va YY AMI STANDARD PRODUCTS February 1993 Features Single-Chip 300 bps, Full Duplex, FSK Modem (Pin Selectable) Frequency Switching RS-232 Control Interface Passthrough Mode for Protocol Independence Low Cost 3.58MHz (TV Crystal) Time Base Digital and Analog Loopback Modes UART Clock Output (4.8KHz) V.25 Tone Generation Bell 103/113 and CCITT V.21 Operation Auto Answer/Originate Operating Modes Manual Answer/Originate Modes No External Filtering Required Phase Continuous Transmit Carrier $3531 Bell 103/V.21 Single Chip Modem Wlcal Applications Board Modems for Office Automation Equipment e Portable Lap Computers Encrypted Data Stream Modem Password Secure Modem * Test Instrument Communications Phone and Modem Combination Smart Vending Machines e Alarm Systems Functional Biock Diagram Pin Configuration Pott TT TTT TTT -7 (La [a } M | DLC] 1 28 |] CLK ! TC] 2 27 [7] 1D _ 22 FSK REC 5 oT DEMOD. FLTER ae EPC] 3 26 [-] AL ep o{ __] | | Yoo L] 4 25 [_] DTR = 21 & ! ENERGY ! RCC] 5 24 [[] OH ato w | t | 8 sar TEST; [] 6 23 [|] CTs oA 044 | 2 a TEST) (] 7 $3531 2217 RD _ 23 RS232 9 _ CTs ol BELL 103 PHONE eee sat] 8 MODEM 211] co ATs O- ef} COT v.21 INTERFACE = sa 25 | TING AGND a 9 20 [] RTS DTR oat to CLK SL a +o tes, Tc [J 10 19 7) A }++o TEST, __ he 1 TIMING AND CONTROL | lop si 18 [) SH | ls osc, [] 12 17] penp I C____________}"9 OSC, ; | 1 6 se, OSCy [] 13 16 [7 cor fe DSR [| 14 15 [7] Vss Yoo Vss-AGND(DGND 63$3531 AMI iZ STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem The $3531 is a Full Duplex FSK Modem integrated cir- cuit which may be operated in Bell 103/113 or CCITT V.21 applications. The S3531 features transmit and receive filtering; answer/originate mode selections; Pin/Function Descriptions February 1993 RS-232 control interface; digital and analog loopback test modes; and generation of both the 4.8KHz UART clock and V.25 Answer Tone. The $3531 is designed for use in stand-alone modem applications and in applica- tions in which the modem function is designed directly into the DTE. Pin # Name Function 1 DL A high level on this input causes the device to enter the digital loopback mode. In this (Digital Loopback) mode the received data from the remote end is internally jooped back to TD and DSR is forced high to signal to the DTE that the modem is not ready for transmission. The re- ceived data is not available on RD during the DL mode. 2 TP Test Pin. Must be connected to either Vss or Vpp for normal operations. (Test Point) 3 EP Output (analog) of the demodulator prior to slicing. Do not load. (Eye Pattern) 4,1 Vop. Vss Positive and negative Power Pins, respectively (+ 5V). 5 RC This analog input is the data carrier received by the data access arrangement from the (Receive Carrier) line. The modem demodulates this signal to generate the receive data bits. A .1pf series capacitor and a 30K pull down resistor is REQUIRED at this input. 6 Test 1 These are test inputs and must be tied to Vsg for normal applications. See table under 7 Test 0 Passthru Mode. 8 SOT A low level on this input enables (turns on) TC (pin 10). A high level disables (turns (Squelch Transmitter) off) TC. This pin can be left floating in order to enable TC. 9 AGND Analog ground (0 Volts). 10 TC This analog output is the modulated transmit data carrier. Its frequency depends upon (Transmit Carrier) whether the modem is in the answer or originate mode and if a mark or space condition is being sent (Table 1). Typically the output level is at 9dBm. (275mVRMS into 10KQ.) 11 SL A high level on this input selects the CCITT V.21 data transmission format. Applying a (Select) low level selects the Bell 103 data transmission format. 12 OSC, These are terminals for connecting an external 3.579545MHz TV crystal. Al! internal 13 OSCo clock signals are derived from this time base. Feedback resistor and capacitors are in- tegrated on the chip but additional 20pF caps to Vgs from each pin are required. 