Device Communication

The PLC runtime communicates with external devices (I/O racks, VFDs, sensors) through native function blocks. Device types are derived from YAML profiles and appear as normal function blocks in your ST code — with full IDE support (code completion, type checking, debugger variable expansion).

How it works

You create a device profile (YAML file) that describes the device’s registers — field names, data types, directions, and Modbus addresses.
The runtime discovers profiles in your project’s profiles/ directory and makes them available as function block types.
In your ST program, you declare instances of these types and call them each scan cycle. The runtime handles the actual serial communication.

Quick example

PROGRAM Main
VAR
    serial   : SerialLink;
    io_rack  : MyIoModule;
END_VAR
    (* Open the serial port — runs once on first call *)
    serial(port := '/dev/ttyUSB0', baud := 9600, parity := 'N',
           data_bits := 8, stop_bits := 1);

    (* Read/write I/O every 50ms *)
    io_rack(link := serial.port, slave_id := 1, refresh_rate := T#50ms);

    (* Use the I/O fields *)
    io_rack.DO_0 := io_rack.DI_0;
    io_rack.AO_0 := io_rack.AI_0 * 2;
END_PROGRAM

Supported protocols

Protocol	Status	Link type	Crate
Simulated	Working	None (in-memory)	`st-comm-sim`
Modbus RTU	Working	RS-485/RS-232 serial	`st-comm-modbus`
Modbus TCP	Planned	TCP/IP	—
EtherNet/IP	Planned	TCP/IP	—

Architecture

┌──────────────────┐     ┌────────────────────┐
│  ST Program       │     │  Device Profile     │
│                   │     │  (YAML)             │
│  serial(...)      │     │  - register map     │
│  io_rack(...)     │     │  - field types      │
│  io_rack.DO_0 :=  │     │  - scaling/offset   │
└────────┬──────────┘     └─────────┬───────────┘
         │                          │
         ▼                          ▼
┌──────────────────────────────────────────────┐
│  NativeFb Registry                           │
│  (built from profiles at startup)            │
│                                              │
│  SerialLink → opens /dev/ttyUSB0             │
│  MyIoModule → Modbus RTU FC01-FC10           │
└──────────────────────────────────────────────┘
         │
         ▼
┌──────────────────────────────────────────────┐
│  Serial Transport (RS-485 bus)               │
│  - inter-frame timing                        │
│  - bus access mutex (multiple devices share)  │
│  - CRC16 validation                          │
└──────────────────────────────────────────────┘

Diagnostic fields

Every device FB includes diagnostic fields that update automatically:

Field	Type	Description
`connected`	BOOL	TRUE if the device is responding
`error_code`	INT	0 = OK, non-zero = error
`io_cycles`	UDINT	Number of successful I/O cycles
`last_response_ms`	REAL	Last round-trip time in milliseconds

IF NOT io_rack.connected THEN
    (* Handle communication failure *)
    alarm := TRUE;
END_IF;

Next steps

Device Profiles — how to create a YAML profile for your hardware
Modbus RTU — detailed Modbus RTU setup and wiring guide

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