8000 Zone2.pdf

Analog Input Modules - 4-20mAGeneralThe 4–20mA AI modules provide digitiseddata and status information from 4–20 mAcurrent loop sensors.HART ® capabilityAI modules “with HART” can obtaininformation from HART instruments ofprotocol revision 5.0 or later. Each channelcan communicate with a single HARTinstrument. HART universal command 3 isused to gather up to 4 dynamic variablesand status from each HART instrument. Thisprovides more process information to thecontrol system from each device. Greateraccuracy can also be achieved byeliminating A/D and D/A errors.In addition, HART pass-through may be usedfor device configuration, calibration andadvanced diagnostics.Input samplingThe AI modules have eight user-channels thatare sampled every 27ms (2/2) or 33ms(2/1).Data formatThe input signal is stored as a 16-bitunsigned value. In this range 0 is equivalentto 0mA and 65,535 is equivalent to 25mA.Any digital HART data is stored in itsoriginal IEEE754 floating point format.FilteringThe Analog Input modules use a first-ordersoftware filter that provides 12dB attenuation atthe Nyquist frequency of the algorithm. Thefilter supports a set of options that can bematched with control algorithm execution rates.Input alarmsFour configurable alarm levels are providedfor each channel—two high and two low(see figure below). When an input valueexceeds an alarm limit a flag is set and theBIM gets a new alarm status.Alarm deadbandThe Alarm Deadband prevents the alarmfrom tripping on and off because of systemnoise. It can be configured for each channeland is always set on the ‘inner’ side of thealarm limit to be, typically, greater than thesystem noise in the plant. If an alarm isactivated, it will remain until the input movesthe full extent of the deadband towards a“safer” value.The Hi-Hi and Lo-Lo alarms support theNAMUR recommendations, i.e. if the alarmlimit is set less than 3.6mA (Lo-Lo), or greaterthan 21.0mA (Hi-Hi), the alarms must beactive for 4 seconds before the alarm is set.The Deadband does not apply to NAMURalarms. If the alarm limits are set at valuesbetween the NAMUR limits, the alarmsfunction normally.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 be reported.Module operating statesNormal/Failsafe modeThe AI modules support failsafe mode asdefined in the earlier I/O module introductorysection. When not in failsafe the moduleadopts Normal mode.Channel Active/InactiveA channel can be made active or inactiveindividually. When a channel is madeinactive inputs will not be processed.InputValueHi Hi NAMUR Alarm set (4s delay)Hi Hi Alarm setHi Alarm setLo Alarm limitMonitoredChannel ValueHi Alarm clearedLo Alarm setDefault/Power-up conditionsThese modules use the following valueswhen they power 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 FilteringDisabled.PassthroughPassthrough messages to HART instrumentsare always allowed.Visual indicatorsChannel “Status” LED (yellow)OnOffFlashingAn error – i.e. a flashing LED – could be asa result of any of the following conditions:a) a loss of HART signal,b) an error in the A/D converter,c) a NAMUR alarm ord) a Hi (-Hi) or Low (-Low) alarm.Hi Hi NAMUR Alarm clearedHi Hi Alarm clearedLo Alarm clearedSensor loop OKOpen circuit sensorand channel inactiveOpen circuit sensorand channel activeOR Error conditionLo Lo Alarm clearedHi Hi Alarm limitHi Alarm limitDeadbandsLo Lo Alarm limitLo Lo Alarm setLo Lo NAMUR Alarm set (4s delay)Lo Lo NAMUR Alarm clearedTimeHART ® is a registered trademark of the HART Communications Foundation.EUROPE (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