    





    CALL ESTA8LISHMENT
    
    Elelo~e two modems can exchange data, an electrical connecoon
    thraugh the phon. system must be establ~shad. Although it may
    assist in call oststHishment, a rr~rJdem ty~cally does not play a
    malor role. A call may be originated manually or au(omalically and
    it may ba answered manually or automatcally.
    
    Manual Calling - Manual calling is pertormed t~y a person who
    dials the number. waits tor an answer, Ihen places the calling
    rnodern into data hansmission mode.
    
    Automatic Calling - Automalic calling i5 typ cally pertormr~d by
    an automatic calling unit [ACU) which generates the appropriate
    dialing pulse or dual-lone sequence required to call the remotr,
    (called) modem. The ACU also has the ability to detect an answer
    tone trom the called modem and place the calling morJem inlo
    data transmission mode.
    
    Manual Answering - Manual answering is pertormed by a person
    who hears tha phona ring, liKs tha receiver. causes the called
    modem to send an answer tone /o the calling mradem, and places
    ttie calied rnodam inlo dala transmission rnode.
    
    Autornabc Answering - Autornatic answering is pertorn~ed by
    a raltett rnor~rn with a data acress arrangement (DM). The
    DM detects a nngmg signal, takos the phone circuit ort-hook
    (COrrespOrKting to litting tha retedver) and ins~uots the called
    modem to commence tha auto-answer sequence. Next the
    called rnodern sands out silenca on the Fr~a, toFowed by an
    answer tone. When this tone is detected by the caHing modern,
    the connectic~ is considerod to hava been established.
    
    The Am79t0 provides assistance lo; automatic answering
    through the RING signal as tollows, Observe the upper right-hand
    portion ol Fi~ure B(a). Assurne that DATA TERMINAL READY
    (DTR) has recentty been asserted to cause exit Irom Ihe initial
    conditions. Note that i' DtR remains OFF, RING is ignored.
    Assurne also that RTS and EIRTS are OFF and that the mode
    control lir~es (MCO-MC4) sobct a norrnal rr~odem r,onfiguration,
    r~ot a loopback rnr~te. Automabc answering is inibated by recsipt
    ot a LOW tsvol at the RING input, causing errtranco to the auto-
    answer sequence dapicied in hgure B(c).
    
    The Arn7910 outpu~ silence (0.0 volts) at rts TRANSMITTED
    CARRIER (TC) output lor a bmo, tS'L, tdiowed by t,ho answer
    tono f® a time, tAT. The CARRIER DETECT (CD) pn is clemped
    OFF and ~e RECEIVED DATA (RD) signal is therelore clemper.t
    to a MARK (HIGH) during the autr~answer 6equence. Upon
    Compk~tK'n ot the answer tone, Z~ k releasod. 11 tho mode lines
    (NICaMC4) select a 202 or V.23 rnode, the transmit filters are set
    to tho forward channel anr.t the recsive hlters are set to the back
    channel duAng the auto answer sequence.
    
    At the end d the autr~answer sequence, rdum is made to poinl A
    in thc loop d tho upper right-hano portion ot Figura 8(a). Note tha
    s~ the answer llag has been set, tho auto-answer sequence
    cennot be entered agein unless ~i is first tumed OFF, ~en ON.
    At this point the phone Binc coru#ction has been established and
    dala transmission or recoption may begin.
    
    Tha RING input may be activated trorn a conditior~ed DAA Ring
    Indkator ou~put lor outomatic answeri~ it may be activated by
    a swilch tor manual answering. Tying RtNG HIGH will disable the
    auto-answer lunction ot the Am7910.
    
    DATA TRANSIlISSilON
    
    Foll Oupbs
    
    Folbwing call edablishrncnt, lull dupbx data transmission cen be
    ~ I startad by ei~ar the caBled or Caning rr~m. In other words, it the
    O |cor~c~ has b~n esta~ishad and ~o rnodem is hoping
    B~rough pdnt ~ h h~gure 8(a), it no br~r rnaRars whid~ is mO
    
    calied and which Is tbe calEng modem. Dala transmission Is
    initiated by assening REOUEST TO SEND (RTS), At th~s bmo tt~
    TRANSMtTTED DATA (TO) inpu1 will be releasod and a mc~d-

    





    ulated carrier can appaar at tho TRANSMITTED CARRIER (TC)
    outpuh Following ~ delay. IRCON. CLEAR TO SEND (CTS) wii'
    turn ON. A! this tima. Cata may be transmitied through tho TO
    input It is a common protocol tor the user to alvvays present a
    MARK al the TD input before RTS is asserned and during the
    IRCON delay.
    
