RS485 Quick Guide - Analog Devices

1 What Distance Can Be Achieved?The achievable distance is a function of the cable. The longer the cable, the greater the attenuation. Because attenuation increases with frequency, cables also exhibit a lowpass filter behavior so that achievable distance diminishes with data rate. The distances recommended by the RS485 standard are shown in the graph to the right. Many cables are capable of higher speed and distance. Consult the cable manufacturer s typical performance curve of 0 to 50% rise time vs cable is the Failsafe Receiver Output State with No Input Signal?That depends on the failsafe type of the receiver. Type 1 Devices (see over) output a guaranteed 1 state when the receiver inputs are left open, but the output is undetermined when the inputs are shorted.

2 Type 2 Devices output a guaranteed 1 state whether the receiver inputs are left open, shorted or terminated but not is the Proper Way to Terminate the Cable?The cable should be terminated at each end with a resistance equal to the cable s characteristic Quick GuideRS485 conveys data differentially over a terminated twisted pair, permitting up to 10 Mbps data rates. The standard specifies electrical characteristics of a driver and receiver, and does not specify any data protocol or connectors. RS485 is popular for inexpensive local networks, multidrop communication links and long haul data transfer over distances of up to 4,000 feet. The use of a balanced line means RS485 has excellent noise rejection and is ideal for industrial and commercial applications.

3 You ll find RS485 in applications as diverse as monitoring oil wells and linking POS terminals, to alarm systems, motion control and HVAC controls. Extended capability transceivers offer data rates up to 100 Mbps and up to 256 nodes, as well as 6500 VRMS isolation and fault protection up to RS422RS485 Mode of OperationDifferentialDifferentialNumber of Drivers and Receivers Allowed on One Line1 Driver, 10 Receivers32 Drivers, 32 ReceiversMaximum Cable Length4000 Feet4000 FeetMaximum Data Rate10 Mbps10 MbpsMaximum Voltage Applied to Driver Output to 6V 7V to 12 VDifferential Driver Output SignalMinimum Loaded 2V Unloaded 5V 5 VTermination100 120 Receiver Input Voltage Range 7V 7V to 12 VReceiver Input Sensitivity 200mV 200mVReceiver Input Resistance4k (Min)12k (Min)DATA RATE (bps)CABLE LENGTH (FT)

4 10k1M10M100k100 MPRACTICALLIMIT FOR20 MbpsDEVICES1001k1010kRS485 SPECIFIEDLIMITSTIA/EIA-485-A StandardL, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog Devices , Inc. All other trademarks are the property of their respective NumberSupply (V)Max Data Rate (Bits/s)# Dr# RecDuplexSHDNESD (kV)FailsafeCommentsTemp GradePackageGalvanic IsolationLTM2885520M11 FullYes 15 Type 26500 VRMS Isolation, No External Components, Isolated 1W DC/DCC, I, H22 9 BGALTM 2881-3 15 Type 22500 VRMS Isolation, No External Components, Isolated 1W DC/DCC, I, H, MP15 LGA, 15 BGALTC15355250k11 FullNo 8 Type 22500 VRMS IsolationC, ISO(W)-28 60V Fault ProtectionLTC2862-1/-23 to 15 Type 2 Pin-Compatible with LT1785AC, I, H, MPSO-8, 3 3 DFN-8 LTC2863-1/-23 to 15 Type 2C, I, H, MPSO-8, 3 3 DFN-8 LTC2864-1/-23 to 15 Type 2 Pin-Compatible with LT1791AC, I, H, MPSO-14, 3 3 DFN-10 LTC28653 to 15 Type 2 Logic Supply Pin, SLO PinC, I, HMSOP-12, 4 3 DFN-12LT1785A5250k11 HalfYes 15 Type 2 Pin-Compatible with LTC485C, I, HSO-8, DIP-8LT1791A5250k11 FullYes 15 Type 2 Pin-Compatible with LTC491C, I, HSO-14, DIP-14 Integrated Switchable 120 25 Type 2 Low PowerC, I, H3 3 DFN-10 LTC2859520M/250k11 HalfYes 15 Type 2 Slew Rate Control, Low PowerC, I, H3 3 15 Type 2 Low PowerC, I, H4 3 DFN-12.

5 SSOP-16 LTC2861520M/250k11 FullYes 15 Type 2 Slew Rate Control, Low PowerC, I4 3 DFN-12, Supply 15 Type 2 Low PowerC, I, HSO-8, MSOP-8, 3 3 15 Type 2 Low PowerC, I, HSO-8, MSOP-8, 3 3 15 Type 2DE and RE Pins, Low PowerC, I, HSO-14, MSOP-10, 3 3 1 Low PowerC, ISO-8, DIP- 8 Low PowerLTC2856-1/-2520M/250k11 HalfYes 15 Type 2 Hot Swap CapableC, I, HMSOP-8, 3 3 DFN-8 LTC2857-1/-2520M/250k11 FullNo 15 Type 2 Hot Swap CapableC, I, HMSOP-8, 3 3 DFN-8 LTC2858-1/-2520M/250k11 FullYes 15 Type 2 Hot Swap CapableC, I, HMSOP-10, 3 3 DFN-10 LTC169055M11 FullNo 15 Type 2C, IMSOP-8, SO-8, 10 Type 1C, ISO-8, DIP-8 LTC148254M11 HalfYes 15 Type 2 Carrier DetectC, IMSOP-8, SO-8, DIP-8 LTC14835150k11 HalfYes 10 Type 1 Low EMIC, ISO-8, DIP-8 LTC148454M11 HalfYes 15 Type 2C, IMSOP-8, SO-8, DIP-8 LTC1485510M11 HalfNo 10 Type 1C, ISO-8, DIP-8 LTC14875250k11 HalfYes 10 Type 1 Low EMICSO-8, 4 Type 1C, I, MSO-8, DIP-8, 2 Type 1C, ISO-8, 2 Type 1DE and RE PinsC, ISO-14, DIP-14 High SpeedLTC1685552M11 HalfNo 4 Type 2C, ISO-8 LTC1686552M11 FullNo 4 Type 1C, ISO-8 LTC1687552M11 FullNo 4 Type 1DE and RE PinsC, ISO-14 Quad Drivers and ReceiversLTC1688/895100M40No 4 Hot Swap Capable, 1/2 DE PinsC, ISO-16 LTC486/87510M40No 2 Type 1 Low Power, 1/2 DE PinsC, ISO(W)-16, DIP-16 LTC1518/19552M04No 4 Type 2C, ISO-16 LTC488/89510M04No 10 Type 11/2 DE PinsC, ISO(W)

6 -16, DIP-16 LTC1520550M04No 4 High Speed, LVDS-CompatibleCSO-16RS232/ RS485 MultiprotocolLTC28703 to 26 Type 2 Two RS232 TransceiversC, I4 5 QFN-28, TSSOP-28 LTC28713 to 16 Type 2 Two RS232 TransceiversC, I5 7 QFN-38, TSSOP-38 LTC28723 to 16 Type 2 Four RS232 TransceiversC, I5 7 QFN-38 Type 1 = Open; Type 2 = Idle, Open, ShortLinear Technology RS485 /RS422 n 1-800-4-LINEAR