The Trend Log Multiple object extends Trend Log to record several properties at once into each log entry. All monitored properties (listed in log_device_object_property) are sampled on the same trigger (periodic or explicit), producing a single record that contains the values of every member at that instant. This makes it ideal for capturing correlated data (e.g. supply/return temp + valve position + airflow at the same moment).

The object offers the same rich configuration as a regular Trend Log: clock-aligned logging, time windows, buffer management, enable/disable, and optional BUFFER_READY intrinsic reporting when intrinsic_reporting: true. Buffer Retrieval is performed with the BACnet.Protocol.Services.ReadRange service.

Object Description (ASHRAE 135)

A Trend Log Multiple object monitors one or more properties of one or more referenced objects, either in the same device as the Trend Log Multiple object or in an external device. When predefined conditions are met, the object saves ("logs") the value of the properties and a timestamp into an internal buffer for subsequent retrieval.

Trend Log Multiple objects that support intrinsic reporting shall apply the BUFFER_READY event algorithm.

Behaviour and Operation

Trend Log Multiple objects are like Trend Log but log a multiple of properties on each sample (all properties in log_device_object_property are captured). This produces correlated snapshots useful for analyzing cause-effect relationships.

The local logging engine must sample all referenced properties (local or remote) at the same trigger instant (periodic or explicit trigger) and store a single BACnet.Protocol.LogMultipleRecord containing the values.

Retrieval, buffer management, clock alignment, start/stop windows, and BUFFER_READY intrinsic reporting (when enabled) work the same as for a regular Trend Log.

Developer Implementation Notes (geared to device server / application authors)

The generated code handles storage + basic mechanics (validation, implicit_relationships, readonly annotations as hints to your server, etc.). You must drive "special" live properties and side effects yourself, analogous to maintaining present_value on inputs via update_property/3 (never direct mutation). Read notes below + generated tables for details.

Special / live properties and expected developer behaviour

(Very similar to TrendLog; see its notes for the full engine description.)

• log_device_object_property (now a BACnetArray of refs for multiple): The properties being trended (can be on local or remote devices). Dev must: Your logging task(s) must periodically (polled) or on change (COV) read all the referenced properties (issuing ReadProperty or using COV subs for remotes) and produce LogMultipleRecord entries containing the array of values.

• log_buffer ([LogMultipleRecord.t()]): The history. Dev must: Own the sampler: on timer or notification, read current values (with status), build record with timestamp + the array of values + status, append or overwrite per stop_when_full using update_property on the buffer (or manage internally and write whole). Enforce buffer_size.

• record_count, total_record_count (some readonly): Counters. Dev must: Increment record_count on appends; total is cumulative (never resets). Special update overrides allow reset of record_count to 0 under conditions.

• trigger: For logging_type :triggered. Dev must: Your code writes true to trigger a sample on demand.

• logging_type, log_interval, cov_resubscription_interval, client_cov_increment: Control sampling mode. Dev must: Your engine reacts to changes (the object has overrides that e.g. flip logging_type on log_interval 0/non0 for compat). Re-schedule your task or COV subs accordingly. property_writable? prevents some changes while enabled.

• buffer_size, stop_when_full, enable, start_time/stop_time, align_intervals etc: Config. Dev must: Your writer must respect enable/stop/start windows, and for aligned use proper next sample time calculation. buffer_size writes only allowed when !enable (enforced by override).

• notification_threshold + intrinsic event fields (when enabled): BUFFER_READY. Dev must: On appends, manage records_since_notification, emit when crosses threshold.

• reliability etc: Source or buffer full issues. Dev must: Set when a referenced prop can't be read, or buffer issues.

Your logging engine is responsible for all sampling and buffer management; the object is the config + storage + enforcement container. See TrendLog notes for more on clock aligned, remote sources, performance, etc.

The BACnet.Stack.TrendLogger module does a lot of this work.

Intrinsic Reporting

When intrinsic_reporting: true is passed to create/4, BUFFER_READY intrinsic reporting is enabled.

Examples

Creating a Trend Log Multiple:

iex> alias BACnet.Protocol.{DeviceObjectPropertyRef, ObjectIdentifier, BACnetArray}
iex> ref = %DeviceObjectPropertyRef{object_identifier: %ObjectIdentifier{type: :analog_input, instance: 1}, property_identifier: :present_value, property_array_index: nil, device_identifier: nil}
iex> {:ok, tlm} = BACnet.Protocol.ObjectTypes.TrendLogMultiple.create(1000, "MultiTrend", %{log_device_object_property: BACnetArray.from_list([ref]), buffer_size: 100, logging_type: :polled, log_interval: 60}); tlm.object_name
"MultiTrend"

With special options:

iex> alias BACnet.Protocol.{DeviceObjectPropertyRef, ObjectIdentifier, BACnetArray}
iex> ref = %DeviceObjectPropertyRef{object_identifier: %ObjectIdentifier{type: :analog_input, instance: 1}, property_identifier: :present_value, property_array_index: nil, device_identifier: nil}
iex> {:ok, tlm} = BACnet.Protocol.ObjectTypes.TrendLogMultiple.create(1001, "AlignedMulti", %{log_device_object_property: BACnetArray.from_list([ref]), buffer_size: 50, logging_type: :polled, log_interval: 300, align_intervals: false, interval_offset: 0}, intrinsic_reporting: true, clock_aligned_logging: true); tlm.object_name
"AlignedMulti"

See Also

• BACnet.Protocol.LogMultipleRecord
• BACnet.Protocol.EventAlgorithms.BufferReady
• BACnet.Protocol.Services.ReadRange
• BACnet.Stack.TrendLogger

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