    





    TRANSB4ITTED CAFtRIER (TC)
    
    This anatog output is lhe modulated carrier to be cond~tioned
    and sen'over the phona line.
    
    RECEIYED CARRIER (RC)
    
    This inpul is the analog signal reccived Irom the phone line The
    modem extracls the information conla~ned in Ihis modulaled
    carrier and convens il inlo a sedal dala stream lor preseniat~on
    at Ihe RECEIVED DATA (BACK RECEIVED DATA) oulput
    
    RING
    
    This input signai permits auto-answer capability by respond~ng
    to a ringing signal trom a dala access arrangement. 11 a finging
    
    signal is detecied (RING LOW) and DTR is LOW the modem
    beg~ns a sequence to generale an answer lone al the TC output
    
    XTAL1 XTAL2
    
    Master hming ol the modem is prov~ded by ei~her a crystal con-
    necied to these two inputs or an exiernal ciock inserted inlo
    XTAtl. The vatue ol the cryslal or the erlernai ciock trequency
    musl be 2 4576MHz -.01~.
    
    Vcc
    + 5 volt power supply. ~ ~ S.~)
    
    vBe
    - 5 voll power supply. ( ~ 5.r)
    
    TABLE 1.
    
      ~tC4~4C314C2   IIIC1       MCo   . . ~
    
       0   0  0   0  0Bell 103 Originala 300bps ~ull duplex
       0   0  0   0  18ell 103 Answer 300bps tull duplex
       0   0  0   1  0Belt 2021200bps hait duplex
       0   0  0   1  1Bell 202 wilh equalizer 1200bps halt dupleK
       O   O  1   O  OCCITT V 21 Orig 300bps tull duplex
       0   0  1   0  1CCITT V.21 Ans 300bps tull duplex
       O   O  1   1  OCCIT1 V23 Mode 21200bps halt duplex
       O   O  1   1  1CCITT V.23 Mode 2 with equalizer 1200bps haH duplex
       b   1  0   0  0CCITT V.23 Mode 1.600bps hait duplex
    
    O 1 O 1 1
    
       0   1  t   0 t           Reserved
       O   1  1   1 O
       O   1  1   1 1
                  _
       1   O  O   O OBell 103 Orig loopback
       1   0  0   0 1Bell 103 Ans loopback
       1   0  0   1 0Bell 202 Main bopback
       1   0  0   1 1Bell 202 wilh equalizer loopback
       1   0  1   0 0CCITT V.21 Orig looPback
       1   0  1   0 1CCITT V.21 Ans loopack
       1   0  1   1 0CCITT V23 Mr~de 2 main toopback
       1   0  1   1 1CCITT V.23 Mode 2 with equalizer loopback
       1   1  0   0 0CCITT V.23 Mode 1 main loopback
       1   1  0   0 1CCITT V 23 Back loopback
    
       1  1   1   OO I neserved
       1  1   1   1    0 1
       1  1   1   1      1
    
    Flourc 3. Loopback Contiguratlons
    
    ~L
    
        ~7~.0
    ~ Ä-
    ~..
    
        TE                         ~ú~arm ~ ~ 1 | ~L  ~E
    

    





    c6r~r~ll _ Rs
    
    O1Z~,
    
    DGND
    
    Dlpila s~gnal g oJnd p,n
    
    AG ND
    
    Analog s~g~al g~ound pin (lor TRANSrAlTTED CARRIER and
    RECEIVED CARRIER)
    
    CAP1. CAP2
    
    Connecl~on po,nts ol eK'ernal capac~tor/res~stor required lor
    prope~ operat,on ot c,n chip analog Io d~g~tai cor~verter
    
    Recommended vatues are C = 2000pF -10øo,
              R= 10011 ~ 10~o
    
    RESET
    
    Tnis input s~gna! is lor a reset circui'which operates en either ol
    two modes It automatecally resets when power is appl,ed to the
    device. or it cen be acl~vated by appl,cat~on ol an externa' actrve
    low TTL pulse
    
    THEORY OF OPERADOH
    
    The Am7910 MODEM consists o1 Ihree main sections shown in
    1he bioel' d~agram ot F.gure 2 - Transminer' fleceiver. and
    Inler/ace Control.
    
