8000 Zone2.pdf

Analog Input Modules - THC and RTDGeneralThese modules provide digitised data andstatus information of analog measurementsfrom thermocouples, mV sources, RTDs andresistance sources.Thermocouple modules provide four or eightchannels for monitoring input signals fromthermocouples or mV sources. The functionof the module is set up duringconfiguration. Cold junction compensationfor thermocouple applications is provided bymeans of a sensor in the field terminal. Onlythe recommended field terminals can beused with these modules.RTD modules provide four or eight channelsfor monitoring input signals from RTD orresistance sources. The function of themodule is set up during configuration. TheRTD can be 2-, 3- or 4-wire type. Only therecommended field terminals can be usedwith these modules.Input samplingThermocouple modules sample at intervalsof 60ms per channel. In addition, themodule has cold junction temperaturecompensation that is refreshed every 1.8seconds for 4-channel modules and every2.4 seconds for 8-channel modules. Thesampling technique for the RTD module issimilar where samples of the voltage across,and the current through, the RTD aremeasured at intervals of 60ms per channel.Compensation methods reject the effect ofresistance in the cable conductors for 3-wireand 4-wire RTD/Resistance.Data formatThe 8105/6 4-channel modules store dataas 15-bit plus sign integers (–32768 to+32768). The 8205/6 8-channel modulesstore data as 16-bit unsigned integers (0 to65535).FilteringAn Infinite Impulse Response (IIR) filter isused on the input data before it reachesthe A/D converter. Depending upon thecoefficients selected, the output from thefilter will be:a) the input value (filter OFF)b) an average of the last two readings(filter ON - setting 1)c) a running average of readings (filter ON- setting 2)The coefficients can be selected individuallyfor each channel.Input alarmsThe modules provide two configurable alarmlevels for each channel—a high limit and alow one. See figure.When an input value exceeds an alarm limitthe appropriate alarm bit (high or low) is setin the channel status byte. In addition, the“new data” signal is set to allow thecontroller to collect the new alarm statusinformation and the affected channel LEDwill flash.Alarm deadbandThe alarm deadband (not shown on thediagram) is fixed at 1%.Dead zoneEach channel has a definable "dead zone".This is to reduce the need for the module toreport to the BIM every minor change ininput value. If the input value differs by theamount defined by the Dead Zone, or more,then the new value is reported, otherwise itis not. This reduces traffic on the internal buswhich improves the system response time. Ifthe Dead Zone value is set to zero (thedefault), then every input value read will seta 'New Data' flag, and bereported.Open sensor detectionWhen configured to do so, the modules willdetect an open circuit sensor and report itwithin 10 seconds. When this occurs a statusbit is set in the module and the affectedchannel LED flashes. The detection optionsfor the two module types are configurable asfollows:THC and mVOff, drive upscale or drive downscaleRTD and resistanceOff or drive upscaleThese choices can be made for each channel.InputValueHi Alarm setMonitoredChannel ValueHi Alarm clearedModule operating statesNormal/Failsafe modeThe THC and RTD modules support failsafemode as defined in the earlier I/O moduleintroductory section. When not in failsafe themodule adopts Normal mode.Channel Active/InactiveA channel can be made active or inactiveindividually. When a channel is madeinactive inputs will not be processed.Power-up conditionsThe module uses the following values when itpowers up.Module modeNormal (not “failsafe”)Active/inactiveAll channels power up in the active state.AlarmsAll alarms are made inactive by havingtheir values set to high or low extremes,as appropriate.Dead zone0 (i.e. all changes of A/D data arereported for an active channel)Software filteringDisabledChannel typeType K thermocouple or3-wire RTD - Pt100O/C sensorOffVisual indicatorsChannel “status” LED (yellow)OnOffFlashingSensor loop OKOpen circuit sensorand channel inactiveOpen circuit sensorand channel activeOR Error conditionHi Alarm limitLo Alarm limitLo Alarm setLo Alarm clearedTimeEUROPE (EMEA) Tel: +44 (0)1582 723633 Fax: +44 (0)1582 422283AMERICAS Tel: +1 603 926 0090 Fax: +1 603 926 1899ASIA PACIFIC Tel: +65 487 7887 Fax: +65 487 7997E-mail: enquiry@mtl-inst.com Web site: www.mtl-inst.comOct 2006