EPICS Records for Single Scalar Variables

This guide shows how to configure EPICS records for accessing individual single scalar variables on the OPC UA server.

General INP/OUT Link Field Format

@<session_name> ns=<namespace_index>;<identifier_type>=<identifier> [<option>=<value>...]

<session_name>: Name of the OPC UA Session.
<namespace_index>: Numerical namespace index.
<identifier_type>: s for string identifier, i for numerical identifier.
<identifier>: The string or numerical ID of the OPC UA Node.

Note

Escape quotation marks (\") if the identifier string contains them (e.g., "dataBlock"."myItem" becomes \"dataBlock\".\"myItem\").

Available Options

Option	Default	Description
`sampling`	`-1`	Sampling interval in ms [double; -1 = use publishing interval]
`qsize`	`1`	Server-side queue size [int; 1 = no queueing]
`cqsize`	`1.5 * qsize`	Client-side queue size [int; minimum 3]
`discard`	`old`	Discard policy on queue overrun: [`old`/`new` (drop oldest/drop newest)]
`register`	`n`	Register item with server for performance [`y`/`n`]
`deadband`	0.0	Deadband filter for subscriptions [double; 0.0 = no deadband]
`bini`	`read`	Behavior at init: `read`, `ignore`, `write`
`monitor`	`y`	Enable monitoring. For outputs, enables readback
`timestamp`	`server`	Source of timestamp: `server`/`source`

Example: Output Records with Monitor (Bidirectional Mode)

These are output records (bo, ao, longout, mbbo,…) that send data to a single scalar variable (Item) on the OPC UA server and also monitor it, updating the record if the value changes. They require an OPC UA Subscription.

# Boolean Output
record(bo, "$(P)FOO:BOOL") {
    field(DTYP, "OPCUA")
    field(OUT, "@SLOW ns=3;s=\"FOO\".\"BOOL\"")
    field(ZNAM, "OFF")
    field(ONAM, "ON")
}

# Analog Output
record(ao, "$(P)FOO:REALA") {
    field(DTYP, "OPCUA")
    field(OUT, "@SLOW ns=3;s=\"FOO\".\"REALA\"")
    field(DRVH, "100.0")
    field(DRVL, "0.0")
    field(HOPR, "90.0")
    field(LOPR, "10.0")
}

@SLOW: The name of the Subscription to which the Item belongs. The Session is implicitly defined by the Subscription.
monitor=y is the default for all records expecting updates from the OPC UA server, so it is often omitted.
The SCAN should be Passive, as usual with output records, which is the default, so it is often omitted.

Example: Unmonitored Output Records

These are output records (bo, ao, longout, mbbo,…) that send data to a single scalar variable (Item) on the OPC UA server, but don’t require monitoring of the remote value.

# Boolean Output (unmonitored)
record(bo, "$(P)BAR:BOOL") {
    field(DTYP, "OPCUA")
    field(OUT, "@PLC0 ns=3;s=\"BAR\".\"BOOL\" monitor=n")
    field(ZNAM, "OFF")
    field(ONAM, "ON")
}

# Analog Output (unmonitored)
record(ao, "$(P)BAR:REALA") {
    field(DTYP, "OPCUA")
    field(OUT, "@PLC0 ns=3;s=\"BAR\".\"REALA\" monitor=n")
    field(DRVH, "100.0")
    field(DRVL, "0.0")
    field(HOPR, "90.0")
    field(LOPR, "10.0")
}

@PLC0: The name of the Session to which the Item belongs. Without monitoring, there is no Subscription necessary.
monitor=n: Explicitly disables monitoring for these unmonitored variables.

Example: Input Records with Monitor

These are input records (bi, ai, longin, mbbi,…) pointing to a single scalar variable (Item) on the OPC UA server that get updated when the value changes. They require an OPC UA Subscription and should be I/O Intr scanned.

# Binary Input
record(bi, "$(P)BAZ:BOOL") {
    field(DTYP, "OPCUA")
    field(INP, "@SLOW ns=3;s=\"BAZ\".\"BOOL\"")
    field(SCAN, "I/O Intr")
    field(ZNAM, "OFF")
    field(ONAM, "ON")
}

# Analog Input
record(ai, "$(P)BAZ:REALA") {
    field(DTYP, "OPCUA")
    field(INP, "@SLOW ns=3;s=\"BAZ\".\"REALA\"")
    field(SCAN, "I/O Intr")
    field(HOPR, "90.0")
    field(LOPR, "10.0")
}

@SLOW: The name of the Subscription to which the Item belongs. The Session is implicitly defined by the Subscription.
monitor=y is the default for all records expecting updates from the OPC UA server, so it is often omitted.
The SCAN should be set to I/O Intr as usual for input records that process on data changes received from the device.

Example: Unmonitored Input Records

These are input records (bi, ai, longin, mbbi,…) pointing to a single scalar variable (Item) on the OPC UA server, but don’t require monitoring of the remote value.

# Binary Input (unmonitored)
record(bi, "$(P)QUX:BOOL") {
    field(DTYP, "OPCUA")
    field(INP, "@PLC0 ns=3;s=\"QUX\".\"BOOL\" monitor=n")
    field(ZNAM, "OFF")
    field(ONAM, "ON")
}

# Analog Input (unmonitored)
record(ai, "$(P)QUX:REALA") {
    field(DTYP, "OPCUA")
    field(INP, "@PLC0 ns=3;s=\"QUX\".\"REALA\" monitor=n")
    field(SCAN, "I/O Intr")
    field(HOPR, "90.0")
    field(LOPR, "10.0")
}

@PLC0: The name of the Session to which the Item belongs. Without monitoring, there is no Subscription necessary.
monitor=n: Explicitly disables monitoring for these unmonitored variables.

Without a subscription, these records will only update (doing an OPC UA read operation) when they are processed.

Setting a periodic SCAN rate is inefficient and should be avoided; use monitored Items on subscriptions instead.

A valid use case for this type of connection are static status variables, which do not change during a session (e.g., server information: name, vendor, version, start date/time, …). They only get updated once after the connection is established.