DDE Request works but DDE Advise does not work.

[JadeRabbit]

I am using dynamic data exchange (DDE) to send data from an instrument driver to Labview. The instrument driver is set as the server and Labview as the client. The DDE request VI does work. But when I am trying to change to DDE advise VI which is able to track the changes an error pops up in the DDE Advise Start VI.(Error Code 14012).

One thing very interesting: for DDE request, the ref num is the same as DDE Open VI; for DDE advise VI, the ref num is different from DDE Open VI.

I am wondering if anyone could give some advise. Thanks in advance!

[JadeRabbit]

I figured out a way to get round this problem myself. What I did is the following:

Reset my instrument driver as the client and Labview as the server, then made the instrument driver to poke the Labview server and send the data to Labview. The DDE server check VI works flawlessly this time.

Additional comment: It is true that Dynamic Data Exchange (DDE) is an out-dated protocol and ActiveX is preferrable. However, in the science labs some old instruments are still being used and their drivers does not support ActiveX. So DDE is still useful. 

[Dennis_Knutson]

What sort of interfaces to the computer do these instruments have? The preferred protocol is to directly communicate to serial and GPIB instruments with the VISA API. Have you looked for LabVIEW drivers or considered writing your own against the day that DDE is no longer supported by the os?

[JadeRabbit]

Hi Dennis,

Thank you for the reply.

The interface of the instrument to the computer is serial. In fact, the initial idea is to do what you suggested: communicate directly with the instrument. But I am not very familiar with VISA and Labview driver for the instrument is not available. 

What would be a good place to start on VISA? I would definitely look into that.

[Dennis_Knutson]

Go to http://www.ni.com/downloads/instrument-drivers/ and you'll find several links including communication fundamentals and how to write drivers.