QtEDM Expression Channels

An Expression Channel is a QtEDM-only logical widget that evaluates an EPICS calc expression and publishes the numeric result as a process-local soft PV. Use it for display-local derived values, thresholds, fan-out signals, and small calculation chains that do not justify adding IOC-side logic.

Expression Channels are visible and selectable in EDIT mode. In EXECUTE mode they become invisible, subscribe to their configured inputs, and publish their results for other widgets in the same QtEDM process.

Create and connect an Expression Channel

1. Enter EDIT mode and choose Object Palette > Monitors > Expression Channel.
2. Draw the logical widget on the display.
3. Select it and configure its fields in the Resource Palette.
4. Give Variable a unique, explicit name, such as expr:temperatureDelta.
5. Enter an EPICS calc expression using A through D, then assign the corresponding source PVs to Channel A through Channel D.
6. Set another widget's normal channel field to the Variable name.
7. Enter EXECUTE mode. The consuming widget first receives Initial Value, then receives calculated updates according to Event Signal.

The published variable is not an IOC PV and is not visible to caget, camonitor, or another QtEDM process. It can be consumed by other displays opened in the same running QtEDM process, including embedded and tabbed displays.

Resource Palette fields

Field	Meaning
Foreground / Background	EDIT-mode appearance of the logical widget. The widget is hidden in EXECUTE mode.
Variable	Name of the process-local numeric soft PV that receives the result. A blank value creates a private __expr_... name; use an explicit name whenever another widget must subscribe.
Calc	EPICS calc expression. Inputs A, B, C, and D correspond to the four channel fields. QtEDM also accepts == and != and normalizes them for the EPICS calc engine.
Channel A-D	Numeric source channels. Each may be a CA PV, a pva:// PV, a supported provider channel, or another Expression Channel variable. Unused inputs default to zero.
Initial Value	Value published immediately when EXECUTE mode starts, before a successful calculation is published.
Event Signal	Determines which successful evaluations replace the published value; see the table below.
Precision	Saved numeric presentation setting. A consuming widget's own limits and precision determine how it renders the soft-PV value.

Input channels are subscribed in real-time mode. A missing or nonnumeric value is treated as zero for evaluation; a non-finite value is ignored. An invalid or empty calc expression does not publish calculated updates, but the Initial Value is still available. Calculations are limited to 79 characters after operator normalization, protecting the legacy calculator's fixed-size stacks. This limit also applies to visibility and image-frame calculations.

Event Signal behavior

Resource Palette choice	ADL value	Publication rule
Never	never	Publish only Initial Value; evaluate input changes without publishing their results.
On First Change	onFirstChange	Publish the first successful evaluation after startup, then hold that value.
On Any Change	onAnyChange	Publish whenever the calculated result differs from the last published value. This is the default.
Trigger 0->1	triggerZeroToOne	Publish when consecutive evaluated results cross from <= 0 to > 0. The first evaluation establishes the starting state and does not trigger.
Trigger 1->0	triggerOneToZero	Publish when consecutive evaluated results cross from > 0 to <= 0. The first evaluation establishes the starting state and does not trigger.

The trigger modes detect a zero/nonzero boundary in the calc result; they do not require that the expression return exactly 0 or 1.

ADL example

This block publishes the sum of two IOC PVs as expr:sum:

expression_channel {
  object {
    x=20
    y=66
    width=120
    height=40
  }
  variable="expr:sum"
  calc="A+B"
  channelA="source:one"
  channelB="source:two"
  channelC=""
  channelD=""
  initialValue=0
  eventSignal="onAnyChange"
  clr=30
  bclr=4
  precision=3
}

A consuming Text Monitor uses the result like an ordinary channel:

"text update" {
  object {
    x=180
    y=78
    width=150
    height=16
  }
  monitor {
    chan="expr:sum"
    clr=14
    bclr=4
  }
}

Expression Channels may be chained. For example, set a second node's Channel A to expr:sum and its Calc to A*2. QtEDM pre-registers declared outputs when entering EXECUTE mode, so ordering across the loaded display tree is deterministic. Use unique Variable names: two active producers with the same name publish into the same registry entry and can overwrite each other.

Troubleshooting

• The consuming widget tries to connect through CA. Confirm that its channel exactly matches the Expression Channel's Variable and that both displays are open in the same QtEDM process. Do not add a ca:// or pva:// prefix to the soft-PV name.
• Only the Initial Value appears. Check the calc syntax and source channel names, then confirm Event Signal is not Never. QtEDM logs invalid and empty calc expressions to standard error.
• A trigger never fires. Trigger modes need two successful evaluations and only publish when the result crosses the zero boundary in the configured direction.
• A first-change value arrives sooner than expected. Each input monitor event can cause an evaluation; On First Change does not wait for all four channels to connect. Design the expression and initial value with startup ordering in mind.
• The value changes unexpectedly. Search all loaded displays for duplicate Variable names and for calculation chains that feed the same name.
• The variable cannot be inspected later. A blank Variable is assigned a generated private name. Set an explicit name when another widget or the PV Information dialog must refer to it consistently.

Reference fixture and validation

The maintained example is tests/test_ExpressionChannel.adl. It covers direct publication, chaining, On First Change, and Trigger 0->1. Parser coverage is in qtedm/tests/test_adl_parser.cc, and IOC cases are listed in tests/qtedm_ioc_cases.json.

From the repository root, run:

make test-qtedm-unit
make test-qtedm-ioc

For a focused manual check, open tests/test_ExpressionChannel.adl against the repository test IOC, change channelA_PV through channelD_PV, and confirm the four displayed soft-PV outputs follow the instructions printed on the display.