The SERCOS interface is a set of standard specifications that may be incorporated into any company's products, with each control and drive maintaining its own functions and features. Because it is an international standard, it allows any manufacturer's SERCOS interface-compatible digital control to talk to any other SERCOS interface-compatible digital servo drive, digital spindle drive, hydraulic system, digital I/O or sensors over a well-defined fiber optic link or standard Ethernet technology (SERCOS-III). Controls and drives conforming to the standard comply with a standard medium for transmission, topology, connection techniques, signal levels, message (telegram) structures, timing and data formats. Note that SERCOS I/II and SERCOS III differ in terms of their physical layers, but share a common set of standardized parameters. Many of the other digital drive interfaces on the market are proprietary and can only be used with the manufacturer's own drives or have been "opened," but have been developed and are controlled by a single manufacturer, with a small number of other firms providing products using that bus. The SERCOS interface is standardized, guaranteeing both a standardized hardware platform and a standardized protocol open to use by all manufacturers.