Data Acquisition

As individual signal conditioning modules require external power, they are typicallydesigned to plug into a mounting board with on-board power supply as shown in Figure3.14.DAC BoardMulti-coreCablePower SupplyHost ComputerThermocouples Strain gauges RelaysFigure 3.14Board mounted modular signal conditioningA single connector on the mounting board is used for easy cable connection betweenthe mounting board and the I/O of the plug-in data acquisition board. Cables are typicallya multi-core twisted-pair. This allows many different types of transducers to be interfacedto the latest plug-in data acquisition boards, but does not facilitate distributed I/O.3.3.3 Distributed I/O – digital transmittersOften sensors must be remotely located from the personal computer in which theprocessing and A/D conversion of the analog data takes place. This is especially true inindustrial environments where sensors such as thermocouples and strain gauges arelocated in hostile environments over a wide area, possibly hundreds of meters away. Innoisy environments, it is very difficult for the very small signals received from sensors,such as thermocouples and strain gauges (in the order of mV), to survive transmissionover such long distances, especially in their raw form, without the quality of the sensordata being compromised.An alternative to running long (and possibly expensive) wires from the transducersdirectly, or from two-wire transmitter modules, is the use of distributed I/O. DistributedI/O is available in the form of signal conditioning modules that are remotely located fromthe host PC, near the sensors to which they are interfaced. One module is required foreach sensor used, allowing for high levels of modularity (single point up to hundreds perlocation). While this can add a reasonable expense to systems with large point counts, thebenefits in terms of signal quality and accuracy may be worth it.One of the most commonly implemented forms of distributed I/O is the digitaltransmitter. These intelligent devices perform all the functions of simple signal conditioningmodules (two-wire transmitters) but also contain a micro-controller and A/Dconverter to perform the digital conversion of the signal within the module itself.Converted data is transmitted to the computer via an RS-232 or RS-485 communicationsinterface. The simplified functional block diagram of a typical digital transmitter is shownin Figure 3.15.