Read alarm logs

Use historical browsers to find alarm transitions and operator changes across current and dated files.

Search a time interval

1. Open the alarm-log or operator-log browser from View.
2. Set From and To using local time. The entire final minute is included.
3. Enter optional With text. Matching is a case-sensitive substring search.
4. Click Search (ShowSelected in the classic appearance). Use Stop to cancel a long-running search.
5. Review results in chronological order and read any search warnings.

The browser searches the selected current log and its .yyyy-MM-dd siblings. It recognizes modern ALH, legacy ctime, and XML-like timestamps. The With filter searches complete records, including continuation lines in older multiline logs. Results stop at 100,000 records or 10 MiB, with an explicit message when truncated. Unreadable files and unrecognized timestamps are reported.

Choose the right record

View	Use it for
Alarm log	Alarm status/severity transitions and global acknowledgement changes. Value-only updates do not create alarm records.
Operator log	Operator changes, actions, and runtime errors. This file appends independently of the bounded alarm ring.
Current alarm history	The ten most recent in-memory history entries, separate from the files.
Live log-file viewer	Viewing the current file separately from a historical search.

Choose View → Alarm Log File or Operation Log File for a live tail of up to 1,000 records or 256 KiB. The viewer follows changes to the log destination and daily rollover with -T. A valid alarm checkpoint preserves insertion order after the ring wraps, including records with identical timestamps. Without a matching checkpoint, the viewer scans the file and retains the newest timestamps before applying its limits; equal timestamps retain physical order. Use the historical browser to search older records.

Read current history

Choose View → Current Alarm History for the most recent in-memory entries. The newest entry is listed first. Legacy ALH lists the oldest first and the newest last. The modern appearance shows a table with Time, Process Variable, Status, Severity and Value columns, with severity cells in the alarm colors; the classic appearance shows aligned fixed-width text. Outside Motif mode, the font-size shortcuts also work while this dialog has focus.

Change log files

In a runtime, Setup → New Alarm Log File… and Setup → New Operation Modification Log File… switch this runtime's alarm or operation log to the file you choose; Cancel keeps the current file. Without -T or -L, choosing an existing alarm log empties it, as described under Understand circular files below. With -T, the date suffix is added to the chosen name, as in ALH. If you choose an existing dated file such as alarm.log.2026-10-17, QtALH asks whether to Use Log Series (continue the dated series of alarm.log) or Use as Basename (keep the date as part of the name and add the current date after it).

Understand circular files

The default alarm file holds 2000 records unless -L is used, which defaults to unlimited retention as in ALH. An explicit -m count overrides either default (0 means unlimited). Without -T or -L, starting QtALH or choosing another file with Setup → New Alarm Log File… empties that alarm file, as ALH does; with -T or -L, an existing alarm file is reopened without being emptied. If such a file is bounded (-T, or -L with an explicit -m) and already holds more records than the limit, the first alarm keeps only its newest records; ALH appends instead. Copy the file first or use -m 0 to keep every record. After wrapping, physical line order is not chronological. The historical browser sorts recognized timestamps; events sharing one-second timestamps may still have ambiguous relative order.

PV values and descriptions containing line breaks appear as \n and \r in text logs, or 
 and 
 in XML logs. Each new alarm remains one physical line. Bounded-log recovery also repairs older multiline records when their record boundaries can be recognized.

XML operation logs escape special characters in facility labels, names, and messages, so text such as RF & vacuum remains valid XML. Alarm records retain their structured XML fields; plain text logs keep the original text.

For retention limits, dated files, checkpoints, destination changes, and multiple writers, see Logging & shared operation.