I need to cross validate two cells, E must be blank for F to have data, and vice versa, AND E can only accept the letters, M,V,A: Cell F can only accept SM, SA or SV. I have not used Drop down menus, because my staff, has had problems with simple yes/no drop down in another spread sheet.

I can get part of the formula to work for F is =OR(F12="SM",F12="SA",F12="SV"), Message "Use correct Coding"

but when I try to add the second criteria for the E must be blank it doesn't work.
=AND(ISBLANK(E12),OR(F12="SM",F12="SA",F12="SV")) Did block text in E if F had data but allowed any letter to be entered. Also tried this as an IF statement.

I have also tried that if E has M, A, V
=(OR(F12="SM",F12="SA",F12="SV"))=NOT(OR(E12="M",E12="A",E12="V")) wouldn't allow any data in F

=AND(E12="M",E12="A",E12="V",OR(ISTEXT(F12))) Did block text in E if F had data but allowed any letter to be entered.

Thank you so much for any help.