This paper presents the design of InfiniWrite, the implementation of a lightweight communication interface called RWAPI over the InfiniBand interconnect for clusters of PCs. Since the specifications of the InfiniBand interconnect provide many ways to transfer data, we are discussing some issues regarding the choices between InfiniBand capabilities. We implemented RWAPI on top of the grid-oriented architecture called GRWA and evaluated the communication performance. GRWA has been developed to provide performance to higher applications on a wide variety of architectures. We obtain a very low latency and a throughput very close to the maximum user bandwidth for messages as small as 4 kilo-bytes.