14 DSR This output, when low, indicates to the data terminal that the modem is ready to (Data Set Ready) transmit data. 16 CDT Applying a variable voltage level between 0 and 5V at this pin allows control of the (Carrier Detect Threshold) recieve carrier detection threshold. This will override the internally determined threshold. If CDT is set to a voltage between +1.5 and +2.0V the AGC will be disabled during the test modes of pins 6 & 7. 7 DGND Digital ground (0 Volts). 18 SH This input is used to manually place the device in the originate mode. The device will (Switch Hook) make the OH output low and start the originate sequence if SH input is low (5V) and DTR is on. This can be a level or a momentary low-going pulse input (min. 54msec). A pulse duration of less than 27 msec will not be detected. Ri should be high if SH is to be exercised. Once Ri has been activated RTS has no effect. 64AMI Z STANDARD PRODUCTS $3531 February 1993 Bell 103/V.21 Single Chip Modem Pin/Function Descriptions (continued) Pin # Name Function 19 Ri This input, when high, permits auto answer capability. The data access arrangements (Ring Indicator) should apply a low level (5V) to Rl when a ringing signal is detected. The level should be low for at least 107msec. The input may remain low during data transmis- sion, but must be reset before DTR. Similarly, in manual mode, the answer mode is entered by applying a low level to this input (unless RTS is high). 20 RTS A high level on this input with the DTR input in the on condition causes the device to (Request to Send) enter the originate mode. OH will go low to seize the phone line. Auto dialing can be performed by turning the RTS input on and off to effect dial pulsing. This input must re- main high for the duration of data transmission. (Auto answer will not function if RTS is high). RTS should follow OTR by no less than 1msec. 21 CD This output goes to a low level to indicate that the receive data carrier has been received (Carrier Detect) at a level of 43dBm. It turns off if the received data carrier falls below the carrier detec- tion threshold of 48dBm [Both values are +2dB]. 22 RD The device presents data bits demodulated from the received data carrier at this output. (Received Data) This output is forced high if the OTR input or the carrier detect output is off. 23 CTS This output goes to a iow level at the completion of the handshaking sequence and (Clear to Send) turns off when the modem disconnects. It is always turned off if the device is in the digital loopback mode. Data to be transmitted should not be applied at the TD input until this output turns on. 24 OH This output goes to a low level when either the SH or the RTS input is on in the (Off Hook} originate mode and when a valid ring signal is detected on the RI input in the answer mode. This output is off if DTR is off or if the disconnect sequence has been completed. 25 DTR A high level on this input enables all the other inputs and outputs and must be present (Data Terminal Ready} before the device will enter the data mode either manually or automatically. The device will enter an irreversible disconnect sequence if the input is turned off (low level) for more than 14msec during a data call. A pulse duration of less than 6msec will not be detected. To reset the chip before each call, this pin should be held low for greater than 14msec. 26 AL This input allows the data terminal to make the telephone line busy (off hook) and im- (Analog Loopback) plement the analog loopback mode. A high level on this input while DTR is high causes the device to make the OH output low and to enter the analog loopback mode. The receive filter center frequency is switched to correspond to the transmit filter center fre- quency and the transmit data carrier output is internally connected to the receive data Carrier input as well as being available at TC. 27 TD Data bits to be transmitted are presented to this input serially by the data terminal. A (Transmit Data) high level is considered a binary 1 or MARK and a low level is considered a binary 0 or SPACE. The data terminal shouid hold this input in the MARK state when data is not being transmitted. During handshaking this input is ignored. 