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ExarCorporation48720KatoRoad,FremontCA,9453850-668-707www.exar.comSP509_00_06080
ThefourthtypeofdriversareV.35differential
drivers.Thereareonlythreeavailableonthe
SP509fordataandclock(TxD,TxCE,and
TxCinDCEmode).Thesedriversarecurrent
sources that drive loop current through
a differential pair resulting in a 550mV
differential voltage at the receiver. These
drivers also incorporate fixed termination
networksforeachdriverinordertosetthe
V
OHandVOLdependingonloadconditions.
Thisterminationnetworkisbasicallya“Y”
configuration consisting of two 51 resistors
connected in series and a 124 resistor
connected between the two 50 resistors
andaV35TGNDoutput.Eachofthethree
drivers and its associated termination will
haveitsownV35TGNDoutputforgrounding
convenience. Filtering can be done on
thesepinstoreducecommonmodenoise
transmitted over the transmission line by
connectingacapacitortoground.
Thedriversalsohaveseparateenablepins
which simplifies half-duplex configurations
forsomeapplications,especiallyprogram-
mableDTE/DCE.Theenablepinswilleither
enableordisabletheoutputofthedrivers
according to the appropriate active logic
illustrated on Figure 47. The enable pins
haveinternalpull-upandpull-downdevices,
dependingontheactivepolarityofthere-
ceiver,thatenablethedriveruponpower-on
if the enable lines are left floating. During
disabledconditions,thedriveroutputswill
beatahighimpedance3-state.
ThedriverinputsarebothTTLandCMOS
compatible.Alldriverinputshaveaninternal
pull-upresistorsothattheoutputwillbeat
a defined state at logic LOW (“0”). Unused
driver inputs can be left floating. The inter-
nalpull-upresistorvalueisapproximately
500k.
Receivers
The SP509 has eight enhanced inde-
pendent receivers. Control for the mode
selection is done via a three-bit con-
trol word that is the same as the driver
control word. Therefore, the modes for
thedriversandreceiversareidenticalinthe
application.
Like the drivers, the receivers are prear-
ranged for the specific requirements of
the synchronous serial interface. As the
operatingmodeofthereceiversischanged,
the electrical characteristics will change
to support the required serial interface
protocols of the receivers.Table 2 shows
themodeofeachreceiverinthedifferent
interface modes that can be selected.
Therearetwobasictypesofreceivercir-
cuits—ITU-T-V.28(RS-232)andITU-T-V.,
(RS-422).
TheRS-232(V.28)receiverissingle-ended
andacceptsRS-232signalsfromtheRS-
232 driver. The RS-232 receiver has an
operatinginputvoltagerangeof+5Vand
canreceivesignalsdownsto+3V.Theinput
sensitivitycomplieswithRS-232andV.28
at+3V. The input impedance is 3k to 7k
inaccordancetoRS-232andV.28.The
receiveroutputproducesaTTL/CMOSsignal
witha+2.4Vminimumforalogic“”anda
+0.4Vmaximumforalogic“0”.TheRS-232
(V.28)protocolusesthesereceiversforall
data,clockandcontrolsignals.Theyarealso
usedinV.35modeforcontrollinesignals:
CTS,DSR,LL,andRL.TheRS-232receiv-
erscanoperateover20kbps.
Thesecondtypeofreceiverisadifferential
type that can be configured internally to
support ITU-T-V.0 and CCITT-V.35
depending on its input conditions. This
receiver has a typical input impedance
of 10k and a differential threshold of
less than +200mV, which complies with
the ITU-T-V.11 (RS-422) specifications.
V. receivers are used in RS-449/V.36,
EIA-530,EIA-530AandX.2asCategoryI
signalsforreceivingclock,data,andsome
controllinesignalsnotcoveredbyCategory
IIV.0circuits.ThedifferentialV.trans-
ceiverhasimprovedarchitecturethatallows
over40Mbpstransmissionrates.
Receiversdedicatedfordataandclock(RxD,
RxC,TxC)incorporateinternaltermination
forV..Theterminationresistoristypically
120connectedbetweentheAandBinputs.
Theterminationisessentialforminimizing
crosstalk and signal reflection over the
transmission line .The minimum value is
guaranteed to exceed 100, thus complying
with the V.11 and RS-422 specifications.