About Data Input Modules
Data input modules receive signals from devices such as sensors and often provide voltage and current excitation along with signal conditioning for subsequent analog-to-digital conversion. They plug into backplanes or motherboards, or interface directly with computer buses. Some data input modules receive AC and DC voltages, AC and DC currents, charges from piezoelectric elements, or frequency inputs. Other devices receive signals from accelerometers, thermocouples, thermistors, resistance temperature detectors (RTDs), strain gauges, linear rotary differential transformers (LVDTs), and rotary variable differential transformers (RVDTs). Encoders, tachometers, timers, relays, and switches can also provide inputs for data acquisition input modules. Optional features include random access memory (RAM), data storage, and configurable application software for real time graphing and mathematical or statistical functions.
Device specifications for data input modules include the number of analog, differential, and digital I/O channels. When single-ended inputs are available, suppliers often specify the maximum number of analog channel inputs as twice the number of differential inputs. Differential channels, which use the difference between two signals as an input, filter out common mode. In some data input modules, differential inputs are combinations of two single-ended inputs. Digital or discrete I/O signals are used in communication, user interface, and control applications. Other device specifications for data input modules include sampling frequency, resolution, and accuracy. Resolution refers to the degree of fineness of the digital word representing the analog value. Accuracy, which is expressed as a percentage of the full range, depends on factors such as signal conditioning linearity, hysteresis, and temperature.
Data input modules often include an integral filter that attenuates some signal frequencies and allows others to pass. Analog filters are used in low-noise applications. Digital filters such as finite impulse response (FIR) and infinite impulse response (IIR) filters approach ideal band pass characteristics. Low pass, high pass, band pass, band stop, and all pass filter functions are available. Filter types include Bessel, Butterworth, Cauer or Elliptic, Chebyshev, and linear phase. Some data input modules include a filter with both a low pass cutoff frequency and a high pass cutoff frequency. Other modules include a user-programmable or anti-aliasing filter. Devices with integral amplifiers differ in terms of programmable gain, bandwidth, input impedance, and common mode rejection ratio.
Data input modules vary in terms of host and network connections. For host connections, devices can use a direct backplane interface; RS232, RS422, or RS485 serial connections; parallel connections; universal serial bus (USB) or general-purpose interface bus (GPIB); small computer system interface (SCSI), or transistor-transistor logic (TTL). Network connections include CANbus, DeviceNet, smart distributed system (SDS), VersaModule Eurocard (VME) Bus, Foundation Fieldbus, Profibus, ARCnet, serial real-time communication system (SERCOS), Interbus-S, Seriplex, AS-I, and Beckhoff I/O. Ethernet and IEEE 1394 (FireWire®) are available for both host and network connections. FireWire is a registered trademark of Apple Computer, Inc.
Data input modules are available in a variety of form factors. Some devices mount on integrated circuits (ICs), standard DIN rails, or printed circuit boards (PCBs). Others bolt into walls, cabinets, or enclosures. Rack-mounted units fit inside a standard 19” telecommunications rack. Modular styles include stackable units that dock in bays, slots, or boxes. Benchtop or freestanding data acquisition input modules often feature full casings or cabinets and integral interfaces.
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