RS485 Quick Guide

RS485 Quick Guide
TIA/EIA-485-A Standard
RS485 conveys data differentially over a terminated twisted pair, permitting up to 10Mbps 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. 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 100Mbps and up to 256 nodes, as well as 2500VRMS isolation and fault protection up to ±60V.
Specification
RS422
RS485
Differential
Differential
1 Driver,
10 Receivers
32 Drivers,
32 Receivers
4000 Feet
4000 Feet
10Mbps
10Mbps
–0.25V to 6V
–7V to 12V
Minimum Loaded
±2V
±1.5V
Maximum Unloaded
±5V
±5V
100Ω
120Ω
Mode of Operation
Number of Drivers and Receivers
Allowed on One Line
Maximum Cable Length
Maximum Data Rate
Maximum Voltage Applied to Driver Output
Differential Driver Output Signal
Termination
Receiver Input Voltage Range
±7V
–7V to 12V
Receiver Input Sensitivity
±200mV
±200mV
Receiver Input Resistance
4kΩ (Min)
12kΩ (Min)
What 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. Type 2 devices output a guaranteed 1
state whether the receiver inputs are left open, shorted or terminated but not driven.
What is the Proper Way to Terminate the Cable?
The cable should be terminated at each end with a resistance equal to
characteristic impedance.
L, LT, LTC, LTM, Linear Technology and the Linear logo are
registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
10k
CABLE LENGTH (FT)
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 length.
1k
100
10
10k
RS485
SPECIFIED
LIMITS
PRACTICAL
LIMIT FOR
20Mbps
DEVICES
100k
1M
10M
DATA RATE (bps)
100M
Linear Technology RS485/RS422 Transceivers
Part Number
Supply
Max Data
ESD
(V)
Rate (Bits/s) # Dr # Rec Duplex SHDN (kV) Failsafe Comments
Temp
Grade
Package
2500VRMS Isolation
LTM®2881-3/-5
LTC1535
3.3/5
20M
1
1
Full
Yes
±15
Type 2
No External Components
Required, Isolated 1W DC/DC
Converter, Switchable, 120Ω
Termination, UL File #E151738
5
250k
1
1
Full
No
±8
Type 2
UL File #E151738
Pin-Compatible with LT1785A
C, I, H, MP
C, I
15 × 11.25 × 2.8 LGA,
15 × 11.25 × 3.4 BGA
SO(W)-28
±60V Fault Protection
LTC2862-1/-2
3 to 5.5
20M/250k
1
1
Half
Yes
±15
Type 2
LTC2863-1/-2
3 to 5.5
20M/250k
1
1
Full
No
±15
Type 2
C, I, H, MP
SO-8, 3 × 3 DFN-8
C, I, H, MP
SO-8, 3 × 3 DFN-8
LTC2864-1/-2
3 to 5.5
20M/250k
1
1
Full
Yes
±15
Type 2
Pin-Compatible with LT1791A
LTC2865
3 to 5.5
20M/250k
1
1
Full
Yes
±15
Type 2
Logic Supply Pin, SLO Pin
C, I, H
LT1785A
5
250k
1
1
Half
Yes
±15
Type 2
Pin-Compatible with LTC485
C, I, H
SO-8, DIP-8
LT1791A
5
250k
1
1
Full
Yes
±15
Type 2
Pin-Compatible with LTC491
C, I, H
SO-14, DIP-14
3 × 3 DFN-10
C, I, H, MP
SO-14, 3 × 3 DFN-10
