Data Acquisition Output Modules

Data acquisition output modules transfer amplified, conditioned, or digitized signals to processing systems or devices. They plug into backplanes or motherboards, or interface directly with computer buses. User interface features include touch screens or front panels and displays, handheld or remote programming units, and connections to supervisory or host computers. Some data acquisition output modules output ranges of voltage or current. Other devices transmit frequencies or pulse signals such as amplitude modulation (AM), frequency modulation (FM), or pulse width modulation (PWM). Timers, relays, switches, and potentiometers can also receive outputs. 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 acquisition output modules include the number of analog, differential, and digital I/O channels. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs. Differential channels, which use the difference between two signals as an input, filter out common mode. In some data acquisition output modules, differential outputs are combinations of two single-ended outputs. Digital or discrete I/O signals are used in communication, user interface, and control applications. Other device specifications for data acquisition output modules include resolution and accuracy. Resolution refers to the degree of fineness of the digital word representing the analog value. Accuracy, which is expressed as percentage of the full range, depends on factors such as signal conditioning linearity, hysteresis, and temperature.

Data acquisition output 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 acquisition output 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 output modules often feature full casings or cabinets and integral interfaces.

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