Fiberplex FOM-1090 Uživatelský manuál stáhnout pdf (Strana 14)

DIPSwitchSettings

CompatibilitywithMIL‐STDcircuits

ThedriversandreceiversforV.11signalsarecompatiblewiththeMIL‐STDbalancedspecificationsandthe

V.10interfaceissimilartoMIL‐STDunbalanced.MIL‐STD‐100signalsusethesamenegativeMARKcondition

asTIAcircuits,sothereisnoneedtoinverttheTDandRDsignals.MIL

‐STD‐188‐114Aisset‐upforusercontrol

oftheMARKlevel,sotheneedfordatainversionattheFOMwillneedtobemadeonacase‐by‐casebasis.

IftheMIL‐STDinterfaceusesanyunbalancedsignals,suchasMIL‐STD‐188C(notethatthis

standardusesa

positiveMARK),thenprovisionswillneedtobemadetoexternallybiasonesideofthereceiversontheFOM

tousethemsingle‐ended.NotethattheMILstandardsareonlyanelectricalspecificationanddonotspecifya

pinoutorconnectortype,soacustom

cablewillberequiredinmanycases.

Asynchronous,Isochronous,orSynchronousoperation

TheFOMistransparenttodataandclockingformats,sotherearenoswitchsettingsfordistinguishingthe

differentmodesofoperation.WhenapairofFOM‐1090unitsisusedasmodemlinkbetweentwoDTEs,allof

theinputsignalsaretransferredtothecrossed‐overcorrespondingoutputs(i.e.

‐TTfromtheDTEisprovided

asRToutoftheFOM‐1090)attheoppositeend.WhentheFOMisusedinSendTimingapplications,certain

switchoptionsmaybeusedtoeliminateclockingissuesthatmayarise.Thoseoptionsareexplainedbelow.

SendDataRegenerationwhenusingSendTimingfromtheDCE

ThetypicalSendTimingset‐uphastheDCEsupplyingallclocks.TheSTsignalisgeneratedattheDCEandthen

carriedtotheremoteDTE.Inreturn,theremoteDTEthenclockstheSendDataoutofitselfontherisingedge

ofthesuppliedSTsignal.TheSend

DataiscarriedbacktotheDCEwhereitisclockedin,samplingthedatabit

onthefallingedgeofthegeneratedSTclock.Alignmentproblemsariseduetopropagationdelaywhencertain

combinationsofdatarateandcabledistance(bothcopperandfiber)resultintheSendData

transitions

occurringnearthefallingedgeoftheSTsignalattheDCE.

Asanexample,usingaroughnumberof4nsdelaypermeterofcable,25metersofcablewitha2.5mHzclock

willcausea180degreeshiftintheST‐SDrelationshipattheDCE

interface.(Thereisactually50metersof

propagationdelaysincetheclocktravels25metersinonedirectionandthedatatravels25metersinthe

other).ThisiswithouttakingintoaccountthedelaysofthelinedriversandreceiversintheDTE.

TheFOM‐1091regenerationoptionsmake

upforthisintwoways.ThefirstistocorrectforanySD‐ST

misalignmentduetopropagationdelayfromtheFOM‐1091totheDCEbydelayingtheSDsignaloutofthe

FOMbyonehalfofaSTclockcycle.ThesecondistheFOMhasthe

abilitytoretimetheincomingSDdata

internallywiththeincomingSTsignal,whichremovesanysamplingjitterfromtheSDsignalaswellas

correctingforpropagationdelays.

WhilethefallingedgeoftheSTsignalfromtheDCEisideallylocatedmid‐bitoftheSDsignalcoming

intothe

DCE,itisnotnecessaryforittobemid‐bit.Infact,it’susuallynotmid‐bitduetodelays.Thisisoften

misunderstood.Theactualrequirementisthattheset‐uptimefortheregisterthattheSDsignalisbeing

loadedintobemetandthis

isusuallyafractionoftheavailablebittime.Theonlytimethereisaproblemis

whenthefallingedgeoftheSTistooclosetotheSDtransitionsattheDCEinterfaceandthispreventstheset‐

uptimefrombeingmet.IftheSTfallingedge

isfarenoughawayfromtheSDtransitionedges(whenthenew

SDdatabithasmettheset‐uptime),theDCEwillstillclockthedatareliablyeventhoughitisn’tmid‐bit.Thisis

whymanySTtimingset‐upswillworkwithnoregenerationrequiredatall.

Whentheedgesaretoocloseat

theDCEtheFOMwillneedtoretimethedataontheoppositeedgeoftheSTsignalbysett ing switch 1.8to

ON.Thiswillallowthatwhenthepropagationdelaysaretakenintoaccounttheedgeshaveskewedenoughto

meet

theset‐uptimeattheDCE.

Notethatininstallationswherethe dataratemaybech angedorthecablelengthsmaychangeduetopatch

panelroutingofequipment,it’sentirelypossiblethatacombinationofswitchsettingsthatworksinone

scenariowillnotworkinanother.The

onlysolutiontothesesituationsistoinsertmoredelayinoneormore

oftheconfigurationsbyaddingtocablelengthuntilallofthescenarioswillworkwiththesameswitch

settings.

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Fiberplex FOM-1090 Uživatelský manuál Strana 14

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