A high-performance and ease-of-use implementation of the Modbus protocol written in Java by Infinite Automation Systems Inc.. Supports ASCII, RTU, TCP, and UDP transports as slave or master, automatic request partitioning and response data type parsing.
For commercial licenses see www.InfiniteAutomation.com
Get the most recent code here: https://github.com/infiniteautomation/modbus4j
For support: http://infiniteautomation.com/forum/category/11/modbus4j-general-discussion