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Final Electrical Specifications
LTC1535 Isolated RS485 Transceiver
August 1999
FEATURES
s s s s s s
DESCRIPTIO
s s s s s
UL Rated Isolated RS485: 2500VRMS Half- or Full-Duplex Eliminates Ground Loops 350kBd Maximum Data Rate Self-Powered with 400kHz Converter Fail-Safe Output High for Open or Shorted Receiver Inputs Short-Circuit Current Limit Slow Slew Rate Control 68k Input Impedance Allows Up to 128 Nodes Thermal Shutdown 8kV ESD Protection On Driver Outputs and Receiver Inputs
The LTC(R)1535 is an isolated RS485 full-duplex differential line transceiver. Isolated RS485 is ideal for systems where the ground loop is broken to allow for much larger common mode voltage ranges. An internal capacitive isolation barrier provides 2500VRMS of isolation between the line transceiver and the logic level interface. The powered side contains a 400kHz push-pull converter to power the isolated RS485 transceiver. Internal full-duplex communication occurs through the capacitive isolation barrier. The transceiver meets RS485 and RS422 requirements. The driver and receiver feature three-state outputs, with the driver maintaining high impedance over the entire common mode range. The drivers have short-circuit current limits in both directions and a slow slew rate select to minimize EMI or reflections. The 68k receiver input allows up to 128 node connections. A fail-safe feature defaults to a high output state when the receiver inputs are open or shorted.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s s s s
Isolated RS485 Receiver/Driver RS485 with Large Common Mode Voltage Breaking RS485 Ground Loops Multiple Unterminated Line Taps
TYPICAL APPLICATIO
** CTX02-14659 1/2 BAT54C
+
10F 2
1/2 BAT54C 2 VCC 10F 1 RO 28 RO 3 ST2 400kHz
2 11 GND2 14 VCC2
+
1
VCC
ST1
A R B RO2
16 15 17 TWISTED-PAIR CABLE
RE DE DI
27 26 25 4 1
RE DE DI GND D Y Z SLO
13 12 18
1535 TA01
LOGIC COMMON 1
FLOATING RS485 COMMON 2
** TRANSFORMER COILTRONICS (561) 241-7876
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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1
LTC1535
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW VCC 1 ST1 2 ST2 3 GND 4 28 RO 27 RE 26 DE 25 DI
VCC to GND ................................................................ 6V VCC2 to GND2 ............................................................ 8V Control Input Voltage to GND ...... - 0.3V to (VCC + 0.3V) Driver Input Voltage to GND ........ - 0.3V to (VCC + 0.3V) Driver Output Voltage (Driver Disabled) to GND2 .............. (VCC2 - 13V) to 13V Driver Output Voltage (Driver Enabled) to GND2 ............... (VCC2 - 13V) to 10V Receiver Input Voltage to GND2 ............................ 14V Receiver Output Voltage .............. - 0.3V to (VCC + 0.3V) Operating Temperature Range .............. 0C TA 70C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LTC1535CSW
GND2 11 Z 12 Y 13 VCC2 14
18 SLO 17 RO2 16 A 15 B
SW PACKAGE 28-LEAD PLASTIC SO WIDE TJMAX = 125C, JA = 125C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL VCC VCC2 ICC ICC2 VOD1 VOD2 IOSD1 PARAMETER VCC Supply Range VCC2 Supply Range VCC Supply Current VCC2 Supply Current Differential Driver Output Differential Driver Output Driver Short-Circuit Current VOUT = HIGH VOUT = LOW Logic Input High Voltage Logic Input Low Voltage Input Current (A, B) Receiver Input Threshold Receiver Input Hysteresis Receiver Input Impedance RO Output High Voltage RO Output Low Voltage
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. 0C TA 70C, VCC = 5V, VCC2 = 5V unless otherwise noted.
