Specifications

6
Troubleshooting
106
OnlytheswitchingportionoftheSCMsactinActive‐Activemode.TheShMCsareneverin
dualactivestateunlessthere'sac atas trophicbusfaultthatdoesnotallowany
communicationbetweentheSCMs.TheShMCsarbitratetobecomeactiveorstandbyassoon
astheyinitialize.Ifthey
everbecomedual‐active,theyareconstantlyprobingeachother's
statesandwouldquicklynoticethestatemismatchanddore‐arbitration.
TheIPMBbusesAandBareredundantandnosynchronizationoccursbetweenthetwo
buses.TheShMCsthatareconnectedtomultipleIPMBsfollowaround‐robinarbitration
protocol.Requestsaretransmittedinround‐robinmanneronthetwoIPMBs.
IfanShMCfailstoreceivearesponsemessagefromanyslot(forexampleslot#3),it
retransmitstherequestmessageuptothreetimesat250msecintervals.The250msec
intervalissuggestedbyPICMGtoallowformanagedFRUcommunication,thusitcantakeup
to750msecstotimeout.
IfaclientcannotaccessIPMCrightaway(duetoBUSbusy),theIPMCkeepstryingtoarbitrate
onthebusuntilitgetsaccessandcantransmitthebytes.Otherfunctionsarenotpendingin
thisstate.Amessagingqueueismaintainedandmessagesthatneedtogettransmittedare
putinthisqueue.Thelow‐levelI
2
Cdriverpicksthemupfromthisqueuefortransmission.All
otherIPMCfunctionscarryonwhilethisishappening.Thetasksofhot‐swap,sensor,FRU
data,etc.areallhandledinaseparatethread.
IPMB bus redundancy
Thesamemessageisnottransmittedsimultaneouslyonthesamebus.Messagesare
transmittedinaround‐robinmanner.Soifmessage1istransmittedonbusA,message2will
beonbusB,message3willbeonbusAandsoon.TheIPMCkeepstrackofwhatbuswas
usedforthelasttransactionandusestheotherbusforthenext.Ifabusisisolatedoranerror
isdetected(reportedbytheIPMB‐0sensor)thenalltransactionshappenontheother
availablebus.
IPMBIsolationcausesarenotedinthePICMGspecitselfwhereitdefinestheIPMB‐0sensor
eventstates.Themostcommoncausesareasfollow:
1h=UnabletodriveclockHI
2h=UnabletodrivedataHI
3h=UnabletodriveclockLO
4h=UnabletodrivedataLO
5h=Clocklowtimeout
6h=Undertest(theIPMControllerisattemptingtodetermineifitiscausingabushang)
7h=UndiagnosedCommunicationsFailure