EPICS Records for Structured Data

Added in version 0.3: The opcuaItem record type and support for structured data.

This guide shows how to configure EPICS records for accessing structured data on the OPC UA server. This is achieved using the opcuaItem record type in combination with regular EPICS records.

The opcuaItem record connects to the Item that represents the structure. Leaf elements are accessed through regular EPICS records, one for each element that is interfaced.

Creating the Root opcuaItem

INP Link Field Format for the opcuaItem Record

This format is the same as for the INP/OUT links of single scalar variables.

@<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]
bini	read	Behavior at init: read, ignore, write
monitor	y	Enable monitoring. For outputs, enables readback
timestamp	server	Source of timestamp: server/source/@<top_element> [top_element points to source of timestamp]

opcuaItem Record Configuration

The opcuaItem record connects to a structured variable (Item) on the OPC UA server. It requires an OPC UA subscription.

The opcuaItem record is bidirectional. Explicit read and write operations can be triggered by writing to the (pseudo) fields READ or WRITE.

record(opcuaItem, "$(P)FOO:OPCUA-ITEM") {
    field(DTYP, "OPCUA")
    field(INP, "@SLOW ns=3;s=\"DB1\".\"FOO\"")
    field(SCAN, "I/O Intr")
    field(DEFACTN, "read")
    field(WOC, "immediate")
}

• @SLOW: The subscription name.

• ns=3;s=\"DB1\".\"FOO\"": Points to the structured OPC UA Item.

• SCAN, "I/O Intr": Ensures the record processes when the structured data changes.

• DEFACTN specifies the default action when the record is processed: read or write.

• WOC controls the writing behavior when single elements of the structure are changed. manual (default) requires explicit processing of the record to write the structure. immediate processes the record (writes the structure) as soon as an element changes. This can be used to avoid writing inconsistent data while multiple elements of the structure are written in a transactional fashion, e.g., when restoring a snapshot or loading a configuration.

Linking Element Records to opcuaItem

Individual elements within the structured item are accessed by other regular input/output records.

INP/OUT Link Field Format for Elements

@<opcuaItem_record_name> [element=<element_name>] [<option>=<value>...]

• <opcuaItem_record_name>: The name of the opcuaItem record managing the structured data.

• element=<element_name>: The path to the specific element within the structure. Use . as a hierarchy separator.

Available Options

Option	Default	Description
element		Path to element inside an opcuaItem structure [. = hierarchy separator; <empty> = root element if no structure]
bini	read	Behavior at init: read/ignore/write
monitor	y	Enable monitoring. For outputs, enables readback
timestamp	server	Source of timestamp: server/source/data

Example: Input Records for Elements of $(P)FOO:OPCUA-ITEM

# Binary Input for a boolean element
record(bi, "$(P)FOO:BOOL") {
    field(DTYP, "OPCUA")
    field(INP, "@$(P)FOO:OPCUA-ITEM element=STRUCT.BOOL")
    field(SCAN, "I/O Intr")
    field(ZNAM, "OFF")
    field(ONAM, "ON")
}

# Analog Input for a real element
record(ai, "$(P)FOO:REALA") {
    field(DTYP, "OPCUA")
    field(INP, "@$(P)FOO:OPCUA-ITEM element=STRUCT.REAL")
    field(SCAN, "I/O Intr")
}

• element=STRUCT.BOOL: Refers to the BOOL sub-element inside the STRUCT element of the structure that the opcuaItem record points to.