Single-Chip Modem Provides Low-Cost Data Communication simple mod

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Single-Chip Modem Provides Low-Cost Data Communication simple mod

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Single-Chip Modem Provides Low-Cost Data Communication
Single-Chip Modem Provides Low-Cost Data Communication
simple modem provides inexpensive link your compatible computer with remote system modem which transmits data asynchronously baud easy build features both autodial autoanswer Using simple design build inexpensive modem your compatible computer Based single chip that transforms digital computer data into analog frequencies modem lets transmit data asynchronously telephone lines baud circuit also uses DTMF (dual-tone multiple-frequency) tone-dialer chip (data-access arrangement) telephone-line interface UART that handles autoanswer autodial functions Configured operate your PC's address space this modem circuit uses equality detector 3to-8-line decoder decode address lines strobes strobes modem design based MM74HC942 modem chip Figure illustrates chip's architecture eliminating external amps on-chip line driver 2-to4-wire hybrid simplify task interfacing telephone line line driver drives 600X line through external 600X terminating resistor thus providing nominal impedance match between modem phone line When programmable level-adjust resistor connected between line driver transmits levels approaching access driver externally through pins (pins input provides gain
National Semiconductor Application Note Wesley July 1986
about input allows achieve adjustable gain reaching series resistor gain approximately Because circuit communicates 2-wire telephone lines both incoming outgoing signals enter HC942 on-chip line hybrid rejects outgoing signal while passing incoming signal demodulator Under most phone-line conditions hybrid provides 20-dB transmit-carrier rejection receive filter demodulator section reduces outgoing signal negligible levels LOOK MM74HC942 MODEM CHIP MM74HC942 modem lets simple support circuitry requires relatively little circuit-board area consumes little power Although chip functionally similar others market includes adjustable line driver programmable carrier-detection circuitry operates from supplies chip's analog loopback (ALB) function power-down mode provide self-testing power conservation function holding input high which causes modulator shunt output input demodulator providing simple diagnostic self-test power-conservation feature makes HC942 power down when both inputs high chip typically draws less than power-down mode certain applications might have battery backup power Either input bring chip power-down mode HC942 bus-like configuration Because chip's inputs outputs offer 3state levels either input serve chip select when them MODULATOR CONVERTS DATA modulator section modem frequency synthesizer generates clock frequencies sine-wave synthesizer During transmission digital data from UART enters input HC942 which converts incoming ones zeros mark space frequencies respectively Originate Answer input sets these frequencies into upper lower transmit bands (Figure
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FIGURE MM74HC942's 2-to-4-wire hybrid line driver simple circuitry interface your modem telephone line
FIGURE HC942 modem chip converts incoming ones zeros mark space frequencies when digital data enters chip's input
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Printed previously C1995 National Semiconductor Corporation 8691 RRD-B30M105 Printed
high signal lets originate call originate mode HC942 transmits lower band receives upper band answer mode chip transmits upper band receives lower band chip's sine-wave synthesizer which essentially converter decodes output frequency synthesizer producing output that's compatible with Bell standard Table shows frequency assignments both modes operation Using 9-pole switched capacitor demodulator passes incoming tones same time that virtually eliminates locally transmitted signals demodulator's receive filter then transmits output frequency discriminator which detectors sense mark space frequencies converting each frequency logical zero receive filter also passes output carrier detector which then compares output filter with externally adjustable voltage leave this floating connect ground voltage threshold nominally When carrier exceeds upper threshold carrier detector activates demodulator after preset timing delay forcing output Timing capacitor sets carrier-detect delay When carrier level drops below lower threshold point demodulator returns disabled state TABLE Bell Tone Allocation Data Space Mark Originate Mode Transmit 1070 1270 Receive 2025 2225 Answer Mode Transmit 2025 2225 Receive 1070 1270
