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ExarCorporation48720KatoRoad,FremontCA,94538•50-668-707•www.exar.comSP508E_00_07242

ThefourthtypeofdriversareV.35differential

drivers.Thereareonlythreeavailableonthe

SP508Efordataandclock(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

Thisterminationnetworkisbasicallya“Y”

configuration consisting of two 51Ω resistors

connected in series and a 124Ω resistor

connected between the two 50Ω 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 45. 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

500kΩ.

Receivers

The SP508E 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 3kΩ to 7kΩ

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 10kΩ 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

over20Mbpstransmissionrates.

Receiversdedicatedfordataandclock(RxD,

RxC,TxC)incorporateinternaltermination

forV..Theterminationresistoristypically

120ΩconnectedbetweentheAandBinputs.

Theterminationisessentialforminimizing

crosstalk and signal reflection over the

transmission line .The minimum value is

guaranteed to exceed 100Ω, thus complying

with the V.11 and RS-422 specifications.