Item rev field mandatory for line type "inventory" on a Purchase order.

Hello Gurus, We have a requirement to make the item revision (Rev) mandatory on a purchase order only when the PO line type is "Inventory". So once the line type is selected as inventory and the item is entered in the item field, the rev field must automatically become a mandatory field. The same functionality must work when multiple requisitions are autocreated to the purchase order. The rev field must NOT be mandatory for other line types for example. expense, outside processing etc. Please let me know how to create the personalization in order to achieve this solution. Please attach