determined time lower three address lines through access 8250's internal registers addressing UART directly 8250 HC942 provide serial communications link between host remote computers During transmission UART converts parallel data from serial data HC942 HC942 receives this data input transmits output output During data reception HC942 receives incoming tones RXA1 input demodulates tones sends serial data through output 8250 Because HC942 doesn't require external line driver hybrid modem circuit requires only passive components Resistor sets line driver's transmit level (Table same time that adjusts carrier-detect timing circuit This circuit waits predetermined time (the time period function time constant) before responding carrier sending (Carrier Detected) output high addition circuit delays short time before dropping output again following equations find appropriate delay times TOFF-TO-ON TON-TO-OFF
interface hardware betweeen HC942 compatible computer four major parts decoding logic communication link tone dialer (Figure decoding logic addresses address locations that reserved asynchronous communications primary location 3F8HEX 3FFHEX secondary location 2F8HEX -2FFHEX When decoding logic uses these locations this interface design compatible with software (For other address locations consult documentation your computer other add-in cards your computer using interface circuit employs HCT688 equality detector HCT138 3-to-8-line decoder When connect strobes NAND configuration circuit generates strobe while direct-memory-access (DMA) strobe acts output from equality detector compares corresponding inputs checks whether equals When equal output selects HCT138 decoder HCT245 bidirectional driver HCT138 further decodes address lines selecting many eight devices although enables only TP5088 tone dialer 8250 UART Output directly enables 8250 output however indirectly enables TP5088 HC123 dual one-shot This output triggers one-shot which activates TP5088 pre-
where seconds first equation gives delay before transmission begins second gives delay before transmission ends Note that these times differ considerably When modem preparing transmit must delay long enough ensure that carrier stable carrier stability critical when modem preparing stop transmitting HC942 uses common 579-MHz crystal (without loading capacitors) generate internal timing modulated sine waves crystal also drives TP5088's oscillator doesn't however drive 8250's oscillator because software requires 8250 8432-MHz crystal adjust carrier-detection trip points applying external voltage (CarrierDetect Adjust) When double nominal voltage trip points increase Similarly when voltage half trip points decrease TABLE Standard Resistor Values Transmit Level (dBm)
Transmit Level Adjust Resistor R12(X) OPEN
UNIVERSAL SERVICE ORDER CODE
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FIGURE interface hardware between HC942 computer compatible) comprises four major parts decoding logic communication link tone dialer data-access arrangement (DAA)
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FIGURE
Because modem operates full-duplex mode most handshake signals Data Terminal Ready (DTR) Clear Send (CTS) operate general-purpose control signals need only (Data Carrier Detect) (Ring Indication) signals control modem UART switches HC942 into originate answer mode squelch mode (modulator disabled) through outputs OUT1 8250 also closes off-hookswitch relay through output 8250 receives status information from carrier-detection ring-detection circuits When carrier present internal carrier detector HC942 sends Carrier Detected signal input port 8250 This signal gives begin data communication Similarly ring detector sends signal input port during ring signal DTMF tone dialer operates summing tones from group from high group (Table III) tone dialer then sends composite tone telephone company's central office TP5088 which directly decode 4-bit binary code becomes active when Tone Enable receives low-to-high transition remains active long input stays high This input connected HC123 one-shot which sets duration DTMF tone requires that tone duration have minimum cycle time pulse duration minimum interdigit interval maximum interdigit interval TABLE Functional Truth Table Keyboard Data Inputs Tone Tones Mute Equivalent Enable (Hz) (Hz) 1209 1336 1477 1209 1336 1477 1209 1336 1477 1336 1209 1477 1633 1633 1633 1633
