Volume 1 • Issue1
Spring 1999

EXTENSIONtheA Technical Supplement to c o n t r o l NETWORKTEN RECOMMENDATIONS WHEN
IMPLEMENTING EIA-485
NETWORKS

Understanding EIA-485 Networks

1. Recognize that EIA-485 is only a physical layer
standard and connectivity between two machines eachwith an EIA-485 interface is not ensured.

INTRODUCTION

2. Since EIA-485 is basically a specification for driver,receiver and transceiver chips, the manufacturer of theequipment needs to specify cabling, grounding,termination, fail-safe bias and connectors.
3. Data rate and segment lengths are not addressed in thestandard and, therefore, must be specified by the
manufacturer. A high-speed EIA-485 design could bequite different from a low speed design.
4. EIA-485 is intended to be cabled as a linear bus withdaisy-chain connections. Stubs may or may not beallowed. Do not cable in a star topology.
5. Carefully review grounding practice. EIA-485 transceiverdamage is usually due to excessive common mode
voltage caused by unequal ground potentials at thevarious devices. Sometimes a third-wire groundconnection must be carried to all nodes to ensure thatthe common mode voltage remain within limits.
6. Device protection circuitry can certainly minimize device
failures, however, verify that high data rates can be
maintained with protection applied.
7. Although optically isolated EIA-485 will not protect thetransceivers themselves, it will provide a level ofprotection to the attached equipment. Be sure to
run a common wire between all optically-isolatedtransceivers.
8. The design of EIA-485 repeaters is tricky. Use only
those repeaters recommended by the manufacturer.
9. Be careful when applying termination and fail-safe bias
so as to not introduce excessive loading. Termination is
only applied at each end of the network while bias is
applied per the manufacturer’s recommendation.
10. EIA-485 can be quite an effective network as long as it
is applied properly. Here are our recommendations:• If the devices are in the same control panel, use
DC coupled EIA-485 and individual earth (chassis)
connections for ground reference.
• If the devices are in separate control panels within
the same building use optically-coupled EIA-485 anda separate reference ground wire.
• If the devices are in separate panels in different
buildings, use fiber optics if at all possible.

EIA-485 networks are usuallyconfigured in a daisy-chain fashion
with termination at each end.

One of the more popular technologies
for interconnecting devices on a
network is EIA-485. Known throughout
industry as RS-485, the proper title forthe standard is TIA/EIA-485-A“Electrical Characteristics of Generators
and Receivers for Use in Balanced
Digital Multipoint Systems.”1 The
EIA-485 standard is misunderstood tomean more than what it defines.
According to the standard it specifies
the characteristics of the generatorsand receivers used in a digital
multipoint system. It does not specify
other characteristics such as signal
quality, timing, protocol, pin
assignments, power supply voltages oroperating temperature range. A
multipoint system consists of two or
more generators and one or more
receivers. A generator is the same as atransmitter and since two or moretransmitters can exist on the same
electrical bus, EIA-485 is suitable formultimaster systems.
The standard itself is very shortconsisting of only 17 pages. Actuallymore guidance is available from its
sister publication TSB89 “Application
Guidelines for TIA/EIA-485-A.”2 An
EIA-485 bus usually consists of two or
more communication controllers eachpowered by a separate power source.

At a minimum, a single shielded or
unshielded twisted-pair cableinterconnects the various controllers ina daisy-chain fashion. In someinstances a short stub is allowed;
however, higher speed networks
usually do not allow stubs. A star
topology is definitely notrecommended. Termination is usuallyapplied to the ends of the network.
EIA-485 is basically a specification forthe drivers, receivers and transceivers
attached to the network. Therefore,
parameters such as unit loads, output
drive, short circuit current andcommon mode voltage are specified.
Basically a driver must be able tosource at least 1.5 volts differentiallyinto 60 ohms (two 120 ohm
terminators in parallel along with 32
unit loads) under a common mode
voltage range of –7 to +12 Vdc. Data
rates are not specified and there are awide range of devices that conform to
the standard but are intended either
for high speed (up to 50 Mbps) or low
speed (skew rate limited). So do notassume that all driver, receiver andtransceiver chips are all the same.
Some receivers and transceivers havehigher input impedance thereby
representing less than one unit loadto the driver.