28 CLK A 4.8KHz LSTTL compatible square wave output is provided for supplying the 16X clock (Clock) signal required by a UART for 300 bits/sec. data rate. This output facilitates the in- tegration of the modem function in the data terminal. 65$3531 IZ AM STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem Absolute Maximum Ratings February 1993 Supply Voltage (Vpp Vss) Operating Temperature Storage Temperature Power Dissipation + 12.0V D.C. Electrical Operating Characteristics: 1, = 0c to +70C; (Vpp-Vss) = 10V; +5% (+5.0V) Symbol Parameter/Conditions Min. Typ. Max. Units Vop Positive Supply Voltage (ref. to DGND, AGND; both at OV)} +4.75 +5.0 +5.25 VDC Vss Negative Supply Voltage (ref. to DGND, AGND) 4.75 5.0 5.25 VDC Pp Power Dissipation, Operating (@+5V) 110 200 mW Rin Input Resistance 8 MQ Cin input Capacitance 15 pF Analog Signal Parameters: 1,=0C to 70C; +5 VDC. fosc= 3.58MHz Symbol Parameter/Conditions Min. Typ. Max. Units fosc Oscillator Frequency 3.579545 +0.02% MHz ft Transmit Frequency Tolerance +1.2 Hz ty cane on Harmonic Attenuation with respect to 50 dB Tout Transmit Output Level into 10KQ min., 25pF max. 245 275 (9dBm) 400 mVRMS Carrier Input Range (CDT open) 48 +2 6 dBm DNR Dynamic Range (CDT open) 42 dB Bit Jitter (Input = 30dBm) 100 psec Bit Bias 1 % Bias Distortion 3 % Carrier Detect] Off to On -43 dBM Trip Points | On to Off 48 dBM Hysteresis 3 dBM Signal Input and Output Compatibility Table Pin Voltage Level Logic Family lot lou Name No. Input Output Low (Max.) High (Min.) Compatibility Miliamps | Miliamps SH 18 X -3 +3 CMOS Ri 19 X 3 +3 CMOS TESTg 7 X 3 +3 CMOS TEST, 6 X 3 +3 CMOS SQT 8 Xx +1 +4 CMOS 0H 24 X +0.4 +2.4 LSTTL 0.4 0.02 CLK 28 X +0.4 +2.4 LSTTL 0.4 0.02 CD 21 X +0.4 +2.4 LSTTL 0.4 0.02 RD 22 X +0.4 +2.4 TTL 1.6 0.4 CTS 23 X +0.4 +2.4 TTL 1.6 0.4 66AMI iZ STANDARD PRODUCTS $3531 February 1993 Bell 103/V.21 Single Chip Modem Signal Input and Output Compatibility Table (continued) Pin Voltage Level Logic Family lo lou Name No. Input Output Low (Max.) High (Min.) Compatibility Mikiamps | Miliamps DSR 14 X +0.4 +2.4 LSTTL 0.4 0.02 RTS 20 X +0.8 +2.0 TTL* TD 27 X +0.8 +2.0 TTL* DTR 25 X +0.8 +2.0 TTL* AL 26 X +0.8 +2.0 TTL* DL 1 X +0.8 +2.0 TTL* SL 11 X +0.8 +2.0 TTL* *These inputs are high impedance CMOS inputs that respond to TTL voltage levels. What is a 300 Baud Modem and What Does It Do? A modem acts like a translator between a computer and the telephone system. Computers work with data in the form of binary pulses but telephones were designed to transmit analog audio waveforms. The modem converts binary data from the computer into analog signals that the phone lines can carry. In the receive mode the modem demodulates the analog signals from the phone line, converting them back to binary form for the computer. 300 Baud modems are among the most common data communications devices in use today. Modems are used for exchanging information between home com- puters, personal computers, banks, offices and main- frames to name just a few posible applications. 300 Baud modems are used anywhere that a normal telephone line exists. Modems based on the $3531 have the advantages of full duplex operation using either BELL 103 or CCITT V.21 Protocols, a built-in in- terface to the industry standard RS232 serial data port, very low system part count, and low power CMOS single chip construction. Both BELL 103 and CCITT V.21 modems use FSK modulation for data transmission over standard phone lines. FSK modulation simply means Frequency Shift Keying or the transmission of frequency A for binary 1 and frequency B for binary 0. Full duplex FSK occurs when two-way transmission happens simulta- neously between two modems. A simple protocol exists to prohibit both the originating modem and the answering modem from transmitting simultaneously on the same frequency. 