line driver HC942 raise 5088's minimum output level Transistor raises output level line driver maximum shorting transmit-level adjustment resistor
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FIGURE Although TP5088 tone dialer allows ignore distortion limits must boost output levels conform with limits shown here PERFORMS DUAL FUNCTIONS both protective device functional modem interface Although simple should submit your final design qualified testers ensure that your circuit conforms rules (part part protective capacity provides surge protection from natural hazards (like lightning) from induced voltages generated relay coils also provides isolation between phone line modem modem-interface functions include ring detection off-hook control DAA's ring-detection circuit sends signal 8250 (shown Figure when incoming ring occurs off-hook control takes place relay that 8250 controls with signal three sections transformer relay ring detector circuit's 600X transformer isolates modem from phone lines preventing line interference voice data communication transformer must have flat frequency response from 3300 with little harmonic distortion from Figure current sink connected parallel with transformer diode drop across shunts excess current current flowing through telephone lines varies from your design requires transformer that can't handle least you'll need current sink shunt excess current from phone line preventing transformer saturation course were transformer that could handle more than wouldn't need current sink transformer would both heavy expensive relay provides off-hook control closing relay switches modem off-hook connects telephone line signal from 8250 controls this relay switching This transistor allows current flow through relay close contacts dpdt relay completely isolates relay-control circuit when modem on-hook
Before coupling tone dialer phone lines must make sure dialer meets certain guidelines instance must maintain specified voltages loop currents loop lengths match impedance phone line provide tones within specified amplitude distortion limits designing correctly make your tone dialer meet first these guidelines Because 5088 generates tone distortion ignore distortion limit However must boost 5088's output levels output levels shown Figure Using inter-
ring-detector circuit Figure sends signal 8250 when incoming ring occurs Because incoming ring signal nominally can't couple directly ring detector includes zener diode capacitor (C10) optocoupler zener diode prevents noise from tripping ring detector creates high impedence that ring detector won't affect other circuits Note that must able accommodate high voltages such high amplitude ring signals optocoupler provides isolation translates high-voltage ring signals into digital levels optocoupler toggles during ring producing series low-going pulses network (R21 C11) smoothes these pulses single pulse 8250 receives this pulse (ring indication) input port detects ring polling this port servicing generated interrupt EMULATES DUMB TERMINAL communication program modem (the listing begins next page) consists three routines (for autodial autoanswer terminal emulation) first routines establish connection either dialing number answering call main terminal routine then configures dumb terminal that transmits phone line characters enter from keyboard displays incoming characters program menu driven select either autodial autoanswer routine these routines control jumps either terminal program main menu ``table equates'' beginning program initialization parameters communication program frequently accesses modem registers control outputs OUT1 also polls line status register read status-input ports When select autoanswer option from menu routine begins polling modem's status register detect incoming calls When ring occurs this goes high modem does immediately off-hook however because ring signal's high voltage causes arcing between relay contacts Instead program delays answering lifting modem off-hook between rings modem goes off-hook setting bits