MSOP-12, 4 × 3 DFN-12
Integrated Switchable 120Ω Termination
LTC2854
3.3
20M
1
1
Half
Yes
±25
Type 2
Low Power
C, I, H
LTC2859
5
20M/250k
1
1
Half
Yes
±15
Type 2
Slew Rate Control, Low Power
C, I, H
3 × 3 DFN-10
LTC2855
3.3
20M
1
1
Full
Yes
±15
Type 2
Low Power
C, I, H
4 × 3 DFN-12, SSOP-16
LTC2861
5
20M/250k
1
1
Full
Yes
±15
Type 2
Slew Rate Control, Low Power
C, I
4 × 3 DFN-12, SSOP-16
3.3V Supply Operation
LTC2850
3.3
20M
1
1
Half
Yes
±15
Type 2
Low Power
C, I, H
SO-8, MSOP-8, 3 × 3 DFN-8
LTC2851
3.3
20M
1
1
Full
No
±15
Type 2
Low Power
C, I, H
SO-8, MSOP-8, 3 × 3 DFN-8
C, I, H
SO-14, MSOP-10, 3 × 3 DFN-10
LTC2852
3.3
20M
1
1
Full
Yes
±15
Type 2
DE and RE Pins, Low Power
LTC1480
3.3
2.5M
1
1
Half
Yes
±3.5
Type 1
Low Power
C, I
SO-8, DIP- 8
Low Power
LTC2856-1/-2
5
20M/250k
1
1
Half
Yes
±15
Type 2
Hot Swap™ Capable
C, I, H
LTC2857-1/-2
5
20M/250k
1
1
Full
No
±15
Type 2
Hot Swap Capable
C, I, H
MSOP-8, 3 × 3 DFN-8
LTC2858-1/-2
5
20M/250k
1
1
Full
Yes
±15
Type 2
Hot Swap Capable
C, I, H
MSOP-10, 3 × 3 DFN-10
LTC1690
5
5M
1
1
Full
No
±15
Type 2
C, I
LTC1481
5
2.5M
1
1
Half
Yes
±10
Type 1
C, I
SO-8, DIP-8
LTC1482
5
4M
1
1
Half
Yes
±15
Type 2
Carrier Detect
C, I
MSOP-8, SO-8, DIP-8
LTC1483
5
150k
1
1
Half
Yes
±10
Type 1
Low EMI
C, I
SO-8, DIP-8
LTC1484
5
4M
1
1
Half
Yes
±15
Type 2
C, I
MSOP-8, SO-8, DIP-8
LTC1485
5
10M
1
1
Half
No
±10
Type 1
C, I
SO-8, DIP-8
LTC1487
5
250k
1
1
Half
Yes
±10
Type 1
LTC485
5
2.5M
1
1
Half
No
±4
Type 1
LTC490
5
2.5M
1
1
Full
No
±10
Type 1
LTC491
5
2.5M
1
1
Full
No
±10
Type 1
Low EMI
C
C, I, M
DE and RE Pins
MSOP-8, 3 × 3 DFN-8
MSOP-8, SO-8, DIP-8
SO-8, DIP-8
SO-8, DIP-8, CERDIP-8
C, I
SO-8, DIP-8
C, I
SO-14, DIP-14
High Speed
LTC1685
5
52M
1
1
Half
No
±4
Type 2
C, I
SO-8
LTC1686
5
52M
1
1
Full
No
±4
Type 1
C, I
SO-8
LTC1687
5
52M
Quad Drivers and Receivers
1
1
Full
No
±4
Type 1
DE and RE Pins
C, I
SO-14
LTC1688/89
5
100M
LTC486/87
5
10M
LTC1518/19
5
52M
LTC488/89
5
10M
LTC1520
5
50M
RS232/RS485 Multiprotocol
4
4
0
0
0
0
0
4
4
4
No
No
No
No
No
±4
±4
±4
±10
±4
Type 1
Type 2
Type 1
Hot Swap Capable, 1/2 DE Pins
Low Power, 1/2 DE Pins
C, I
C, I
C, I
C, I
C
SO-16
SO(W)-16, DIP-16
SO-16
SO(W)-16, DIP-16
SO-16
1/2 DE Pins
High Speed, LVDS-Compatible
LTC2870
3 to 5.5
20M/500k
1
1
Both
Yes
±26
Type 2
Two RS232 Transceivers
C, I
4 × 5 QFN-28, TSSOP-28
LTC2871
3 to 5.5
20M/500k
1
1
Both
Yes
±16
Type 2
Two RS232 Transceivers
C, I
5 × 7 QFN-38, TSSOP-38
LTC2872
3 to 5.5
20M/500k
2
2
Both
Yes
±16
Type 2
Four RS232 Transceivers
C, I
5 × 7 QFN-38
Type 1 = Open; Type 2 = Idle, Open, Short
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