67 Figure 1. Frequency Modulation (FSK) 0 0 1 1 0 1 0 0 0 1 0 Figure 2. Full Duplex, 300 bps, Bell 103 fag 300 1070 1270 2025 2225 3300 1170 2125 fc fc Frequency (Hz) Data: Serial, binary, asynchronous, full duplex Data Transfer Rate: 0 to 300 bps Modulation: Frequency shift-keyed (FSK) FM Figure 3. Full Duplex, 300 bps, CCITT V.21 inal 980 1180 1650 1850 1080 1750 fc fc Frequency (Hz)$3531 AMI iZ STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem The protocol breaks the telephone frequency spectrum into two bands; a high band, and a low band. Each band has its own mark frequency corresponding to a binary 1 and its own space frequency corresponding to a binary 0, for a total of four transmitting frequencies. The protocol states that the originating modem must transmit on the low band and receive on the high band while the answering modem must transmit on the high band and receive on the low band. Obviously the ability of a modem to separate the high band from the low band is important for correct decoding of the transmitted data. Figures 4, 5, 6, and 7 of the February 1993 $3531 transmit and receive filters show a sharp 20 db cutoff within just a few hundred Hertz of the edges of the high and low bands. Block Description The block diagram of the FSK Modem is shown on page 1. The input to the modulator is the TD (Transmit Data) signal, which is the digital data to be converted to analog form. This input would typically be provided by the RS-232 interface or a UART. The modulator generates a square wave whose frequency is shifted in response to the Transmit Data input. Figure 4. Transmit Filter Bell 103 1 4 4 + 1 2000 3000 FREQUENCY (Hz) 1 j 4000 5000 ++_+_}-_+++ D 1000 Figure 5. Transmit Filter V.21 5000 +t+ + ++-+-+ 2000 3000 4000 FREQUENCY (Hz) A 0 1000 Figure 6. Receive Filter Bell 103 4 + 4 1 1 4 j 1 4 4 | 2000 3000 4000 5000 FREQUENCY (Hz) Figure 7. Receive Filter V.21 4 +4. + 4 + 2000 3000 FREQUENCY (Hz) 4 5000 +44 4000 0 1000 68AMI iZ STANDARD PROOUCTS $3531 February 1993 The transmit filter outputs a Frequency Shift Keying signal at the TC (Transmit Carrier) output. The fre- quency of the FSK signal corresponds to the fun- damental frequency of the square wave at the input of the filter. The transmit carrier TC can be enabled and disabled by the SQT control input. A low level on this pin enables (turns on) TC, and a high level disables (turns off) TC. On the receive side, the receive filter whose input is the Receive Carrier, rejects the adjacent channel energy and improves the Signal to Noise Ratio of the received signal. The output of the receive filter is fed into the demodulator where the data is converted back into digital form. The next block is the energy detect circuit. It detects energy levels at which reception and demodulation of data is considered reliable, controlling the CD signal. The last block is the timing control and handshake logic, which besides controlling all the other blocks, also implements the RS-232 interface protocol and controls the BELL 103 and CCITT V.21 operations. Transmit Filter The function of the transmit filter is to produce an FSK signal from the phase continuous, frequency shifted, square wave input. The prime objective of the transmit filter is to pass the square wave fundamental component while atten- uating its harmonics. These harmonics could be located in the receive band. Unless attenuated by the transmit filter, they would be coupled back through the hybrid, unattenuated by the receive filter, thus causing degradation of bit error rate. The transmit filter was designed to have a zero at the third harmonic of the square wave, to alleviate the above problem. The second objective of the transmit filter is to at- tenuate the out of band energy. This is necessary since the modulation process produces energy over a broad spectrum and not just at the mark/space frequencies. The fundamental component is attenuated by 24 dB to produce a signal at 9 dBm at the TC (Transmit Car- rier) output. 