Honeywell XNX - B�1 Modbus And The Xnx Transmitter

XNX Universal Transmitter
Appendix B - Modbus Protocol
184
B.1 Modbus and the XNX transmitter
TheXNX
®
gassensormaybettedwiththeoptionalModbus
®

interfacecard(P/NXNX-MB)�Authoritativeinformationon
theModbusprotocolcanbefoundatwww�modbus�org�
TheXNXsupportsModbus/RTUoveranRS-485physical
layer�Theinterfaceisisolatedandincludesaswitchable120
Ohmterminationresistor�Baudratesfrom1200to38,400are
supportedwith19,200asthedefault(8databits,evenparity,1
stopbit)�
MostoftheoperationsthatarepossiblewiththeHART
®
and
localuserinterfacescanalsobeperformedusingtheModbus
interface�Thisincludestest,calibrationandconguration
operations�ThisappendixdescribesonlyhowtomonitorXNX
statususingModbus�
Thezerocalibrationprocedureshouldbeperformedpriorto
thespancalibration�ThecalibrationprocedureisinSection
3�2�1�
SomeoftherelevantModbusholdingregistersarelistedinthe
followingtable�Inmostinstallations,theXNXtransmitterreads
onlytherstveregisters(fourdata)�Theassignmentoftherst
eightregisters(orsixdata)isidenticaltotheHoneywellAnalytics
XCDgassensor�
BuildinganeffectiveModbusautomaticgasdetectionsystem
requirescheckingforfaults(usingiFaultWarnNumberor
iAlmFltLev)andcheckingiMonitoringStatetoconrmthatthe
XNXisnotinhibitedorincalibration�Thepseudocodeexample
inFigure247suggestscomputationthatshouldbemadein
externalautomationequipment�
SeeSection2�3�4forinformationoninstallingtheoptional
Modbushardware�SeeSection2�5�1forinformationon
settingtheModbusbaudrateandaddressusingthelocal
userinterface�SeeSectionA�1�1forinformationonsettingthe
ModbusparametersusingtheHARTinterface�
if(
((fCurrentConc < TLV) or (iAlmFltLev & 3 == 0))
// low concentration
and
((iFaultWarnNumber < 1000) or (iAlmFltLev & 64 == 0))
// no fault
and
((iMonitoringState == 1) or (iMonitoringState == 7))
// not inhibited
and
(
(Transport layer SW indicates good Comm.)
or
(iHeartBeat changes every 5 seconds)
) // Modbus link healthy
) Then the area is safe.
Figure 247. Modbus Pseudo Code Example
Modbus connections are shown in the ollowing gure.