We just published 3 packages for interfacing, using Modbus TCP communication,your ROS workstation with some industrial component like the famous In-Sight camera from Cognex and the Siemens S7 PLC.

You'll find a link and a quick tutorial for all those three packages:
http://www.generationrobots.com/blog/en/2015/04/cognex-siemens-plc-modbus-pkg/