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The modules can be categorized as generic and application-specific. As an example, discrete inputs (contact inputs) are used in virtually all applications and differ primarily in their voltage rating. Other considerations in selecting a module are its redundancy, isolation (group or point), terminal block type, availability for safety applications (IEC 61508), and approval for hazardous locations. A typical application-specific module is a servo module that is used for fast closed-loop control of a turbine’s servo valve actuator or a complete emergency over-speed trip system for a turbine. These unique modules will not be described in the following tables. However, some applicationspecific modules such as a vibration module is commonly applied in monitoring radial and axial shaft displacement of rotating machinery in plant distributed control systems and will be described in a separate table.