RS232&RS485.pdf
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2020-09-09
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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?
Abstract: Designers of industrial datacom systems often ask, 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 original publishing of this application note used the MAX3469 to demonstrate RS-
485 performance, and that data is still valid. However, Maxim has raised the performance of RS-485 to
100Mbps with the introduction of products such as the MAX22500E. This application note shows how to go
farther and faster.
Introduction
The various serial-datacom protocols range from RS-232 (EIA/TIA-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 (EIA/TIA-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.
Throughout this application note, we will use the “RS” nomenclature to refer to the respective ANSI EIA/TIA
standards.
Protocol Definitions
What is RS-485? How does it compare to other serial protocols, and for what applications are they best
suited? The following overview compares the characteristics and capabilities of the RS-485 PHY with those
of RS-232 and RS-422.
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 up to 20kbps, and later enhanced to 1Mbps.
Other RS-232 specifications include nominal ±5V transmit and ±3V receive (space/mark) signal levels, 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 only. 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.
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