Data Acquisition

Channel commands allow the user to modify specified channels for:• Input configuration• Sensor excitation• Defining channel constants such as resistive shunts and attenuation factors• Identifying reference channels for thermocouples, Wheatstone bridges• Scaling of the channel data by spans, polynomials, factors specifyingstatistical analysis and histogram extraction• Specifying progressive difference, rate of change, integral assignment ofchannel data to temporary storage registers• Assigning unique names to channels• Specifying format and resolution of the channel data• Specifying whether data is returned to the host PC, logged or displayedlocally.Some of these channel commands are discussed in more detail below.As we have shown, many sensors require some form of excitation in the form of a voltagesupply or constant current source, to enable them to output a signal. Excitation channeloptions inform the stand-alone device of the excitation that is required for a particularchannel.Channels can be read frequently but produce a statistical summary at longer intervals.This summary is returned, logged or displayed at intervals determined by the pre-definedschedules. Channels that require statistical sampling must include a channel option toindicate the statistical information to report. The following statistical channel options aretypically available:• AverageThe sum of all the channel readings divided by the number of readings sincethe last statistical report. It is very useful in reducing sensor noise byaveraging out cyclical noise, such as mains hum.• Standard deviationThis is the measure of variability of data concerning the average. The variationmay be due to noise or process changes.• Maximum and minimumThis returns the maximum or minimum for the scan period and the time anddate that it occurred.• IntegralThis returns the integral (or area under the curve) with respect to the time inseconds using a trapezoidal approximation. The units of integration are thoseof the original reading multiplied by seconds.Data manipulation and scaling of data read from a particular channel can be performedautomatically before the data is stored, by using some of the following utilities: