Sheet Update Problem

[Gary Desroches]

I have a LARGE excel file with more than 100 active cells,
buttons, conditional formating etc.
These cells receive information through RSLinx from
various PLC modules in our facility.

My Problem:
a few of these cells need to be used as "triggers"
When the value changes I want to open a msgbox alerting
the user of a change on that sheet or another sheet not
currently visible. Currently I can only get this to work
if the cell value changes due to direct user input from
the keyboard.
If a formula updates say from 0 to 1 the cell "value" is
still the formula typed into the cell, so the function:

Private Sub Worksheet_Change(ByVal Target As Range)
' do something
End Sub

does not fire. Likewise the function:

Private Sub Workbook_SheetCalculate(ByVal Sh As Object)
' do something
End Sub

does not fire. (These are built in excel functions I
found in the help files.)
Am I trying to to do something that is not possible, or
just doing it wrong? If wrong, can someone show me how to
do it right?

Thanks in advance

Gary