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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 3884
Keywords: RS485, RS422, RS-485, RS-422, Interface, Protocol, Line Drivers, Differential Line Drivers
APPLICATION NOTE 3884
How Far and How Fast Can You Go with RS-485?
Jul 25, 2006
Abstract:
Designers of
industrial datacom systems often ask, what is the most cost-effective
implementation for multidropped, medium-speed, serial data communications? What data rates can be
reliably achieved over what distance, and how? The design trade-off has always been less distance at a
higher rate, or greater distance at a lower rate. So, the crucial question is: how far can you reliably
transmit and receive data at a specified data rate? The MAX3469 is used to demonstrate RS-485
performance.
Introduction
The various serial-datacom protocols range from RS-232 to Gigabit Ethernet, and beyond. Though each
protocol suits a particular application, in all cases you must consider cost and performance of the
physical (PHY) layer. This article focuses on the RS-485 protocol and the applications best suited to that
standard. It also shows the ways that you can optimize data rates as a function of cabling, system
design, and component selection.
Protocol Definitions
What is RS-485? What is Profibus? How do they compare to other serial protocols, and for what
applications are they best suited? To answer these questions, the following overview compares the
characteristics and capabilities of the RS-485 PHY with those of RS-232 and RS-422.
[1]
(Throughout this
article, RS refers to the respective ANSI EIA/TIA standards.)
RS-232 is a standard that originated as a communications guide for modems, printers, and other PC
peripherals. It provided a single-ended channel with baud rates to 20kbps, later enhanced to 1Mbps.
Other RS-232 specifications include nominal ±5V transmit and ±3V receive (space/mark), 2V common-
mode rejection, 2200pF maximum cable load capacitance, 300Ω maximum driver output resistance, 3kΩ
minimum receiver (load) impedance, and 100ft (typical) maximum cable length. RS-232 systems are
point-to-point, not multidroppable. Any RS-232 system must accommodate these constraints.
RS-422 is a unidirectional, full-duplex standard for electrically noisy industrial environments. It specifies a
single driver with multiple receivers. The signal path is differential, and handles bit rates above 50Mbps.
The receivers' common-mode range is ±7V, the driver output resistance is 100Ω maximum, and the
receiver input impedance can be as low as 4kΩ.
The RS-485 Standard
RS-485 is a bidirectional, half-duplex standard featuring multiple 'bussed' drivers and receivers, in which
each driver can relinquish the bus. It meets all RS-422 specifications, but is more robust. It has a higher
receiver-input impedance and larger common-mode range (-7V to +12V).
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