthz`       | Health check with connection and validation status |
| GET    | `/all_tags`      | Read current values for all tags                   |
| GET    | `/read?tag=name` | Read a single tag by name                          |
| POST   | `/write`         | Write a single tag                                 |
| POST   | `/read_batch`    | Read multiple tags at once                         |
| POST   | `/write_batch`   | Write multiple tags at once                        |
| GET    | `/schema`        | Get all tag definitions                            |
| POST   | `/validate`      | Re-run tag validation against PLC                  |

## Troubleshooting

| Symptom                              | Fix                                                                                                                    |
| ------------------------------------ | ---------------------------------------------------------------------------------------------------------------------- |
| "PLC not connected" (Allen-Bradley)  | Verify PLC address format (IP/Slot), check port 44818 is reachable                                                     |
| "PLC not connected" (Modbus)         | Check IP/port (default 502) and verify Unit ID matches the device                                                      |
| "PLC not connected" (Siemens S7)     | Check IP/port (default 102), rack, and slot values; for S7-1200/1500 enable PUT/GET and disable optimized block access |
| "Function refused" (Siemens S7)      | PUT/GET disabled in TIA Portal, or optimized block access enabled on the target DB                                     |
| REAL value reads as garbage (Modbus) | Try the opposite Word Order (big vs. little)                                                                           |
| Validation shows NOT\_FOUND          | Check the PLC program for the exact tag name (case-sensitive)                                                          |


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