Developed as early as 1979 by US PLC maker Gould Modicon (which is now a division of Schneider Electric), the Modbus protocol is considered one of the very first fieldbus systems. Enabling communication between control systems and devices supplied by different manufacturers, the open protocol eventually became something of an industry standard. Modbus is purely an application protocol, i.e. it does not depend on a transmission medium. Conceived and initiated by Schneider Automation, Modbus-TCP draws on the same services and the same object model as the original Modbus varieties, i.e. Modbus ASCII, Modbus RTU (asynchronous data transfer via RS-232 or RS-485), and Modbus Plus (Token Passing). However, this new incarnation uses Ethernet as its data transfer protocol, and uses TCP/IP packets for sending data.