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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 ar
e capable of higher speed and distance. Consult the cable
manufacturer’s typical performance curve of 0 to 50% rise time vs cable length.
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.
RS485 Quick Guide
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 2500V
RMS
isolation and fault protection up to ±60V.
Specification RS422 RS485
Mode of Operation Differential Differential
Number of Drivers and Receivers
Allowed on One Line
1 Driver
,
10 Receivers
32 Drivers,
32 Receivers
Maximum Cable Length 4000 Feet 4000 Feet
Maximum Data Rate 10Mbps 10Mbps
Maximum Voltage Applied to Driver Output –0.25V to 6V –7V to 12V
Dif
ferential Driver Output Signal Minimum Loaded ±2V ±1.5V
Maximum Unloaded ±5V ±5V
Termination
100Ω 120Ω
Receiver Input Voltage Range ±7V –7V to 12V
Receiver Input Sensitivity ±200mV ±200mV
Receiver Input Resistance
4kΩ (Min) 12kΩ (Min)
DATA RATE (bps)
CABLE LENGTH (FT)
10k 1M 10M100k 100M
PRACTICAL
LIMIT FOR
20Mbps
DEVICES
100
1k
10
10k
RS485
SPECIFIED
LIMITS
TIA/EIA-485-A Standard
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