CONDITIONS
q q
MIN 4.5 4.5
TYP
MAX 5.5 7.5
UNITS V V mA mA mA V V V
No Load R = 27, Figure 1 No Load No Load R = 50 (RS422) Note 2 R = 27(RS485), Figure 1 -7V VCM 10V -7V VCM 10V DE, DI, RE VCC = 4.5V DE, DI, RE VCC = 4.5V Note 3 -7V VCM 12V, Note 4 -7V VCM 12V IRO = - 4mA, VCC = 4.5V IRO = -10mA, VCC = 4.5V IRO = 4mA, VCC = 4.5V IRO = 10mA, VCC = 4.5V VIN = 12V VIN = - 7V
q q q q q q q q q q q q q q q q q
13 63 7 2 1.5 75 75 2
18 73 12 5
2 100 100 135 135 0.8 0.25 -0.20
VIH VIL IIN VTH VTH RIN VOH VOL
-200 10 50 3.7
-90 30 68 4.0 3.4 0.4 0.9
-10 70 85
0.8
2
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mA mA V V mA mA mV mV k V V V V
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WW
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LTC1535
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. 0C TA 70C, VCC = 5V, VCC2 = 5V unless otherwise noted.
SYMBOL IOZ VOH2 VOL2 fSW RSWH RSWL IREL IREH VUVL VUVH VISO PARAMETER Driver Output Leakage RO2 Output High Voltage RO2 Output Low Voltage DC Converter Frequency DC Converter R High DC Converter R Low RE Output Low Current RE Output High Current Undervoltage Low Threshold Undervoltage High Threshold Isolation Voltage VCC = 4.5V VCC = 4.5V RE Sink Current, Fault = 0 RE Source Current, Fault = 1 RE Fault = 1, Note 5 RE Fault = 0, Note 5 1 Minute, Note 6 1 Second IRO2 = - 4mA, VCC = 4.5V IRO2 = -10mA, VCC = 4.5V IRO2 = 4mA, VCC = 4.5V IRO2 = 10mA, VCC = 4.5V
q q q q q q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 3.7
TYP 1 3.9 3.4 0.4 0.9
MAX
UNITS A V V
0.8 520 6 5 - 80 130 4.25 4.40
V V kHz A A V V VRMS VRMS
290
420 4 2.5
- 40 80 3.90 4.05 2500 3000
- 50 100 4.00 4.20
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. 0C TA 70C, VCC = 5V, VCC2 = 5V, R = 27 (RS485) unless otherwise noted.
SYMBOL tSJ fMAX tPLH tPHL tr, tf tZH tZL tLZ tHZ tPLH tPHL tPLH tPHL tr, tf tLZ tHZ tSTART tTOF PARAMETER Data Sample Jitter Max Baud Rate Driver Input to Output Driver Input to Output Driver Rise or Fall Time Driver Enable to Output Driver Enable to Output Driver Disable to Output Driver Disable to Output Receiver Input to RO Receiver Input to RO Receiver Input to RO2 Receiver Input to RO2 Receiver Rise or Fall Time Receiver Disable to Output Receiver Disable to Output Initial Start-Up Time Data Time-Out Fault CONDITIONS Figure 8, Note 7 Jitter = 10% Max, SLO = 1, Note 8 DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0, Figure 3, Figure 5 DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0, Figure 3, Figure 5 DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0 DI = 1, SLO = 1, Figure 4, Figure 6 DI = 0, SLO = 1, Figure 4, Figure 6 DI = 0, SLO = 1, Figure 4, Figure 6 DI = 1, SLO = 1, Figure 4, Figure 6 RE = 0, Figure 2, Figure 7 RE = 0, Figure 2, Figure 7 RE = 0, Figure 2, Figure 7 RE = 0, Figure 2, Figure 7 RE = 0, Figure 2, Figure 7 Figure 2, Figure 8 Figure 2, Figure 8 Note 9 Note 9
q q q q q q q q q q q q q q
SWITCHI G CHARACTERISTICS
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MIN 350
TYP 250 600 1300 600 1300
MAX 285 855 1560 855 1560 50 1000 1400 1400 1000 1000 855 855
UNITS ns kBd ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
150
20 500 1000 1000 700 700 600 600 30 30 20 30 30 1200 1200
3
LTC1535
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: RS422 50 specification based on RS485 27 test. Note 3: IIN is tested at VCC2 = 5V, guaranteed by design from VCC2 = GND2 5.25V. Note 4: Input fault conditions on the RS485 receiver are detected with a fixed receiver offset. The offset is such that an input short or open will result in a high data output. Note 5: The low voltage detect faults when VCC2 or VCC drops below 4.2V and reenables when greater than 4.4V. The fault can be monitored through the weak driver output on RE. Note 6: Value derived from 1 second test. Note 7: The input signals are internally sampled and encoded. The internal sample rate determines the data output jitter since the internal sampling is asynchronous with respect to the external data. Nominally, a 4MHz internal sample rate gives 250ns of sampling uncertainty in the input signals. Note 8: The maximum baud rate is 350kBd with 10% sampling jitter. Lower baud rates have lower jitter. Note 9: Start-up time is the time for communication to recover after a fault condition. Data time-out is the time a fault is indicated on RE after data communication has stopped.
