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74VHC943WM Datasheet(PDF) 3 Page - National Semiconductor (TI) |
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74VHC943WM Datasheet(HTML) 3 Page - National Semiconductor (TI) |
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3 / 12 page  Description of Pin Functions Pin Name Function No 1 DSI Driver Summing Input This input may be used to transmit externally generated tones such as dual tone multifrequency (DTMF) di- aling signals 2 ALB Analog Loop Back A logic high on this pin causes the modulator output to be connect- ed to the demodulator input so that data is looped back through the entire chip This is used as a chip self test If ALB and SQT are simultaneously held high the chip powers down 3CD Carrier Detect This pin goes to a logic low when carrier is sensed by the carrier detect circuit 4 CDT Carrier Detect Timing A capacitor on this pin sets the time interval that the carrier must be present before the CD goes low 5 RXD Received Data This is the data output pin 6VCC Positive Supply Pin A a5V supply is recom- mended 7 CDA Carrier Detect Adjust This is used for ad- justment of the carrier detect threshold Car- rier detect hysteresis is set at 3 dB 8 XTALD Crystal Drive XTALD and XTALS connect to a 35795 MHz crystal to generate a crys- tal locked clock for the chip If an external circuit requires this clock XTALD should be sensed If a suitable clock is already avail- able in the system XTALD can be driven 9 XTALS Crystal Sense Refer to pin 8 for details Pin Name Function No 10 FTLC Filter TestLimiter Capacitor This is con- nected to a high impedance output of the receiver filter It may thus be used to evalu- ate filter performance This pin may also be driven to evaluate the demodulator RXA1 and RXA2 must be grounded during this test For normal modem operation FTLC is AC grounded via a 01 mF bypass capacitor 11 TXD Transmitted Data This is the data input 12 GND Ground This defines the chip 0V 13 OA OriginateAnswer mode select When logic high this pin selects the originate mode of operation 14 SQT Squelch Transmitter This disables the mod- ulator when held high The EXI input re- mains active If SQT and ALB are simulta- neously held high the chip powers down 15 RXA2 Receive Analog 2 RXA2 and RXA1 are analog inputs When connected as recom- mended they produce a 600X hybrid 16 RXA1 Receive Analog 1 See RXA2 for details 17 TXA Transmit Analog This is the output of the line driver 18 EXI External Input This is a high impedance in- put to the line driver This input may be used to transmit externally generated tones When not used for this purpose it should be grounded to GNDA 19 GNDA Analog Ground Analog signals within the chip are referred to this pin 20 TLA Transmit Level Adjust A resistor from this pin to VCC sets the transmit level 3 |
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