Link two cells so that edits to either are reflected - vba

Question

I have two cells, "R3" and "S3", in the same Excel sheet, "Data", which I want to link so that a manual entry to either is reflected across to the other with a correcting factor. Circular references prevent using formulae. I searched here and found this VBA snippet:
Private Sub Worksheet_Change(ByVal target As Range)
If target.Address = "$R$3" Then
ActiveWorkbook.Worksheets("Data").Range("$S$3").Value = target.Value*2.55
End If
End Sub
It was originally meant to link two cells on different sheets and in my own case the cells are on the same sheet. This works perfectly for entering in one and seeing the change in the other in one direction only. I can also add additional cell pairs using ElseIf. However, if I expand it to contain a dual reference as in R3->S3 and S3->R3 like this:
Private Sub Worksheet_Change(ByVal target As Range)
If target.Address = "$R$3" Then
ActiveWorkbook.Worksheets("Data").Range("$S$3").Value = target.Value*2.55
Else If target.Address = "$S$3" Then
ActiveWorkbook.Worksheets("Data").Range("$R$3").Value = target.Value/2.55
End If
End Sub
Even though the two sections seem mutually exclusive and cannot both be referenced in the same procedure call, this crashes my spreadsheet every time I try to make a change. Obviously I am doing something hugely wrong! I had been led to believe from the original post that this would not cause a circular reference.
My guess is that I should be approaching this with two separate procedures, or is it just plain impossible based on this code?

Answer 1

You are creating an infinite loop using the change module. By updating one it triggers the event using the other cell
update your change event to look like:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Or Target.Address = "$B$1" Then
Call cellUpdate(Target.Address)
End If
End Sub
and then in a module put:
Sub cellUpdate(Target As String)
Application.EnableEvents = False
With ActiveWorkbook.Worksheets("Data")
If Target = "$R$3" Then
.Range("$S$3").Value = .Range(Target) * 2.55
ElseIf Target = "$S$3" Then
.Range("$R$3").Value = .Range(Target) / 2.55
End If
End With
Application.EnableEvents = True
End Sub
By breaking it out into a separate module and controlling the Application.EnableEvents in the separate module you stop the infinite loop.

Answer 2

You are missing one End If:
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
If Target.Address = "$R$3" Then
ActiveWorkbook.Worksheets("Data").Range("$S$3").Value = Target.Value * 2.55
Else
If Target.Address = "$S$3" Then
ActiveWorkbook.Worksheets("Data").Range("$R$3").Value = Target.Value / 2.55
End If
End If
End Sub