PI FU CTIO S
POWER SIDE VCC (Pin 1): 5V Supply. Bypass to GND with 10F capacitor. ST1 (Pin 2): DC Converter Output 1 to DC Transformer. ST2 (Pin 3): DC Converter Output 2 to DC Transformer. GND (Pin 4): Ground. DI (Pin 25): Transmit Data TTL Input to the Isolated Side RS485 Driver. Do not float. DE (Pin 26): Transmit Enable TTL Input to the Isolated Side RS485 Driver. A high level enables the driver. Do not float. RE (Pin 27): Receive Data Output Enable TTL Input. A low level enables the receiver. This pin also provides a fault output signal. (See Applications Information.) RO (Pin 28): Receive Data TTL Output. ISOLATED SIDE GND2 (Pin 11): Isolated Side Power Ground. Z (Pin 12): Differential Driver Inverting Output. Y (Pin 13): Differential Driver Noninverting Output. VCC2 (Pin 14): 5V to 7.5V Supply from DC Transformer. Bypass to GND with 10F capacitor. B (Pin 15): Differential Receiver Inverting Input. A (Pin 16): Differential Receiver Noninverting Input. RO2 (Pin 17): Isolated Side Receiver TTL Output. SLO (Pin 18): Slow Slew Rate Control of RS485 Driver. A low level forces the driver outputs into slow slew rate mode.
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LTC1535
BLOCK DIAGRA
ST1
1 28
VCC RO
27
RE
26 25
DE DI
4
GND
TEST CIRCUITS
Y R VOD R
1535 F02
Z
1535 F01
Figure 1. Driver DC Test Load
3V DE Y DI Z R
1535 F03
Figure 3. Driver Timing Test Circuit
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POWER SIDE 1 ISOLATED SIDE 1.3
+
2
3 ST2 400kHz DECODE
11 GND2
14 VCC2 A 16
ENCODE R B
EN FAULT
EN RO2 Y
15
17 13 12 18
ENCODE EN
DECODE D Z SLO
EN FAULT
1535 BD
RECEIVER OUTPUT
TEST POINT
S1
1k VCC
VOC
CRL
1k
S2
Figure 2. Receiver Timing Test Load
R
CL1
S1 OUTPUT UNDER TEST 500 S2 CL
1535 F04
VCC
CL2
Figure 4. Driver Timing Test Load
5
LTC1535
SWITCHI G TI E WAVEFOR S
3V DI 0V t PLH Z VO Y VO 0V -VO 80% 20% tr t SJ 80% 20% t SJ tf
1535 F05
1.5V
Figure 5. Driver Propagation Delays
3V DE 0V 5V Y, Z VOL VOH Y, Z 0V t ZH 2.3V 2.3V t ZL 1.5V
Figure 6. Driver Enable and Disable Times
VOH RO VOL t PHL VOD2 A-B -VOD2 0V 1.5V OUTPUT tr 10ns, tf 10ns INPUT t PLH 0V
1535 F07
Figure 7. Receiver Propagation Delays
3V RE 0V 5V RO 1.5V t SJ RO 0V tZH t SJ t HZ t SJ
1535 F08
1.5V
1.5V
Figure 8. Receiver Enable and Disable Times
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t SJ
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tr 10ns, tf 10ns t PHL
1.5V
VDIFF = V(Y) - V(Z)
tr 10ns, tf 10ns t LZ OUTPUT NORMALLY LOW
1.5V
0.5V
OUTPUT NORMALLY HIGH t HZ t SJ t SJ
0.5V
1535 F06
t SJ 1.5V
tr 10ns, tf 10ns tZL t LZ OUTPUT NORMALLY LOW
1.5V
0.5V t SJ
OUTPUT NORMALLY HIGH
0.5V
LTC1535
APPLICATIO S I FOR ATIO
Isolation Barrier and Sampled Communication The LTC1535 uses the SW-28 isolated lead frame package to provide capacitive isolation barrier between the logic interface and the RS485 driver/receiver pair. The barrier provides 2500VRMS of isolation. Communication between the two sides uses the isolation capacitors in a multiplexed way to communicate full-duplex data across this barrier. The data is sampled and encoded before transmitting across the isolation barrier, which will add sampling jitter and delay to the signals. The sampling jitter is approximately 250ns with a nominal delay of 600ns. At 250kBd rate, this represents 6.2% total jitter. The nominal DE signal to the driver output delay is 875ns 125ns, which is longer due to the encoding. Communication start-up time is approximately 1s to 2s. A time-out fault will occur if communication from the isloated side fails. Faults can be monitored on the RE pin.