    Dala transmissioncontinues unbl RTS is tumed OFF Following a
    shon delay, tRCO`F'CTS tums OFF. As soon as RTS r~s OFF,
    the TD inpul i5 ignored ar~d the TC output i5 set to 0.0 volts
    (siler~ce). Aher CTS larns OFF, the state machine relums lo point
    A in F,~ure B(a).
    
    Halt Duplex
    
    When a hal1 doplex mode is selecied (202 or V.23l, dala trans-
    mission can be either on the main channel at 1200!600 baud or on
    Ihe back char~nel at 5175 baud. In normat halt dup~ex operation a
    single rnodem is either transmittrl9 on the main and receiving on
    the back channel or vico vr~rsa In the Am7910 control of tho
    transminer and receiver filters to tho proper channel is perlormed
    by RTS. When RTS is assened, the Iransminer hlters and syn-
    thesizer are sel to transmit on the main channet; the recedver
    lilters are set lo receive on the back channel. Therefore,
    whenever RTS is on, BRTS shouid not be asserted sir~ce the
    transmitter cenhol t~t usad for the back channel. When RTS is
    OFF and a haR doplex rnode is selected, the transrnittet fitters and
    synlhesizer are sel to the back channel the recelver itilters are set
    to the main charmel. It h~ anrJ ~iT3õ are asserted sirnuhane
    
    ously RTS wilt take precedence. Hov~ever, il BRTS is asserted
    bebre RTS and the back channed data transrnission sequence
    has been enlered (Fboure Et(b)). RTS will be ignored until BRTS is
    turned OFF.
    
    The state machine sequences ~r rnain and back diann61 trans-
    mission diHer slighHy and are rJepicied in Figure B" Assume the
    state machine is idling through point A in Fbure 8(a).
    
    hlain Channel
    
    This transmission seqwnc. is entered K a 202 or V.23 mode i5
    selectad and RTS Is asseried. Since the recelver is no~r lorced to
    the back channel, the RECEIVED OATA {RD) signal is dampad
    to a MARK; and the CARRIER DETECT s~nal is damped OFF.
    The TRANSMITTED DATA input (TD) is raleased antt a carrier
    appears at Ihe TRANSMITTED CARRIER output whkh lollows
    the MARKISPACE applied to TD. RTS turning ON initiatos a
    delay, tRCoN. at the end of which the CLEAR TO SEND (CTS)
    output goss LOW. When CTS rgees LOW data may be transrnit-
    ted through inpul TD. Dala transmission contnues unbl RTS is
    turned OFF. At this time soveral evenis are indbsted. First a delay.
    t''COFF, iS initiated at the and ol which CTS turns OFF. The TD
    inpul is ignored as soon as ~ 9Oes OFF. 1' a 202 tnode is
    selected, a soft turn o~l tone appaars at ~ TC output lor a time.
    tSTo. lollowed by slilence (0.0 votts) For both 202 and V 23
    rnodes a squekh peried, tSQ, is Wtiated when RTS r~s OFF.
    Durinrg this perrod the CD outpul is clemped OFF. torcliryt the RD
    outpult to a MARK c~ndition. The squelch perr,od begins as soon
    
    ús RTS goes OFF and thu5 owrtap5 both tRCOFF and ISTO- At
    the and o! the squelch peried, Iha stala machine retums to the idle
    bop at paint A in Figura 8(a).
    
    Tho raasons lor squskh and soK-turnol1 are as blbv~s:
    
    Soh Turn-OtI: Whan RTS is turned OFF st tt. emd oi a tnessago,
    transients occur which rnsg cause spunous sp*cc 5rgnals to bo
    retedvad at a remotei rsodem. Dunng soh turn~on the rnodem
    trans~nits a wh cairrier hequency lor a parliod, tSTo. alter ~S ie
    
    lurned OFF. Th~s resulls in a steady MARK on the RECLIVED
    DAtA (RD) l~ne ot the remote mr~tem
    
    Sr~uelch The iar al recckver musl be turned OFF sher RTS is
    OFF, unt~l Ihe slart ol ca~r~er detecl, so tbaN'ne Irans~enis are not
    dernodulated The proceSs ol disableng Ihe rece~ver aher RTS ~s
    turned OFF ~s called squelch,ng.
    
