Skip to main content

Communication Function Blocks

ZPLC provides specialized Function Blocks (FBs) that interact directly with the runtime's network dispatch layer. These blocks allow you to read/write external devices and publish/subscribe telemetry directly from your IEC 61131-3 logic.

Asynchronous Execution Model​

Unlike simple arithmetic operations, network communication is asynchronous. All communication FBs share a standard asynchronous interface:

  • EN (BOOL): Exec trigger.
  • BUSY (BOOL): True while the network request is in flight.
  • DONE (BOOL): True for one cycle when the request successfully completes.
  • ERROR (BOOL): True if the request failed or timed out.
  • STATUS (DINT): Returns the underlying runtime error code for diagnostics.

[!IMPORTANT] Because these blocks execute asynchronously, avoid placing them inside fast cyclic loops without checking the BUSY flag, as this can overwhelm the network stack.

Supported Function Blocks​

Modbus Communications​

Used to interact with field devices without using implicit auto-mapping.

  • MB_READ_HREG: Read Holding Registers (Function Code 3).
  • MB_WRITE_HREG: Write Holding Registers (Function Code 16).
  • MB_READ_COIL: Read Coils (Function Code 1).
  • MB_WRITE_COIL: Write Coils (Function Code 15).

MQTT Messaging​

Used to send and receive asynchronous payloads to cloud brokers.

  • MQTT_CONNECT: Establishes the broker connection dynamically.
  • MQTT_PUBLISH: Publishes a string or byte payload to a specific topic.
  • MQTT_SUBSCRIBE: Subscribes to a topic.

Cloud Integration Wrappers​

Pre-configured wrappers that simplify connecting to major hyperscalers.

  • AZURE_C2D_RECV: Receive Azure Cloud-to-Device messages.
  • AZURE_DPS_PROV: Azure Device Provisioning Service registration.
  • AWS_FLEET_PROV: AWS IoT Fleet Provisioning.
  • SPB_REBIRTH: Sparkplug B Device Rebirth sequence handling.

Language Availability​

All communication Function Blocks are universally available across ZPLC's supported IEC 61131-3 languages. They can be instanced natively in Structured Text (ST), or dragged and dropped onto networks in Ladder Diagram (LD) and Function Block Diagram (FBD).