OPC DA

OPC DA (Data Access) is the most widely used component of the OPC Classic standards. It is built on Windows COM/DCOM technology and provides real-time data access. It was used as the de facto standard in industrial automation before OPC UA.

inSCADA supports the OPC DA protocol in Client role only.

Note

OPC DA is based on Windows COM/DCOM. Therefore, it only runs on Windows. For a platform-independent solution, OPC UA should be preferred.

OPC DA vs OPC UA

Feature	OPC DA	OPC UA
Platform	Windows only	Platform independent
Technology	COM/DCOM	TCP/IP, HTTP, WebSocket
Security	DCOM security	TLS, certificates, user auth
Discovery	Via DCOM	Browse + Discovery
Status	Legacy	Active development

Data Model

Connection (COM ProgID)
└── Device
    └── Frame (Data Block — Subscription group)
        └── Variable (Item ID)

Configuration

Connection

Parameter	Example	Description
Protocol	OPC DA	Protocol selection
IP Address	192.168.1.100	OPC DA server IP address
Port	135	DCOM port (default: 135)
COM ProgID	Matrikon.OPC.Simulation	OPC server COM programmatic identifier
Separator	.	Tag path separator character (default: dot)
Max Depth	12	Tag tree browse depth
Server Status Check Time	30000 ms	Server status check period

Tip

COM ProgID is the OPC server’s programmatic name in the Windows Registry. Examples: Matrikon.OPC.Simulation, KEPServerEX.V6, RSLinx OPC Server. It can be found from the OPC server documentation or OPC browser tools.

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 (notification on change)
Percent Deadband	0.5	Analog value change threshold (%)

Subscription Mode: When true, the OPC server sends notifications only when values change — reducing network traffic and CPU load. When false, periodic polling is performed.

Variable

Parameter	Example	Description
Name	Temperature	Variable name (also used as OPC Item ID)
Type	VT_R4	OPC DA data type

Supported Data Types (VARIANT Types)

Data Type	Description
VT_BOOL	Boolean
VT_I1	Signed 8-bit integer
VT_UI1	Unsigned 8-bit integer
VT_I2	Signed 16-bit integer
VT_UI2	Unsigned 16-bit integer
VT_INT	Signed 32-bit integer
VT_UINT	Unsigned 32-bit integer
VT_I8	Signed 64-bit integer
VT_R4	32-bit floating point (float)
VT_R8	64-bit floating point (double)
VT_BSTR	Character string

Browse (Discovery)

inSCADA can discover available items by browsing the OPC DA server’s tag tree. This feature makes it easier to find the correct Item ID information when defining variables.

DCOM Configuration Notes

Since OPC DA runs over Windows DCOM, DCOM configuration is required for remote connections:

1. Configure remote access permissions using DCOMCNFG on the OPC server computer
2. Open DCOM ports (135 + dynamic ports) in Windows Firewall
3. Both computers must have the same user account or appropriate authorization

Caution

DCOM configuration is complex and can create security vulnerabilities. Where possible, using OPC UA instead of OPC DA is recommended.