69 Bell 103/V.21 Single Chip Modem Receive Filter The measured frequency response of the receive filter is shown in Figures 6 and 7. The receive filter rejects out-of-band noise so that the filtered signal can be demodulated with a resultant low bit error rate. The filter was designed to reject the adjacent channel energy by 60dB. This is essential since that channel is used for carrier transmission which is coupled back, through the hybrid and into the receive section. Unless attenuated by the receive filter, this component would corrupt the demodulated data and result in excessive bit error rate. The filter was also designed to minimize group delay distortion between the mark and space fre- quencies. The band width of the filter is 500Hz and is centered around the center frequency of the received carrier. The dynamic range of the receive signal is 42dB due to the automatic gain control circuit employed. Timing Control The chip also incorporates a 14 second abort timer. This is necessary for automatic operation. When a call is automatically originated, and the remote device is busy, then the originating device waits for 14 seconds and hangs up. On the other hand, if the modem is called by mistake it will hang up in 14 seconds, unless the appropriate carrier is received. Clock Crystal The $3531 uses the popular low-cost 3.58MHz crystal. This crystal is very popular because it is used in all NTSC color TVs and in many low cost personal com- puters (which require the 3.58MHz to interface with TV monitors). The S3531 can therefore use the same system clock as the display interface to reduce system costs. Operation A. Answer Mode In the answer mode the $3531 stands idle waiting for an incoming call. With DTR high, a low from the ring detector to RI causes the $3531 to set OH and DSR low enabling the hookswitch relay and connecting the modem to the phone line. After 2.1 seconds the $3531 sends a carrier at 2225Hz (mark) to the Originate Modem. If 1270Hz (mark) is returned the $3531 carrier$3531 AMI IZ STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem February 1993 $3531 Modem Timing Chart for 103 Operating Mode MINIMUM RECEIVE | PULSE | ORIGINATE \~ DIALING | 1Smsec +| |~ 70msec MINIMUM BEFORE DIALING MAY BEGIN DIAL TONE RING & ANSWER BILLING DELAY OTR LJ +| * Imsec MINIMUM e [UU rn ae 106ms 200ms 640ms JAIL - LT Li ew LPL co cTs Tc ANSWER < 1270Hz (MARK) >< DATA | OTR __| RI v OH OSA LT Lo Te - i 2.1sec 106ms Lo 14 aN 2225Hz (MARK) x DATA 70WAMI $3531 STANDARD PRODUCTS February 1993 Bell 103/V.21 Single Chip Modem $3531 Modem Timing Chart for V.21 Operating Mode 15msec MINIMUM RECEIVE DIAL TONE RING & ANSWER ORIGINATE <t PULSE DIALING ___-> BILLING DELAY OTR LI | Sh t | Imsec MINIMUM SLPS LS ATS LLL Ly _ . 200 OSR ~~ Lt Per Me jat- -426ms> ms, + 640ms > =I f cTs Ih Sf Te oS fom th 980Hz i DATA ANSWER Sf fo Sf DTR _I RTS tf a Sh | oH th Vs a i Osa 4 tb tt ip ] crs a _ | Tc f 2100Hz K 185082 DATA -~t 2.1sec )~4- 3.4sec [+ 426ms>| ~{ ~-* 80ms 71$3531 AMI Z STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem detect circuit turns on within 106msec, setting CD and CTS low indicating completion of the handshaking se- quence. Data can then be sent and received. Originate Mode in the originate mode with DTR high, a call is initiated by applying a high to the RTS input in auto mode ora negative or low pulse to SH in manual mode. This will cause OH to go low, enabling the hookswitch relay and connecting the phone line. When dial tone is detected, RTS can be pulsed off to provide dial pulses*. The OH will follow the RTS pulses, sending the desired digits over the line. When the answering modem comes on line it will wait 2.1 seconds (billing delay) and then send the 2225Hz answer tone. 106msec later the CD pin will go low indicating carrier received. 190msec later the $3531 will respond with 640msec of 1270Hz. At the end of that time CTS will go low indicating to the terminal side that the communications link has been established. Abort Mode There is an automatic abort feature in the S3531 to avoid tying up a system when there is difficulty establishing a link. If no carrier is detected within 14 seconds of being put into the answer or originate mode it will abort the call by turning off OH and disconnecting the phone line. DSR will also go off (high). This abort time can be extended by pulsing RTS jow for 1msec before the 14 seconds have elapsed. This will reset the abort timer. If time does run out DTR should be pulsed off to reset the $3531. Shutdown Mode Should the received carrier fall below 48 dBm (approx.) during data exchange for more than 213msec the $3531|will terminate the call and go on hook, dis- connecting the phone line. Table 1. 