    6tack Cnannel
    
    This transmission sequence, shown in F'gure Bib). is entered ii a
    202 or V.23 rnode ~s selected, RTS rs OFF, and BRTS es asserted
    Tne BACK CARRIER DETECT (BCD) Oulpul is lorced OFF and

    





    the BACK RECEIVED DATA IBRD) oulpul is clamped to a
    MARK. The BACK TRANSMITTED DATA inpul (BTD) is úe-
    leased and a carrier appears at the TC outpul vvhich tollows the
    MARt<'SPACE eppleed to BTD. Tuming ON BRTS ~nihales a
    delay, tBRCoN at the end o'vvhich ~he BACK CLEAR TO SEND
    (B~) output goes LOW When ~S gnes LOW data may be
    Iransmitted through input BTD. Data Iransmission continues unt~l
    BRTS is turned OFF. The inpul BTD is immect,ateiy igno~ed and
    the TC output is silenced (set to 0.0 volts) Follovving a short
    delay, IgRcoFf. the outpul BGTS goes OFF. The signals BCD
    and BRD are released and the state machine retums to idle at
    paint A ot F'gure 8(a)
    
    In 202 back channel mode, BTD shouid t~e bed HIGH. Then
    EIRTS controls the ON OFF keying modolation When BRTS is
    LOW, 387Hz appears at the TC oulput, when BRTS is HIGH, 0
    votts appaars at TC.
    
    DAtA RECEPTION
    
    Data receplion is conlrolled by state machir~e 2 and depicted in
    hgure 9. At power on the machine enters initial conditions and
    remains there until UtR is asserted. It then t~ops until either
    CARRiER DEfECT(CD) or BACK CARRiER DETECT (BCD)
    occurs.
    
    Full Duplex
    In tull duplex data receplion, CARRIER DETECT may appear
    
    út any time siter the phor~e conner,~;on has been established.
    fleception is independent d transmission. v,nwn the rec~iver
    rdetecLs a valid carrier tor at bast a bme, tCPON. tbe output CD is
    b~rnod ON, the RECEIVE[) DATA (RD) output is released. ar~d
    velid dala cen oe oblained al RD. Data is recsivad unGI the
    recsi'ver de ects bss ol carner lor at tea51 e time. ICDoFF. At Ihis
    (time the CD oulput is turned OFF and RD is dan~ped lo a MAR K.
    The state machir~e relurr~s to the idle bop al point E.
    
                               1
    
    Hall Duplex
    
    As disoussed in d~e data Iransmission section above, when a
    hall duplex mode has been selecled, Ihe signal RTS conlrols
    whelher the main channel is transmitan9 or receiving Tne back
    channel can ooly do the opposite Irom the main. 11 RTS is OFF,
    then CARRIER DETECT may be asserted and the data recep-
    lion sequence is idenncal to that d~scussod above lor tull dup-
    lex reception. As lor~g as RTS remains OFF, BACK CARflIER
    DETEC:T ~'ill never be asserted. 11 RTS is ON, then CARRIER
    DElECT wili never be asserled. Inslead the receiver will look
    for a valid carner in the back channel frequency band. It a vatid
    carrier exisis lor at least a time, tgCDor4. the output BACK
    CARRIEn DETECT (BCD) is turned ON, the BACK RECEIVED
    DATA (BRD) outpul is released and valid dala cen be obtained
    at BRD. Dala is received unlil the receiver delecls loss o'back
    channel receivad signal lor al leasl 0me, tBCDOFF. At this time
    the BCD outpul is turned OFF. Dala outpul, BRD, is clarnped lo
    a MARK il a V.23 mode is selected. For 202 back channel
    m ode, BCD represents the reccived data. The BRD Outpul
    cen be ignored. The stale machine relurns to the idle loop at
    poinl E.
    
    LOOPaACK
    
    Ten modes exist to allow bolh analog and digilal loopback for
    each modem specification met by the Arn7910 When a loopback
    mode is selected, the signal processing (fillers, etc.) lor both the
    transmitter and recelver is set to process the same channel or
    frequency band. This allows the analog output, TRANSMITTED
    CARRIER, and the analog input, RECEIVED CARRIER. to be
    r;onnected lor local analog loopback. Aitematively the digital data
    signals, TD and RD or BTD and BnD. con be connected exter-
    nally, allowing a remole rnodem to test the Jocal mrt~m ~h hc
    digital dala signals looped back.
    
    When a loopback mode is selecied, Iha state machine sequences
    are allered sligh~t. Firsl, auto-answer is disabled. Serx~d. i1 a
    hatf duplex toopback mode is selecIed (202 or V.23), t~e bcal
    CARRIER DETECT/BCD is nol lorced OFF ~rhen RTsitaRTs is
    
    "ce~rt~
    
    The 202 and V.23 main bopback modes anow use in a 4-wire

    





    configuration at 1200 bps
    
    e e~
    
    _ ~"C
    
    ~ ~ ~ Äc - ~
    
    _, ~
    
                                             1              -s
    
    