Abstract This paper presents a module allocation technique to improve on-line testability of a data path. The technique assigns multi-type operations to modules. Types of modules and count of each type of module, needed to synthesize a given scheduled data flow graph (SDFG) must be known a priori. The testing utilizes idle time of modules. The objective is to test each type of operation assigned to a module. Testing time is reduced by minimizing the number of types of operations assigned to a module. Certain operations called idle-time operations are scheduled in the SDFG and assigned to modules in their idle time to enhance testing. Ideally, one idle-time operation is required for each type of operation assigned to the module. The technique minimizes number of types of operations assigned to each module and creates sufficient idle time. Promising results are obtained on benchmark examples.