PB app, locking in Informix 5

Hi, I am developing a multiuser PowerBuilder 4.0.03 application with Informix OnLine 5 as the back end database. In the application, one user will need to exclusively read/update a set of rows in several tables at a time. Other users should be able to read/update the other rows at the same time. I am wondering if anyone knows how to lock a set of rows in several tables at a time. I tried ALTER TABLE tablename LOCK MODE (ROW) but still unsuccessful. I am also wondering if 'repeatable read' will lock the whole table. Answers or pointers will be very much appreciated. Thank you.