DDE Server


Thread Starter

Vijaya chandar

Dear all,

I would like to know the following points:

1. Request from DDE client to DDE server is happening Simultanously or sequentially?

2. If simultanous means how many Data items can be retrieved or refreshed at a time?

3.If it is happening sequentially then it is abosolutely a wrong concept to work with Real time application. Let us consider there are 100 DDE items. When the client is placing the requests, sequentially after every fixed delay, the the retrived data or the data getting refreshed is of all the delayed ones and not the Real time one. am i right or not?

What will be the situation if i use the sequential concept for the real time data logging into the database and using those datas for generating report?

Kindly help me in this regarding.

Thanks in advance.

J.Vijaya chandar.
[email protected]

Raymond van der Tas

1. Depends on the implementation of your DDE server.
If you have a DDE server vendor in mind you should ask for their performance bench marks.

2. You could squeeze a couple of hundred tags out of a DDE server. This was one of the reasons why the OPC (Ole For Process Control) initiative was started back in 1995. DDE did not scale too well and was discontinued by Microsoft. It worked well for projects in the early nineties, but failed when modern requirements asked for processing thousands of tags.

Some companies have tried to expand the live cycle of DDE a bit by multiplexing DDE channels and transferring blocks of data. This would work if both the client and server agree on the special implementation. However, one day Microsoft may remove their first OLE trial from their operating systems.

3. In addition to you remark DDE technology forgot to expose process information such as the time stamp of the tag as well as the data integrity of the tag. As a result your 'realtime' client may be processing dead values where your client has no knowledge about calamities such as a broken thermocouple or communication link.

Again you can put intelligence in both the server as well as the client side to work around this.

Serious data storage and reporting applications should work with technologies where the value is accurately time stamped and data integrity is ensured. For details, please study http://www.opcfoundation.org

Raymond van der Tas