modem's control register outputs turn These outputs then switch HC942 into answer mode closing relay establish connection After modem answers telephone billing delay must occur (according regulations) before transmission begin During this period telephone company's central offices exchange callers' billing information software delay keeps modem's control register ensuring that HC942 engages squelch function during this period After billing delay HC942 disengages squelch function polls modem's status register high carrier present data communication begin autoanswer routine then jumps main program transmit receive data other computer fails respond with carrier autoanswer routine times control returns main menu autodial routine first asks whether want make long-distance local call then enters phone number into buffer BUFF1 Next modem dials phone number fetching digit from buffer sending TP5088 then waits During this period HC123 one-shot enables TP5088 then disables remaining Therefore each digit comprises DTMF tone followed silence After modem dials phone number polls status register carrier present Modem operation then takes place autoanswer mode main program enters either from autodial autoanswer routine polls line status register (LSR) modem's control register (MCR) determine whether characters from keyboard telephone line present detect transmission errors carrier loss When keyboard characters present goes high when line characters present goes high then reads characters from UART Bits indicate transmission errors when such errors occur displays question mark indicates carrier loss main program detects carrier loss during transmission disconnects modem returns main menu
COMMUNICATIONS PROGRAM HC942 Table Equates TP5088 BAUD1 BAUD2 LCNTL LOCPH LDPH 2E8H 2F8H RBR01 RBR01 RBR02 RBR03 RBR04 RBR05 RBR06 RBR07 0F4H ADDR TP5088 RECEIVER BUFFER TRANSMITTER HOLDING LOWER DIVISOR LATCH INTR ENABLE UPPER DIVISION LATCH INTR LINE CONTROL MODEM CONTROL LINE STATUS MODEM STATUS SCRATCH LOWER BAUD RATE DIVISOR UPPER BAUD RATE DIVISOR DATA CONTROL DATA BITS STOP BITS PARITY DIGIT COUNTER LOCAL PHONE DIGIT COUNTER LONG
STACK SEGMENT PARA STACK 'STACK' DUP(0) STACK ENDS DATA SEGMENT PARA PUBLIC 'DATA' BUFF DUP(0) BUFFER PHONE NUMBER MSG1 DIALING MODE MSG2 call long distance MSG3 'Enter long (1-AAA-PPP-SSSS) MSG4 'Enter local (9-PPP-SSSS) MSG5 answer call again MSG6 'Dialing waiting response MSG7 MODEM COMMUNICATIONS PROGRAM Dial number Auto-answer routine Exit return MSG8 Phone line been disconnected MSG9 'Carrier Detected Phone connected MSG10 AUTO-ANSWER MODE 'Strike exit mode MSG11 'Ring detected waiting carrier MSG12 carrier detected phone disconnected Invalid entry re-enter phone DATA ENDS
COMMUNICATIONS PROGRAM HC942 (Continued) CODE SEGMENT PARA PUBLIC 'CODE' PUBLIC START1 PROC Program Prolog ASSUME PUSH PUSH ASSUME CODE DATA DATA START1
Initialize Line Control Register UART data stop bits baud rate parity parity LINE CONTROL PREPARE BAUD RATE LOWER DIVISOR LATCH BAUD1 LOWER DIVISOR UPPER DIVISOR LATCH BAUD2 UPPER DIVISOR LINE CONTROL LCNTL UART DATA CONTROL data control Disable interrupts 8250 UART INTR ENABLE DISABLE INTR
8250
Select Auto-dial Auto-answer routine MENU CALL CALL CALL CALL CALL CALL CALL CALL CALL OFFSET MSG7 DISPLAY DISPLAY DISPLAY DISPLAY INPCHAR DISPCHAR SHOW OPENING MESSAGE
COMMUNICATIONS PROGRAM HC942 (Continued) DIAL ANSW RETURN 'GOTO AUTO-ANS GOTO AUTO-DIAL ROUTINE
AUTO-ANSWER MODE This routine answers phone seconds after ring detected after seconds carrier detected program returns menu ANSW CALL CALL TEST CALL OFFSET MSG10 DISPLAY DISPLAY MENU1 AANS RING INPCHAR MENU DISPLAY AUTO-ANS MESSAGE
AANS
EXIT STRUCK MODEM CONTROL TEST RING INDICATOR RI41
MENU1
CLEAR CHAR
KEYBOARD
Wait seconds after ring detected RING WAIT1 CALL LOOP CALL DELAY WAIT1 OFFSET MSG11 DISPLAY LOAD DELAY COUNTER DELAY RING BEEN DETECTED
Switch MODEN into answer mode
MODEM CONTROL DTR40 RTS40 OUT1 MODEM SQUELCHED
MODE
Wait second billing delay CALL LOOP DELAY WAIT2 LOAD DELAY COUNTER
WAIT2
MODEM CONTROL DTR40 RTS40 OUT140 MODEM UNSQCH
COMMUNICATIONS PROGRAM HC942 (Continued) Poll carrier TEST CALL LOOP Timeout after seconds LOAD TIMEOUT COUNTER
CARR
COMM1
COMM1 DELAY CARR COMM
TEST CARRIER
DCD41
WAIT THEN POLL AGAIN
MAIN PROGRAM
carrier detected after answering phone auto-answer routine CALL OFFSET MSG12 DISPLAY ANSW
hang back
MODEM CONTROL CODE HANG PHONE
DIALING ROUTINE Determine long distance local call DIAL CALL CALL CALL CALL CALL OFFSET MSG1 DISPLAY OFFSET MSG2 DISPLAY INPCHAR DISPCHAR DISPLAY OPENING MESS LONG DISTANCE
INPUT CHAR DISPLAY CHAR CHAR
Enter long distance phone number into buffer CALL CALL CALL OFFSET MSG3 DISPLAY OFFSET BUFF LDPH BUFFER 0FFHK PROMPT LONG CALL