** CTX02-14659 1/2 BAT54C
2
6 VCC2 (V)
1/2 BAT54C 2 VCC 10F 1 4 1 LOGIC COMMON 1 FLOATING RS485 COMMON 2 GND 3 ST2 400kHz
+
1
VCC
ST1
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Push-Pull DC/DC Converter The powered side contains a full-bridge open-loop driver, optimized for use with a single primary and center-tapped secondary transformer. Figure 9 shows the DC/DC converter in a configuration that can deliver up to to 100mA of current to the isolated side using a Coiltronics CTX0214659 transformer. Because the DC/DC converter is open-loop, care in choosing low impedance parts is important for good regulation. Care must also be taken to not exceed the VCC2 recommended maximum voltage of 7.5V when there is very light loading. The isolated side contains a low voltage detect circuit to ensure that communication across the barrier will only occur when there is sufficient isolated supply voltage. If the output of the DC/DC converter is overloaded, the supply voltage will trip the low voltage detection at 4.2V. For higher voltage stand-off, the Coiltronics CTX02-14608 transformer may be used.
ILOAD IEXT
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VCC2 vs ILOAD
+
10F
IVCC2
8
VCC = 5.5V 4 VCC = 5V VCC = 4.5V 2
2 11 GND2 14 VCC2
0 0
1535 F09
50 100 TOTAL LOAD CURRENT, ILOAD (mA)
150
1535 F09a
** TRANSFORMER COILTRONICS (561) 241-7876
Figure 9
7
LTC1535
APPLICATIO S I FOR ATIO
Driver Output and Slow Slew Rate Control
The LTC1535 uses a proprietary driver output stage that allows a common mode voltage range that extends beyond the power supplies. Thus, the high impedance state is maintained over the full RS485 common mode range. The output stage provides 100mA of short-circuit current limiting in both the positive and negative directions. Thus, even under short-circuit conditions, the supply voltage from the open-loop DC converter will not be pulled too low to prevent proper communication across the isolation barrier. The driver output will be disabled in the event of a thermal shutdown and a fault condition will be indicated through the RE weak output. The CMOS level SLO pin selects slow or fast slew rates on the RS485 driver output. The SLO input has an internal 100k pull-up resistor. When SLO is low, the driver outputs are slew rate limited to reduce high frequency edges. Left open or tied high, SLO defaults to fast edges. The part draws more current during slow slew rate edges.
RE
POLL DE FAULT BUFFER
POLL FAULT FAULT INDICATED WHEN RE IS THREE-STATED
1535 F10
Figure 10. Detecting Fault Conditions
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Monitoring Faults on RE The RE pin can be used to monitor the following fault conditions: low supply voltages, thermal shutdown or a time-out fault when there is no data communication across the barrier. Open circuit or short-circuit conditions on the twisted pair do not cause a fault indication. However, the RS485 receiver defaults to a high output state when the receiver input is open or short-circuited. The RE pin has a weak current drive output mode for indicating fault conditions. This fault state can be polled using the circuit in Figure 10 where the control to RE is three-stated and the fault condition read back from the RE pin. The weak drive has 100A pull-up current to indicate a fault and 50A pull-down current for no fault. This allows the RE pin to be polled without disabling RE on nonfault conditions. Both sides contain a low voltage detect circuit. A voltage less than 4.2V on the isolated side disables communication.
