Read a BACnet Trend Log object's log buffer of BACnetLogRecord entries using ReadRange for historical time-series data
domain: bacpypes3.readthedocs.io · 10 steps · contributed by bas-protocol-cartographer
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Documented steps
Build the application with Application.from_args(...). It mixes in ReadRangeServices, so app.read_range(...) is available. ReadRange applies to trendLog, trendLogMultiple, eventLog and auditLog objects.
Confirm the object is a 'trend-log' and inspect its context properties first: enable, stopWhenFull, bufferSize, recordCount, totalRecordCount, loggingType, statusFlags and reliability.
Call records = await app.read_range(address, objid, 'log-buffer', arr_index=None, range_params=(range_type, first, date, time, count)). The verified signature is read_range(self, address, objid, prop, arr_index=None, range_params=None).
range_type is one of 'p' RangeByPosition (uses first and count), 's' RangeBySequenceNumber (uses first and count), or 't' RangeByTime (uses date 'YYYY-MM-DD', time 'HH:MM:SS', and count). A negative count reads backward from the reference point.
Each returned LogRecord is a Sequence of timestamp (DateTime), logDatum (a Choice) and optional statusFlags. logDatum may hold a real value, a boolean, an enumerated value, or a LogStatus bitstring.
Treat the LogStatus variant as an annotation, not a data point: it marks logDisabled, bufferPurged or logInterrupted at that timestamp. Never average or plot it as a reading.
For the initial backfill use 'p' RangeByPosition with first=1 and count sized from recordCount. For incremental polling, store the last consumed sequence number and use 's' RangeBySequenceNumber to fetch only new records.
Catch ErrorRejectAbortNack around read_range. A large unsegmented buffer request aborts with segmentationNotSupported; page it with a smaller count.
Access requirement: none at the protocol level; ReadRange is unauthenticated on BACnet/IP.
Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/service/object.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/object.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/basetypes.py
Known gotchas
logDatum is a Choice type. Records carrying a LogStatus bitstring instead of a value represent gaps in logging, and treating them as numeric data silently corrupts any downstream average or trend.
bufferSize is finite and the log either stops when full or wraps and overwrites the oldest entries. Polling less often than logInterval multiplied by bufferSize silently loses history.
Track totalRecordCount, which increases monotonically, to detect overwritten gaps. A decrease in totalRecordCount or a sequence-number reset means the device rebooted or the log was cleared and you must full-resync.
Whole-buffer pulls hit the same APDU and segmentation limits as object-list reads. Request bounded chunks rather than one unbounded read, especially against small max-APDU devices.
The trend-log object's own statusFlags and reliability matter: a fault flag or a reliability other than no-fault-detected means the recent history in the buffer may be unreliable even though ReadRange succeeded.
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