NOVUS RS422 User Manual

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RS485 & RS422 Basics

RS485 & RS422 Basics

RS485 & RS422 BasicsRS485 & RS422 Basics

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INTRODUCTION

The 422 and 485 standards, as they are known today, are balanced data-transmission schemes that offer robust
solutions for transmitting data over long distances and noisy environments. These standards don’t specify a logical
communication protocol, and are used as the physical layer specification by many protocols such as Modbus, Profibus,
DIN-Measurement-Bus and many others.

All Novus products that have serial bus communication follow RS485 standard, due to the advantages it shows in
industrial environments. For being so well known all over the world, it is easily accepted.

Besides they have been used for a long while, it is still common to find users with some unanswered questions about
RS485 and RS422 based networks. Due to this, we propose this document to present a brief explanation on important
topics for the design, analysis and installation of a communication networks using the RS485 and RS422 standards.

TIA/EIA-422

The TIA/EIA-422 standard, known as RS422, describes a communication interface that uses balanced data transmission
over multiple pairs of wires to establish communication from one transmitter to up to 10 receivers. At least two twisted­pairs of wires are used, one for communication from the transmitter (usually the master) to the receivers (usually the
slaves), and the other for transmission from the slaves back to the master. Since multiple slaves share the same wires
for transmission, they must keep their line drivers turned off (in high impedance state) most of the time. When data from
a slave is requested, it turns on its line driver, transmit data and turn it off again to allow transmission from another slave.
Use of two pairs of wires allows master and one slave to transmit data at the same time, which is called full-duplex
operation.

TIA/EIA-485

The TIA/EIA-485 standard, known as RS485, describes a communication interface that uses balanced data transmission
over one or two pairs of wires to establish communication between 32 “load units”. Usually, each network device
(transmitter and receiver) corresponds to one “unit load”, thus resulting in a 32 devices network. New devices can have
fractional “unit loads”, increasing the allowed number of networked devices. RS485 networks usually communicate using
a twisted-pair of wires, where data flows in both directions. Each device turns on its line driver only when transmitting
data, and keeps it off (in high impedance state) for the remaining time to allow other devices to transmit. Only one device
can transmit at a time, which is called a half-duplex operation.

RS485 networks can also operate using 2 pairs of wires, in full-duplex mode, as described for RS422.

BALANCED DIFFERENTIAL LINES

Both RS485 and RS422 use balanced differential lines for communication, usually twisted pairs of wires. Line drivers and
receivers for these interfaces use as data information the voltage difference between the two lines of the same pair.
Binary data are identified by the polarity of this voltage difference, defining that the data is a logical ‘1’ when the polarity
is positive (voltage level in “+” wire is higher than in “-“ wire) and ‘0’ when the polarity is negative (voltage level in “-” wire
is higher than in “+“ wire). A noise margin of ±0.2 V level is defined to enhance noise immunity. The balanced data
transmission cancels the induced noise, since the same noise is induced in both conductors of the pair, preserving the
voltage difference that carries the information. The radiated noise of a balanced communication bus is also lower than
the one of a non-differential bus.

TERMINATION RESISTORS

Communications theory states that a transmission line must be terminated by an impedance that is equal to the line
characteristic impedance. Proper termination attenuates signal reflections that degrade transmitted data, increasing the
maximum allowed cable length and/or data rate. Some termination methods are presented in the next figure.

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RS485 & RS422 Basics

Unterminated networks are low power, low cost, and simple to build. The disadvantage, of course, is that data rates must
be quite slow or cable length must be short for the network to operate reliably. A network up to 100 m long and
communicating at 19,200 bps is expected to be reliable even when no termination resistors are installed.

A parallel termination offers excellent data rates but is limited to networks that only have one driver. The driver must be
located on one end of the network and the termination resistor must be located on the far end. This is the case for each
pair of a RS422 or full-duplex RS485 network.

The third termination technique is a bidirectional termination, which offers excellent signal integrity. With this technique,
the line drivers can be anywhere on the network. The disadvantage is power consumption. This technique is probably the
most reliable RS-485 termination technique.

The last subject related to termination is what to do with unused conductors in a data cable. Unused conductors will self­resonate and couple noise into the data wires. If the unused cables are left opened, they will resonate at all sorts of
strange frequencies. If they are grounded at one end, they will resonate at L/2 (“L” is the cable length). If they are
grounded at both ends, they resonate at L/4. The best method for minimizing energy on an unused conductor is to
dissipate the energy as heat. In short, terminate both ends of the unused conductor to ground with resistors (a
bidirectional termination). The resistors should be equal to the characteristic impedance of the line. Other option is to use
a cable with no extra wires.

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