Applies To:
  • CitectSCADA 4.x

DDEformu.xls is an Excel macro which uses DDE to grab data out of a running Citect system. When I choose to select a large number of trend samples from Citect using DDEformu.xls I don't get them all. What is happening? 

The problem here is that the 16bit DDE subsystem in Windows 95 has a 64K limitation. This will cause the transfer to terminate at this quantity of data. To figure out how much data you can get in one go, consider the following example.

Each sample space uses nineteen bytes to hold the time and date stamp. Add an extra eleven bytes for every trend you have selected. Multiply this addition to arrive at the total amount of data. Say for instance that you want 1000 samples from 2 trends.

Each sample = 19 + (11 * 2)
Total data  = 1000 * Each sample
            = 41000 bytes

This data set should work fine. However, if you try for 4000 samples of one trend....

Each sample = 19 + (11 * 1)
Total data  = 4000 * Each sample
            = 120000 bytes

You will probably find that this set abruptly ends around sample number 2184, since 65535 / 30 (sample size) = 2184