PLC Relay

Configure PLC Relay to read and write PLC tags over Allen-Bradley, Modbus TCP, or Siemens S7.

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.

PLC Relay is available exclusively for Enterprise customers. Contact the Roboflow sales team to learn more.

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

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.

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

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

Modbus TCP

Siemens S7

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)