OPC Data Access (OPC DA) Versions & Compatibility
What is OPC DA?
OPC DA stands for OPC Data Access. It is an OPC Foundation specification
that defines how real-time data can be transferred between a data source and a
data sink (for example: a PLC and an HMI) without either of them having to know
each other’s native protocol.
Why is OPC DA so popular? How is it different than previous protocols?
The OPC DA Client/Server architecture was the first
architecture defined by the OPC Foundation. Before OPC DA, vendors’ products
(devices, PLCs, HMIs) required any device or applications connecting to them to
have a “custom driver” that translated between the third party connection and
the product in question. There were many problems associated with custom driver
based communications; some of these most common ones were: high cost,
proprietary technology that tied users to a particular vendor, hard to
configure and maintain because each custom driver had its own way of doing
things, hard to keep up-to-date because of the constant release of new devices
and applications. In contrast, OPC DA made it possible to connect to any
real-time data source without a custom connector written specifically for the
data-source/data-sink pair. Hence, reads and writes could be performed without
the data-sink having to know the data-source’s native protocol or internal data
structure.
Is there only one OPC DA Specification?
Yes and no. While the OPC DA specification belongs to the OPC Foundation, it has
gone through a number of revisions. The key ones are:
| Year |
Version |
Comment |
| 1996 |
1.0 |
Initial specification. |
| 1997 |
DA 1.0a |
Data Access (DA) name adopted to
differentiate it from other specifications being concurrently developed.
|
| 1998 |
DA 2.0 - DA 2.05a |
Numerous specification clarifications and modifications.
|
| 2003 |
DA 3.0 |
Further additions and modifications. |
Given there are different versions of the OPC Data Access (OPC DA)
specification, the key question is: are these versions backward compatible? For
example: can an OPC DA 1.0a client communicate with an OPC DA 3.0 OPC Server?
The answer is: depends.
Data Access OPC Client and OPC Server backward compatibility
It is possible and recommended that vendors write OPC Clients and OPC Servers
that are backwards compatible however, the reality is that backward
compatibility is optional rather than mandatory which means: a number of
vendors chose not to follow such advice (and continue to do so) and developed
OPC DA Servers that only recognize one or two of the specifications but not
all. What this means is that while these non-backward-compatible OPC Servers
and OPC Clients still give users the advantage of using OPC… they only work
with specific versions of the specification. The good news is that companies
like MatirkonOPC not only develop fully backward-compatible OPC Servers, they
also offer OPC data management products (ex. OPC Data Manager and OPC Security
Gateway) that sit between the non-backward-compatible OPC Clients and OPC
Servers to enable them to communicate with each other by translating between
OPC DA revisions on the fly.
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