103/V.21 Mark and Space Frequencies February 1993 Reset Protocol By insuring that all control inputs are in their inactive states a minimum of 2msec before the rising edge of DTR, the $3531 will be properly reset. Manual Operation The $3531 can be operated manually as well as automatically. With DTR enabled (high) a negative pulse (5V) of >107msec on RI will put the device in the Answer Mode. Similarly (with DTR high) SH can be pulled low for >54msec to put the $3531 into the Originate Mode. Passthru Mode With the Test 0 and Test 1 lines the $3531 can be put into the Passthru Mode disabling the handshake protocol. The transmit and receive functions are en- abled but become independent of timing and control. CD works as usual and the Answer and Originate Modes are selected manually with Ri and SH. Test 0 Test 1 $3531 PIN 7 PIN 6 STATUS | 1 = +5V (Vpp) 0 0 NORMAL | 0= 5V (Veg) 1 0 PASSTHRU V.21 Mode, CCITT Operation With the SL pin tied high the $3531 functions in the CCITT V.21 Mode but performs the same operations described above. The basic principle is the same but the frequencies and the timings are switched to V.21 specifications. When in V.21 Mode the V.25 answer tone of 2100Hz will be generated upon answering. See the timing charts and Table 1 for additional details. Diagnostic Modes The $3531 has two diagnostic modes for either local or remote testing. By putting the AL pin high while DTR is high, the device enters the Analog Loopback Mode. OH goes low to busy out the phone line. The receive filter center frequency is switched to the transmit Transmit Frequency (Hz) Receive Frequency (Hz) Mode Mark Space Mark Space Bell 103 Originate 1270 1070 2225 2025 Bell 103 Answer 2225 2025 1270 1070 CCITT V.21 Originate 980 1180 1650 1850 CCITT V.21 Answer 1650 1850 980 1180 CCITT V.25 Answer Tone 2100 N/A (Note that OH only follows RTS. The proper timing for dialing must come from the terminal on the RTS line.)AMI IWAN PRODUCTS $3531 February 1993 center frequency and the TC signal is internally con- nected to the RC input. The transmit signal also remains available on the TC pin. Thus any digital data input at TD is coded and sent out via TC, and at the same time back through the analog input, decoded, and out on the RD pin. By putting the DL pin high the $3531 enters the Digital Loopback mode. In this mode any data received from the remote end of the phone line is retransmitted back to its source and DSR is forced high. The digital or decoded data is not available at the RD output in this mode. See Table 2. Table 2. Control Logic During Diagnostic Modes Test Status Lines __ _ Mode | DTR RTS DSR OH CTs co AL On On On On On On DL On On Off On Off Off To establish diagnostic modes in either originate or answer, establish handshaking in the preferred mode (originate or answer), then enter diagnostic modes. Oscillator Details Quartz Crystal Specification (25C + 2C) Operating Temperature Range Frequency...........00000....00000, 0C to +70C 3.579545MHz Frequency Calibration Tolerance .............. 02+ % Load Capacitance .....0 0 ee. 18pF Effective Series Resistance ............ 180 Ohms, max. Drive Level-Correlation/Operating................ 