VCC VCC RO RE LTC1535 DI FAULT GND
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LTC1535
APPLICATIO S I FOR ATIO
High Voltage Considerations
The LTC1535 eliminates ground loops on data communication lines. However, such isolation can bring potentially dangerous voltages onto the circuit board. An example would be accidental faulting to 117V AC at some point on the cable which is then conducted to the PC board. Figure 11 shows how to detect and warn the user or installer that a voltage fault condition exists on the twisted pair or its shield. A small (3.2mm) glow lamp is connected between GND2 (the isolated ground) and the equipment's safety "earth" ground. If a potential of more than 75V AC is present on the twisted pair or shield, B1 will light, indicating a wiring fault. Resistors R3 and R4 are used to
LTC1535 B GND2 2 Z 2
EQUIPMENT SAFETY GROUND EARTH GROUND FLOATING RS485 COMMON 2
Figure 11. Detecting Wiring Faults
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ballast the current in B1. Two resistors are necessary because they can only stand off 200V each, as well as for power dissipation. As shown, the circuit can withstand a direct fault to a 440V 3 system. Other problems introduced by floating the twisted pair include the collection of static charge on the twisted pair, its shield and the attached circuitry. Resistors R1 and R2 provide a path to shunt static charge safely to ground. Again, two resisitors are necessary to withstand high voltage faults. Electrostatic spikes and transients can be limited by the addition of capacitor C1 and discharged through R1-R4.
A Y TWISTED-PAIR NETWORK 2 R1* 470k R2* 470k C1*** 10nF R3** 100k R4** 100k B1 CN2R (JKL) * IRC WCR1206 ** IRC WCR1210 *** PANASONIC ECQ-U2A103MV
1535 F11
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LTC1535
APPLICATIO S I FOR ATIO
DI
Y-Z
Figure 12. Driver Propagation Delay with Sample Jitter. SLO = VCC2
Z
Y
Figure 14. Driver Output. R = 27, VCC2 = 5V, SLO = VCC2
Y-Z
Figure 16. Driver Differential Output. R = 27, VCC2 = 5V, SLO = VCC2
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DI Y-Z
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Figure 13. Driver Propagation Delay with Sample Jitter. SLO = 0V
Z
Y
Figure 15. Driver Output. R = 27, VCC2 = 5V, SLO = 0V
Y-Z
Figure 17. Driver Differential Output. R = 27, VCC2 = 5V, SLO = 0V
LTC1535
PACKAGE DESCRIPTIO
SW Package 28-Lead Plastic Small Outline Isolation Barrier (Wide 0.300)
(LTC DWG # 05-08-1690)
0.697 - 0.712* (17.70 - 18.08) 28 27 26 25 18 17 16 15
0.291 - 0.299** (7.391 - 7.595)
0.005 (0.127) RAD MIN
0.010 - 0.029 x 45 (0.254 - 0.737)
0.009 - 0.013 (0.229 - 0.330)
NOTE 1 0.016 - 0.050 (0.406 - 1.270)
NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS.
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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Dimensions in inches (millimeters) unless otherwise noted.
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
1
2
3
4
11
12
13
14
0.093 - 0.104 (2.362 - 2.642)
0.037 - 0.045 (0.940 - 1.143)
0 - 8 TYP
0.050 (1.270) BSC
0.014 - 0.019 (0.356 - 0.482) TYP
0.004 - 0.012 (0.102 - 0.305)
SW28 (ISO) 1098
11
LTC1535
TYPICAL APPLICATIO
2 VCC 10F 1 RO 28 RO
+
1
VCC
ST1
RE DE DI
27 26 25 4 1
RE DE DI GND D Y Z SLO
LOGIC COMMON 1
RELATED PARTS
PART NUMBER LTC1177 LT1424-5 LTC1485 LTC1531 LT1785/LT1791 LTC1690 DESCRIPTION Isolated MOSFET Driver Isolated Flyback Switching Regulator High Speed RS485 Transceiver Self-Powered Isolated Comparator 60V Fault Protected RS485 Transceiver, Half/Full-Duplex Full-Duplex RS485 Transceiver COMMENTS No Secondary Supply, 2500VRMS Isolation 5% Accurate with No Optoisolator Required 10Mbps, Pin Compatible with LTC485 2.5V Isolated Reference, 3000VRMS Isolation 15kV ESD Protection, Industry Standard Pinout 15kV ESD Protection, Fail-Safe Receiver
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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** CTX02-14659 1/2 BAT54C
+
10F 2
1/2 BAT54C 3 ST2 400kHz
2 11 GND2 14 VCC2
A R B RO2
16 120 15 17
13 120 12 18
1535 TA02
FLOATING RS485 COMMON 2
** TRANSFORMER COILTRONICS (561) 241-7876
Figure 18. Full-Duplex Connection
1535i LT/TP 0899 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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