The I/O module types can be classified as analog input, analog output, digital input and digital output. They are usually categorized as 2, 4, 8 and 16 channel modules. Analog input modules measure voltages as small as 10 mV when connected to strain gauges, thermocouples and wheatstone bridges to voltages as large as 10 V. The voltage inputs can be single ended or differential and can range from 0-5 V, 0-10 V, +/-5 V and +/-10 V. Current inputs are usually 4-20 mA loops and can also source excitation current for passive sensors like thermistors. In addition, some analog I/O modules can communicate to field instruments by modulating the 4-20 mA current using the HART protocol. Analog output modules used to drive actuators and power sensors also use the same voltage ranges i.e. 0-5 V, 0-10 V, +/-5 V, +/-10 V and current ranges i.e. 4-20 mA. Digital inputs and outputs, on the other hand, are typically 24 V and can tolerate up to +/-30 V. They usually have short-circuit and overload protection and some newer products allow communication to field sensors via the IO-Link protocol. All these modules include isolation between the control side and the field side via DC-DC converters and digital isolation between the field devices and the digital backplane.