2mW Shunt Capacitance.....00000 00, 7pF, max. Oscillation Mode....2 22... ee. Fundamental External Drive Requirements To use an external 3.58MHz clock a TTL level, 50% duty cycle, square wave can be applied to pin 12, OSCo through a .1nF capacitor. It must have a 2V P-P amplitude and be AC coupled through the .1pF capacitor. Applications Circuits Two applications circuits are illustrated. The first circuit is for a stand-alone RS-232 interface modem to be used as a peripheral accessory to a terminal or computer. Plug- ging into an RS-232 serial port on one side and into a standard modular phone jack on the other side it is a 73 Bell 103/V.21 Single Chip Modem stand-alone direct connect modem for operation at rates up to 300bps. The second circuit is an add-on modem for building into a computer and connecting to the internal parallel buss structure. The ACIA or UART does the parallel-to- serial and serial-to-parallel conversion required. The edge connector is numbered for an Apple II application but the same interface applies to most uP systems. Both circuits are intended for direct connection to the phone tines. This requires meeting FCC Part 68 requirements for network protection as well as protec- tion of the modem. No suppression components are illustrated on these examples as the design of the in- terface will vary depending on the needs of the designer. After a design is completed it must be sub- jected to Part 68 certification before sale to the public. If one wants to avoid the protection/certification details a certified DAA (Data Access Arrangement) such as the Cermetek GH1810 can be used instead. The DAA is designed to handle the phone line inter- face including the 4-wire to 2-wire function and is already registered with the FCC. Whether using a DAA or not, the $3531 requires very few external components. Hybrid Function In the stand-alone circuit the hybrid 4-wire to 2-wire converter utilizing the dual op amp was configured to provide 1:1 conversion in each direction. A 9dBm voltage level from the Transmit Carrier pin on the $3531 is amplified by the op amp to compensate for the losses in the 3002 matching resistor and the coupl- ing transformer. The transmit carrier is delivered to the line at 9dBm. (For CCITT applications this should be reduced to 13dBm.) In the receive direction the loss in the coupling transformer is compensated for by the other half of the op amp. If there is a 20dBm signal across Tip and Ring then a 20dBm signal is delivered to the Receive Carrier pin on the $3531. The 3002 resistor is to provide the proper termination so that Tip and Ring look like a 6002 AC impedance to the line. The 16KQ resistor from the Transmit Carrier pin to the inverting input of the receive op amp is to provide sidetone suppression. The transmit carrier is provided through the 16KQ resistor 180 out of phase from the transmit carrier presented to the line. Thus, the transmit carrier is cancelled and presented to the Recieve Carrier pin on the $3531 at a reduced level.& sid oO a 2 eI > 2 3 5 Wd HOLIMS dIO SMS-THS _ 2 o a re i Te"A \ AS+ SHS] cot xOF AS- OF S AGS yOVEd001 SOTWNY i HC av SSA VV 370N09 ms) Ton |S ave ~01d0 S 49 vOOPNI E BETNS 8 dH AS- 09 e QOA 02 nS 6 > 89 = 7 S 9 o<}+ = vo oO {>o v SB b - ' <If arate 2 Ty f 9 z= wep WON -[>- fo = fw gorarton 2)" t E eS. oy AS+ ot + ty ON = = e o st =a 9N | Sead : 9 5 22-SU IMT" 00 = 3 Lp NA+ fp ef RR = yor acrtow? rT} | HOLVOIONI = = WaIBHVS OL 2 |: ! : Th ! , 2 |e | = S I 5 yar AS zeeene - t o D atte, ~~ -* |) ~--- = < INS WAINDS HO NVHLOWOIW 99 rT l oF a - Tt e& zs WA | ud <r a = wnt teses Se St pt a | Suu <<} se = v) E | Wy }-Yyv4[o- vt N 3g Ge 60r gOrSP TOW mo is |? oe o Lt) = a =_ 7] 2 ee 74$3531 Bell 103/V.21 Single Chip Modem Schematic for $3531 STANDARD PRODUCTS AMI Suggested Parallel Interface Application y Z a February 1993 Wd HOLIMS dIO SMS-TMS = z gig d3e08 T2A = g 2 g g AS+ B20 535 28 S- wot aye N HOt go e8 3 ag0an aber fyeee S ace | %28d007 90TvNV 5 ij SSA aun & PMS] -wWHON 8 H370N09 Biba -O1d0 8 6ETND dH QQA so oec |p og our 760 16 afi one zed [oz 20 os eo [t2 0 ane ved |22 a ong seo lez sa wo 990 Ive "a ug viov {8 422788 To 9 guy tocos 1s mM 20 aman on Ig ox a AS- N zWwix we Sat ON FWLX oes yt ON Laga ~o ON AS+ =I 9N ae v ov ee HOLO3NNOD 3903 02 achTont IT 3idd ygt AS- o +_- @kt__k} 91> vOOPNT ettel ANSWAINOZ YO NVULOYDIW aOT 402 75$3531 AMI Z STANDARD PRODUCTS Bell 103/V.21 Single Chip Modem Under ideal conditions 20dB or more of cancellation might be achieved, but because telephone lines vary considerably, a cancellation of