Gary Maulding Introduction LT®1785 LT1791 RS485/RS422 transceiver

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Gary Maulding Introduction LT®1785 LT1791 RS485/RS422 transceiver

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RS485 Transceivers Sustain Faults Design Note
Gary Maulding
Introduction LT®1785 LT1791 RS485/RS422 transceivers with ±60V fault tolerance solve real-world problem field failures typical RS485 interface circuits. Modems other computer peripherals point-to-point RS422 connections support higher communication speeds with better noise immunity over greater distances than possible with RS232 connections. Multipoint RS485 networks used LANs industrial control networks. these applications vulnerable unknown, sometimes hostile environment outside controlled, shielded environment typical electrical equipment chassis. Because RS485 transceivers directly line fire, transceiver chips often socketed PDIP packages allow easy field servicing equipment. Field failures standard transceiver circuits caused data-line voltages exceeding absolute maximum ratings transceiver chips. Installation wiring faults,
ground voltage faults lightning-induced surge voltages common causes overvoltage conditions. ±60V Faults electrical standards RS422 RS485 signaling reflect need tolerance ground voltage drops extended network requiring receivers operate with input common mode voltages from 12V. RS485 RS422 transceivers commonly available from various vendors vulnerable damage from fault voltages only slightly outside operating envelope. vendor's RS485 transceivers have absolute maximum voltage ratings 12.5V data pins. Such narrow margins beyond required operating conditions makes such circuits very fragile real-world environment. addition, external protection circuitry ineffective protecting these circuits without corrupting normal operating signal levels. LT1785 LT1791, with ±60V absolute maximum ratings driver output receiver input pins inherently safe most environments that will destroy other interface circuits. Standard pinouts either PDIP packages allow easy upgrades existing RS422/RS485 networks. Whether circuit transmitting, receiving, standby powered off, voltage within ±60V will tolerated chip without damage. Data communication will interrupted during fault condition, circuit will live talk another day. Figure shows characteristics RS485 input/output pins.
registered trademarks Linear Technology Corporation.
1mA/DIV
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Figure LT1785 Input Current
LT1785
LT1785
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Figure Half-Duplex RS485 Network Operation ±60V ±15kV Protected
04/99/203
128-Node Networks 250kbaud addition their unique fault tolerance capabilities, these transceivers feature high input impedance support extended RS485 networks nodes (Figure Controlled slew-rate outputs minimize problems while supporting data rates 250kbaud (see Figure Driver outputs capable working with inexpensive telephone cable with characteristic impedance with loss signal amplitude. grade devices available that ensure "fail safe" receiver outputs when inputs open, shorted signal present.
interfere with normal operation data network. voltage clamping Zeners TransZorbs® available tight voltage tolerances, addition, their internal impedances cause several volts additional potential above their nominal breakdown voltage appear protected device's pins. protect circuit with 12.5V absolute maximum voltage rating would require protection devices with voltage ratings much below required common mode range RS485 networks interfering with normal data transmission. Figure gives example external clamping limiting components extend LT1785's ±60V tolerance peak 120V line voltage. TransZorbs used clamp transceiver's line pins below capability transceiver. During 120V line fault, peak surge currents nearly will flow through limiting resistors PolySwitchlimiters. peak current rating series resistance TransZorbs must considered when selecting clamp device ensure that clamp limiter withstand surge that peak voltage will remain below ±60V limitations LT1785. even high current TransZorbs will exceed their nominal breakdown voltage several volts, making this protection method ineffective with transceiver circuits with only margin above their operating ranges. PolySwitch limiters thermally activated increase resistance many orders magnitude about 10ms. After PolySwitch transition, fault currents only milliamperes. Carbon composite resistors must used limiting initial surge current before PolySwitch transition point. Metal film resistors have effective surge overload ratings will fail before PolySwitch transition drops currents sustainable levels.
TransZorb registered trademark General Instruments, GSI. PolySwitch trademark Raychem Corporation.
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Figure Normal Operation Waveforms 250kbaud
Extending Protection Beyond ±60V While ±60V fault tolerance forgives great number sins, higher voltage demons still lurking. such demon, with voltage spikes into thousands volts. LT1785 LT1791 have on-chip protection ±15kV transients other high voltage faults, such lightning-induced surge voltages line shorts. such high energy faults, external protection must used protect circuits. Typical protection networks voltage clamping current limiting networks. concept, such networks could used with normal RS485 circuits afford extended protection, practice, addition protection networks would
LT1785
±60V
RAYCHEM POLYSWITCH TR600-150 RT,120 0.1µF 300V
120V LINE FAULT
CARBON COMPOSITE 1.5KE36CA
LIMITER NETWORK
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Figure Limiter Network Clamps 120V Fault Voltage Less Than ±60V
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literature Interface products, call 1-800-4-LINEAR. applications help, call (408) 432-1900, Ext. 2453
dn203f LT/TP 0499 340K PRINTED
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408)432-1900 FAX: (408) 434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1999

Gary Maulding Introduction LT®1785 LT1791 RS485/RS422 transceiver

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