I think your request is not so standard, because you have to consider that being able to operate and modify an Excel sheet while a macro (Visual Basic application script) is running, and therefore potentially acting on the same sheet, is dangerous: what happens if both the you and the script are trying to modify the same cells?
In general, the result is not deterministic.
However, warned of the possible side effects, there is a simple way to achieve this result.
You don't need to use callbacks, since FasterCap and FastCap2 do not use callbacks to signal the end of their task. You have to poll the solver instead. You can, therefore, modify the core of the loop that waits for the solver to end its task, to allow the Windows OS to process other events. This can be done with the DoEvents function (you can check the Excel online help for an explanation on how this function works).
The modified code (taken from the FastCap2 Excel automation sample) is reported here below, where the changed part is highlighted in red.
Enrico
Sub FastCap_nonblocking()
' FastCap Automation example
' Enrico Di Lorenzo, 2004/04/15
Dim FastCap2
' Create FastCap object
Set FastCap2 = CreateObject("FastCap2.Document")
For i = 0 To 3
' Try to run FastCap
' Remark: the run path must be surrounded by quotas '"' to support
' also paths containing spaces (quotas in VisualBasic are escaped by
' doubling the symbol, i.e., "" )
couldRun = FastCap2.Run("""" + ActiveWorkbook.Path + "\sphere" + CStr(i) + ".qui""")
' Wait for end of operation, using polling; could also use callback function
startTime = Now
Do While FastCap2.IsRunning = True
DoEvents 'This will allow the OS to process events
Loop
' retrieve capacitance matrix
capacitance = FastCap2.GetCapacitance()
Range("B" + CStr(i + 5)).Value = "Sphere" + CStr(i)
Range("C" + CStr(i + 5)).Value = capacitance(0, 0)
Next
' Quit FastCap
FastCap2.Quit
' Destroy FastCap object
Set FastCap2 = Nothing
End Sub