around 10dB is a more realistic number. The transformer listed is rated to 9OmA loop current. To go to the maximum toop current the Microtran number would be T5115 for 120mA loop current capability. The DC resistance may be slightly different and various com- ponents may need to be adjusted to retain the necessary February 1993 AC and DC specifications. The T2112 is much smaller and lighter because the low end frequency response is not needed. It is a modem transformer, not a voice transformer. NOTE once again, that only minimal transient protec- tion is illustrated in these examples, This must be add- ed to meet the needs of the application and the FCC Part 68 requirements. 76AMI IZA PRODUCTS $3531 February 1993 Modem Glossary Analog Loopback A diagnostic test for the entire in- ternal signal path of the modem chip. The transmitted analog output is internally connected to the analog input. Asynchronous A scheme for transmitting data on a character-by-character basis without a synchronizing clock signal. In general the asynchronous protocol in- cludes a start bit to identify the beginning of a character, the data bits, and stop bit(s). Bandwidth The frequency range of a communica- tions channe!. Normal phone lines have a bandwidth of 3000HZz for voice, from 300Hz to 3300Hz. BPS The speed at which a modem can transmit or receive data, measured in bits per second. 300 bps is roughly equal to 300 words per minute. Bias Distortion Distortion such that the actual mark and space bits are not of equal time duration, thus causing a deviation from the expected 50% duty cycle. CCITT International Telegraph and Telephone Con- sultative Committee. An organization for developing communication system standards. The European equivalent of BELL standards. Data Distortion Bit bias distortion occurs when the width of bits received are not equivalent for both a logic one and a logic zero. Bit bias is easily measured as it stows up as a deviation in average voltage. In a normal data stream of alternating ones and zeros the average voltage is zero. However when bit bias destor- tion is present the duty cycle is not exactly 50% and hence the average voltage is not zero. Excessive bit bias will lead to quality degradation as system UARTs deserialize data correctly only when bit bias distortion is low. Bit jitter distortion is also important for proper opera- tion of all modems. Bit jitter occurs when the actual center of the data bit drifts around the theoretical center. Again, this is important to the proper operation of a modem because UARTs only deserialize data cor- rectly when bit jitter distortion is low. Jitter distortion 77 Bell 103/V.21 Single Chip Modem is important in all asynchronous serial data systems because the edges of the data bits are used to reconstruct ail timing information. DAA Data Access Arrangement. An FCC registered device necessary for correctly connecting a device to the switched telephone network. Refer to Part 68 of the FCCs regulations. DCE Data Communication Equipment. Modem or any other equipment necessary for the transmission and reception of data between computers and terminals. Digital Loopback A diagnostic test for the entire phone line and remote modem. The remote modems digital output to the DTE is connected to the digital in- put from the DTE and fed back to the transmitting modem. Direct Connect Modems Modems that contain a DAA rather than requiring an acoustic coupler ora tie-in toa phone handset mouthpiece. DTE Data Terminal Equipment. The digital equip- ment that attaches to a modem as the end of the data path. Usually a terminal or a computer. FSK Frequency Shift Keying. A modulation method which varies the carrier frequency to correspond with the binary signals to be transmitted. Full Duplex Simultaneous two-way communication (transmission and reception) between two computers or modems. Off-Hook Connected to the telephone line. RS232C A serial communications interface defined by the Electronic Industries Association. Frequently used to connect stand-alone modems to personal computers.