    TRANS.titTER (Nlodulator)
    
    The Iransminer. shown in Figure ~ receiveS oinary digi(al data
    from a source such as A UART andconverts the dala lo an analog
    signat using Irequency shih keying (FSK) modulation. This
    analog signat is apptied lo the phone tine Ihrough a DM or
    acouslic coupler. FSK is a modulation lechnique which encodes
    one bil per baud. A bgiC one applied to Ihe TRANSMITTED
    DATA (TD) inpul causes a sine wave at a given trequency to
    
    ú"ear 81 Ihe analog TRANSMITTED CARRIER (TC) output. A
    bgic tero applied to input TD causes a sine wave ot a ditlerenl
    trequer~cy to appear at the TC r~uput. As lhe dala al the TD inpul
    ~wilches between togical one and 1ero the TC output switrhes
    between the two hequencios. In the Am7910 Ihis swilching be-
    lween Irequencies is phase conlinuGus. The trequencies
    themselves are d~gitally synthesized sine tundions.
    
    The bequencies for aach rnDoarn configurabon available in the
    Arn7910 are lisled in Table 3a.
    
                                                  ~TL$-    ~1
    
     n~ O~A To uARr
    aeo~ncoouP"e
    
    o'~.
    
    The p~ocess ol sw~tch~ng bel~veen ~wo trequenc~es as in FSK
    genc~rales eneigy at many more 8equenc~es Ihan a.e Iwo usod In
    Ihe rnodulat,on Al; the aansmined inlormat~on can be recoveled
    trorn a trequency band B Hz w~de where E] ~s the bit rale or
    matimum lelc o! change ol Ihe digital dala at Ihe TD input Th~s
    bar d is centered abr~ut a trequency. IC
    
    where ic = tt ~ (i2 -- tl)/2
    (tl = lower ol Iv~o FSK tiequenc~es)
    (~2 = htgher ol Iwo FSK trequencies)
    
    In adda~on to this prirr~ary informalion band. there e ~ist side bands
    containing redundant inlorrnation. Il is desirable to anrnuate
    these bands tor two reasons:
    
  1. The phone companies have specificalions on the amount o'
    energy allowed en cartain trequency bands on Ihe l~ne.
    
  2 It Iwo independent information channels are present simul.
    taneously on Ihe hne (e.g. 300 bps lull duplex or 1200 bps
    hall duplell with back) the ~edundant transmitter components
    may tall in the trequency band ot the locat recedver channel

    





    and inlertere with deleclion. In the Am79tO these redundant
    and undesirable components are attenuated by digital band-
    pass hlters.
    
    Following Ihe digital bandpass tilters Ihe tiltered FSK signal is
    converted lo an analog signal by an on-chip DAC operaling at a
    high sample rate. This analog FSK signal is finally smoothed by a
    simple on~chip analog low pass 1iller.
    
    RECEIVER (Demodulelor)
    
    A simplilied block diegram ot the Am7910 FSK receiver Is shown
    in Figure 5 Dala transmined trom a remote site rnodem over the
    phone line is an FSK.modaloled analog carrier. This carrier is
    applied lo the RECEIVED CARRIER (RC) pin via a DM or
    acouslic coupter. The tirst stage ot the demodulalor is a simpie
    on-chip analog bw pass anti-alias filter. The oulput ot this is
    oonverted inlo digitai torm and tiltered by d~gilal bandpass tdilers
    to improve the signal to nOise ratio and roject other independent
    channel trequencies associated with the phone line in Ihe case ot
    tuli duRlex configuralion. The bandpass fillered output rs digilally
    dernodulated to recuver the binary dala. A carrter delecl signal is
    also digitally extrac1ed trom U`e received line carrier lo indicate
    velid dala.
    
    Figure 4. Trenemmer elock DbEir~m
    
    r~c.
    
    nrL">E~ ~}c ~Mr
                                        ACOUsnC
    
    o,
    
    Pigure S. Resetver Block Dbgram
               ~ CÄ~ I
               
    