Modulepool table control line item validation

Dear Sir

I am facing a prob. when trying to validate line item wise in table control.

I am explaining my requirement below.

I have no. of line items which are displaying table control screen output.one field is  SERNR  (seriel number).(Always editable)

If user selects all the line items ex. 7 lines (rows) ..and press check button ..It shoud give an error message

Message ''Serial number should not be blank''' type 'E'... Now user put  enter and system allowing to enter  the

serial number . I put the value ex. 2 in the first line item   ..Then check button press, system again should

give the same error message.. because other 6 line items (SERNR) field  still  blank ..So the error message should follow

all line items check unless and until the SERNR number has hold  some value.

How to achieve  this validation ??Pl. help me ..

Thanks