I'm not familiar with doing this, but I wouldn't have expected a worksheet
change event  or recalculation to be triggered by a cell change  made via DDE.

Is this the only way your PLC can update the spreadsheet? If so, you may
need to put a timed loop into VBA to periodically check the contents of the
cells that the PLC fills, and do a workbook calculate if it has changed from
last time.

However, I'm not even sure that a workbook calculate would do anything, as
it might not think the cells have been changed. You might need to "dirty"
then cell  to get Excel's attention - e.g. from VBA, set recalculation to
manual, set the cell to its value plus one, set the cell back  to what it
should be, set recalculation to automatic, do a workbook calculate. (I
can't remember whether it's workbook calculate or application calculate -
just don't do a worksheet  calculate.)

The other approach is to look for events  that DDE does trigger  - there must
be some. Google will know.