There are three major communications standards used in process control systems. First is the Industrial Fieldbus protocol, based on RS-485/422 and RS-232. This is used for slower speed communications between PLC modules and PLC controllers.  Industrial Fieldbus systems often contains isolation. The second major standard is Industrial Ethernet. Industrial Ethernet is used for communications in DCS systems between local devices and the central host. Industrial Ethernet is also used in PLC systems for communications to a controller. Two common major industrial Ethernet standards are Profinet and EtherCAT. The third major standard is HART.  HART is an FSK-based communication system super-imposed on a current loop for a loop powered device, such as a field instrument. Wireless standards, such as wireless HART, are also beginning to emerge.