pvxs/documentation/ntscalar.rst

NTScalar and NTScalarArray

The "epics:nt/NTScalar:1.0" and related "epics:nt/NTScalarArray:1.0" definitions describe a primary 'value' and supporting meta-data. In the case of NTScalarArray the value is a 1-d array of primative type. In the case of NTScalar the value is a single primative value.

Both variants include time and alarm meta-data fields, and optionally display and control meta-data fields.

using namespace pvxs;

// integer scalar
auto iscalar = nt::NTScalar{TypeCode::Int64}.create();

// real array
auto farray = nt::NTScalar{TypeCode::Float64A}.create();

// eg. access "value" field
iscalar["value"] = 42;

Fields

"value"

The primary field. May be any pvxs::TypeCode other than Struct, Union, StructA, UnionA, or Null.

"timeStamp.secondsPastEpoch"

"timeStamp.nanoseconds"

Time associated with the value. Typically time of measurement. See time_t

"alarm.severity"

"alarm.status"

"alarm.message"

Alarm state associated with value. See alarm_t

"display.description"

Text providing some context about what this value/PV represents.

"display.units"

Text identifying units of value. eg. "V" or "Hz"

Meta-data for numeric types.

"display.limitLow"

"display.limitHigh"

Hints for clients which can indicate inclusive range of possible values. eg. a UI gauge widget. Ignore unless limitLow < limitHigh

"control.limitLow"

"control.limitHigh"

Hints for clients on the inclusive range of values which may reasonably be written to this PV. Ignore unless limitLow < limitHigh Not authoritative.

"control.minStep"

Hint for client of a useful minimum increment for setting.

"valueAlarm.lowAlarmLimit"

"valueAlarm.highAlarmLimit"

"valueAlarm.lowAlarmSeverity"

"valueAlarm.highAlarmSeverity"

"valueAlarm.lowWarningLimit"

"valueAlarm.highWarningLimit"

"valueAlarm.lowWarningSeverity"

"valueAlarm.highWarningSeverity"

Hints for clients on the ranges of values which will result in a alarms of the given severities. Could be used by eg. a UI gauge widget to place markers colored by alarm severity. Actual alarms are signals with "alarm.severity".

Two value ranges are defined. The names Alarm vs. Warning do not have a special significance.

Ignore *Alarm* range unless lowAlarmLimit < highAlarmLimit

Ignore *Warning* range unless lowWarningLimit < highWarningLimit

If a value is within a range [low*Limit, high*Limit] inclusive then no alarm is expected. If a value is < low*Limit then the alarm severity low*Severity is expected. If a value is > high*Limit then the alarm severity high*Severity is expected.

Builder API

pvxs::nt::NTScalar