Formula which gives error when more than 16 Characters entered

[Newton1234]

Hi,

I was wondering if anyone was aware of a formula which will allow an error message to pop up in a cell if more than 16 Characters are entered into another cell ?

However, I cant put an actual limit on the characters for this cell as if another cell is populated with anything other than bacs then it can allow more than 16. Therefore my formula needs to state something like :

=If(A1="BACS", If(B1=">16 characters", "TOO LONG", ""))

If anyone can help I would be very grateful, thanks.