Skip to content
Get started

Modbus Monitor

The modbus_monitor component runs automations for the requests and responses that a Modbus hub handles. On a hub with the server role, that is each request the hub receives and the reply to it, sent by this hub or another device. On a hub with the client role, that is each request the hub sends and the reply to it.

# Example configuration entry
modbus_monitor:
  - on_request:
      - lambda: |-
          ESP_LOGD("modbus_monitor", "Request to %u, function 0x%02X", address, request[0]);
    on_response:
      - lambda: |-
          if (response[0] & 0x80) {
            ESP_LOGW("modbus_monitor", "Exception 0x%02X from %u for function 0x%02X", response[1], address,
                     request[0]);
          }

Configuration Variables

Section titled “Configuration Variables”

  • modbus_id (Optional, ID): The Modbus hub to watch. Automatically assigned when a single hub is configured.

  • on_request (Optional, Automation): Runs for each request. See below.

  • on_response (Optional, Automation): Runs for each response. See below.

At least one of on_request and on_response is required. Several modbus_monitor entries can watch the same hub.

Automation Variables

Section titled “Automation Variables”
  • address (uint8_t): The address the request was sent to. 0 is a broadcast.
  • request (std::span<const uint8_t>): The request PDU: function code followed by data, without address and CRC.
  • response (std::span<const uint8_t>, on_response only): The response PDU, in the same form. An exception response has bit 7 of the function code set (response[0] & 0x80).

The data is only valid while the automation runs, so delay, wait_until and other actions that wait are rejected in it. Copy what you need first. Keep the automation short: it runs in the hub’s loop.

Server Hub

Section titled “Server Hub”

on_request runs for every frame the hub reads as a request, for any address and for broadcasts. For a request to one of this hub’s Modbus Server devices, it runs before the device answers. on_response runs for each reply the hub has sent, after the reply is written to the UART, and for the reply of another device to a request the hub has seen. A reply that is dropped does not run it.

NOTE

The hub tells a request from a reply by the order and shape of the frames. If a device does not answer, the next request that has the shape of a reply is taken for one, whatever its address, for example a single write or mask write (function codes 0x05, 0x06, 0x16). Broadcasts are always read as requests.

Client Hub

Section titled “Client Hub”

on_request runs each time the hub sends a request, broadcasts and retries included, after the request is written to the UART and after the device’s on_sent. A request that was queued but never sent does not run it. on_response runs for each response that matches the request the hub is waiting for, exception responses included, after the device has handled it. A request without a response does not run it.

See Also

Section titled “See Also”