OPC XML-DA

OPC XML-DA (XML Data Access) is the web services (SOAP/XML) based version of OPC DA. Since it runs over HTTP instead of COM/DCOM, it is platform independent and has a firewall-friendly architecture.

inSCADA supports the OPC XML-DA protocol in Client role only.

OPC XML-DA vs OPC DA vs OPC UA

Feature	OPC DA	OPC XML-DA	OPC UA
Platform	Windows only	Platform independent	Platform independent
Transport	COM/DCOM	HTTP/SOAP	TCP, HTTP, WebSocket
Performance	High	Medium (XML overhead)	High
Firewall	Problematic (DCOM)	Easy (HTTP)	Easy
Status	Legacy	Legacy	Active

Note

OPC XML-DA was developed as a platform-independent alternative to OPC DA but has been largely superseded by OPC UA. For new projects, OPC UA should be preferred. OPC XML-DA should only be used for integration with legacy systems that do not support OPC UA.

Data Model

Connection (HTTP URL)
└── Device
    └── Frame (Data Block — Subscription group)
        └── Variable (Item Name)

Configuration

Connection

Parameter	Example	Description
Protocol	OPC XML	Protocol selection
IP Address	192.168.1.100	OPC XML-DA server IP address
Port	8080	HTTP port
Path	/OpcXmlDaService	Web service path (endpoint)
Connect Timeout	5000 ms	Connection establishment timeout
Request Timeout	5000 ms	Request timeout
Max Depth	12	Tag tree browse depth

Device

Parameter	Example	Description
Scan Time	1000 ms	Scan period
Scan Type	PERIODIC	PERIODIC or FIXED_DELAY

Frame

Parameter	Example	Description
Use Subscription Mode	true	Subscription-based data retrieval
Percent Deadband	0.5	Analog value change threshold (%)
Hold Time	0 ms	Server response hold duration
Wait Time	0 ms	Server change wait duration

Hold Time and Wait Time: Control the OPC XML-DA subscription mechanism. The server waits for value changes during the Wait Time and holds the response during the Hold Time. When both values are 0, immediate polling is performed.

Variable

Parameter	Example	Description
Name	Random.Int1	Item name (OPC XML-DA item path)
Type	Float	Data type

Supported Data Types

Data Type	Description
Boolean	Single bit value
SByte	Signed 8-bit integer
Byte	Unsigned 8-bit integer
Int16	Signed 16-bit integer
UInt16	Unsigned 16-bit integer
Int32	Signed 32-bit integer
UInt32	Unsigned 32-bit integer
Int64	Signed 64-bit integer
Float	32-bit floating point
Double	64-bit floating point
String	Character string
DateTime	Timestamp