STARTING OFFSET BUFFER COUNTER LONG NUMBER kCRl ENTER INTO BUFFER
COMMUNICATIONS PROGRAM HC942 (Continued) Enter local phone number into buffer CALL CALL CALL OFFSET MSG4 DISPLAY OFFSET BUFF LOCPH BUFFER DTR40 PROMPT LOCAL CALL STARTING OFFSET BUFFER COUNTER LOCAL NUMBER kCRl
Take phone off-hook 0FFHK
OUT141
OUT240
MODEM CONTROL DTR40 OUT141 OUT240 MODEM POWERED DOWN
Wait CALL LOOP
dial tone DELAY LOOP2 WAIT LOOP BACK TIMES TOTAL 2000
LOOP2
Dial phone number
number from buffer send data TP5088 This continued until encountered DISABLE INTR 8088 DISPLAY MESSAGE6
MOVE STARTING ADDR BUFFER ADDR TP5088 INPUT CHAR FROM BUFFER CHAR DIALING
LOOP3
CALL CALL CALL
OFFSET MSG6 DISPLAY OFFSET BUFF TP5088 0DDH DELAY LOOP3
WAIT
Wait carrier detection timeout CALL CALL ENABLE INTR 8088 MODEM CONTROL TAKE MODEM POWER DOWN MODE WAIT
DELAY DELAY 1000
COMMUNICATIONS PROGRAM HC942 (Continued) LOOP4 TEST CALL LOOP COMM DELAY LOOP4 hang TEST DCD40 WAIT THEN AGAIN MODEM STATUS
answer after
then call again return
MODEM CONTROL CODE UART HANG PHONE OFFSET MSG5 CALL AGAIN CALL DISPLAY CALL INPCHAR INPUT CHAR CALL DISPCHAR ECHO CHAR CALL CHAR BACK BACK MENU BACK MAIN MENU BACK 0FFHK MAIN PROGRAM This communication routine Line Status Register constantly polled incoming outgoing characters COMM CALL CALL TEST TEST TEST TEST OFFSET MSG9 DISPLAY ERROR MAIN MODEM CONTROL REGISTER DTR40 OUT140 MODEM UNSQUELCHED INDICATE THAT CARRIER DETECTED
LINE CONTROL INPUT LINE STATUS FROM UART ERROR CONDITION
MAIN
DATA RECEIVED TRANSMISSION READY MODEM STATUS DCD41 RTRN DATA CARRIER LOST
Check keyboard strike MAIN
POLL AGAIN STRIKE
COMMUNICATIONS PROGRAM HC942 (Continued) Input character from keyboard send UART MAIN
TRANSMISSION HOLDING SEND CHAR UART
Data received routine Read data from UART PUSH
read data from UART display
BUFFER
STRIP SAVE
Display character obtain from UART kCRl RTRN CALL RTRN MAIN then kLFl MAIN
DISPLAY CHAR RESTORE CHAR kCRl CHAR
then hang return main menu OFFSET MSG8 DISPLAY MENU
MODEM CONTROL SEND CODE UART HANG RETURN MAIN MENU
LINE
Error routine ERROR
display there transmission error RECEIVER BUFFER CLEAR BUFFER ERRONEOUS DATA
COMMUNICATIONS PROGRAM HC942 (Continued) MAIN DISPLAY
`BUFFER` subroutine places phone number buffer (BUFF) Entry BX4offset buffer CL4counter phone number digits Output altered
PUBLIC BUFFER BUFFER PROC NEAR INPT CALL INPCHAR CALL DISPCHAR INPT ZERO PUSH CONT INPT EXIT1 OFFSET CALL DISPLAY OFFSET BUFF INPT ZERO PUSH CONT EXIT1 BUFFER ENDP
SAVE DIGIT COUNTER INPUT CHAR ECHO CHAR CHAR CHAR
SAVE CONVERT ASCII BINARY MOVE DIGIT INTO BUFFER
CHAR
kCRl
COUNTER40 DISPLAY ERROR MESSAGE ENTRY RE-ENTER STARTING OFFSET BUFFER RE-ENTER DIGIT COUNTER SAVE CODE DIAL ZERO MOVE `ZERO` DIGIT INTO BUFFER RETURN PHONE DIAL ROUTINE
`DELAY` subroutine waits Entry none Output register preserved
PUBLIC DELAY
COMMUNICATIONS PROGRAM HC942 (Continued) DELAY PROC NEAR PUSH LOOP5 LOOP ENDP 34000D LOOP5 RETURN PHONE DIAL ROUTINE LOOP BACK TIMES
DELAY
`INPCHAR` subroutine invokes BIOS routine input char from keybd Entry none Output AL4character entered from keyboard PUBLIC INPCHAR INPCHAR PROC NEAR INPCHAR ENDP
`DISPLAY` subroutine display character string stored data segment Entry BX4offset message string Output altered
PUBLIC DISPLAY DISPLAY PROC NEAR PUSH PUSH DISP1 CALL DISPCHAR LOOP DISP1 CALL DISPLAY ENDP
CHAR STRING CHAR CHAR FROM DATA DISPLAY CHAR
LONG
RETURN PHONE DIAL ROUTINE
`DISPCHAR` subroutine invokes BIOS routine display character screen Entry AL4character displayed Output altered
PUBLIC DISPCHAR
Single-Chip Modem Provides Low-Cost Data Communication
COMMUNICATIONS PROGRAM HC942 (Continued) DISPCHAR PROC NEAR PUSH DISPCHAR ENDP RETURN PHONE DIALING ROUTINE
subroutine produces carriage return line feed Entry none Output register preserved
PUBLIC PROC NEAR PUSH CALL CALL ENDP
DISPCHAR DISPCHAR
CARRIAGE RETURN LINE FEED
RETURN PHONE DIALING ROUTINE
START1 ENDP CODE ENDS
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LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT NATIONAL SEMICONDUCTOR CORPORATION used herein Life support devices systems devices systems which intended surgical implant into body support sustain life whose failure perform when properly used accordance with instructions provided labeling reasonably expected result significant injury user critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
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