ob07 and fixed exchange rate

Hi Friends,

We have maintained 'M" exchange rate type in OB07 and activated "inverted exchange rate' column only.

So, when user is creating PO with fixed exchange , then is it necessary in OB07 that exchange rate type 'M' should also have "FIXED" column to be ticked.

IF Not- then please can you all let me know where can we configure in SAP so that user can also use fixed exchange rate in PO.

Many thanks in advance,

RD