OPC versus DDE


Thread Starter


We are looking at using an OPC/DDE server to access a PLC and need to decide which way to go OPC or DDE.

What do you think are the key differences between these two technologies DDE and OPC, and how do these differences translate to benefits, both to the system integrator and to end users.

So which is the better technology overall?
OPC is a standard interface which sits above the specific PLC driver. OPC uses the com interface and can pass great amount of data as you are just referencing memory in the server.

DDE is an old standard, initially used by Microsoft to set-up a channel between office apps. It is slow and not really suitable for real-time apps, although a lot of people will beg to differ.

If possible stay away from the DDE route. Obviously the improved performance has a cost.
- OPC is faster
- OPC has a much tighter specification and therefore much better interoperability
- OPC allows you to browse for the data by pointing and clicking. You don't need to know the Topic!| etc.
- OPC is now the standard in automation software
- OPC networks much better than DDE
- OPC supports multiple clients

Clearly you should go OPC. If you still have some client or server which is DDE, don't worry - there are applications that does the conversion.

For OPC, DDE and other automation software technologies take a look at the book "Software for Automation: Architecture, Integration, and Security". Buy online:

Jonas Berge
[email protected]
Learn fieldbus and industrial Ethernet at your own pace:
Learn OPC and automation software at your own pace: www.isa.org/autosoftware
I agree with the first comment. What you can do is use and OPC sever to get and put info from and in the PLC, and then if you want to, e.g., show the info in a VB HMI, you can use DDE. I don not have the sample codes in this PC but if you need them you can send me an email and I'll try to send it.

Curt Wuollet

And OPC will preserve the status quo and prevent innovation for another 10 years.