Physical Layer Standard
In terms of the Open Systems
Interconnection Reference Model (OSI),
EIA-485 only defines the lowest layer—
the physical layer. It is used by Allen
Bradley’s DH-485,3 Profibus,4 BACnet’s
Master/Slave Token Passing option5
and ARCNET6 as well. Each of these
implementations is different
demonstrating that EIA-485 is not an all
encompassing standard.
APPLICATIONPRESENTATIONSESSION
TRANSPORTNETWORKDATA LINKPHYSICAL

In terms of the OSI model, EIA-485
only addresses the lowest layer.
There are several key topics that must
be considered when deploying EIA-485
networks such as termination, fail-safe
bias, connectors, grounding, cablingand repeaters.

TERMINATION
Terminating a data cable with a value
equal to its characteristic impedance
reduces reflections that could cause
data errors. However, if the data rate is
low or the cables are short, termination
may be unnecessary. As data rates
increase, termination becomes
important. Since any device on the bus
can transmit, it is probable that a node
within the middle of the bus willtransmit requiring that termination beapplied to both ends of the bus
segment. National Semiconductor offersa highly in-depth discussion on
termination in application note AN-9037
and offers several alternatives. The
most popular approach is DC
termination although this approach
results in higher power dissipation.
Resistive terminators typically havevalues of 120 to 130 ohms although
twisted-pair cable impedances can be
as low as 100 ohms. An 100 ohm
terminating resistor is too low for the
EIA-485 drivers. A value closely
matching the cable impedance must be
applied at some convenient location
closest to the ends of the cablesegment as possible. One possibility is
to provide the resistor within a nodewith a jumper to disable this option if
termination is not required. The
problem with this approach is thateach node will be configured
differently since only two nodes shouldhave terminators. Care must be
exercised to ensure that only theproper modules have termination
invoked in order not to cause
excessive bus loading.
Another approach is to use externaltermination outside the node. Profibus
uses this approach. Both terminating
and bias resistors are located in theshell of a DB9 connector. DIP switcheswithin the shell are used to disable thisfeature. The advantage of thisapproach is that all nodes on the
network are the same while connectors
are used to properly configure the
network.
Allen-Bradley’s DH-485 uses a slightly
different approach. Althoughterminating resistors are located within
all nodes, an external jumper appliedat the connector invokes termination.
Again, this keeps the configuration in
the connector and not within the node.

FAIL-SAFE BIAS
EIA-485 is a multipoint standard whereindividual devices transmit and receive
data sharing a common two-wire
medium. The opportunities forcollisions (two transmitters on at thesame time) are immense and a methodof medium access control (MAC) is
required. The 485 standard does notprovide a bus arbitration scheme since
this is not a requirement of thephysical layer but is a requirement of
the data link layer. With a master/slave
protocol such as Profibus DP, bus
arbitration with a single master andmultiple slaves may not be a problem
since the master directs all the traffic.
Slaves are always listening and only
respond to the master’s request thereby
avoiding collisions. During this time,
the bus will "float" enabling noise tofalsely trigger one of the bus receivers.
This can occur because the receiver’s
output is undefined when thereceiver’s input voltage is less than 200
mv which could happen when the bus
floats. To ensure that the bus assumes
a defined state when all transmitters
are off, fail-safe bias must be applied.

Disconnects are provided for
fail-safe bias and termination.
National Semiconductor’s AN-8478
application note fully discusses theneed for fail-safe bias and recommends
the proper biasing resistors needed toensure that the bus differential voltage
will not dip below 200 mv when idle.
This note recommends a pull-up
resistor to +5 volts attached to one
signal line and a pull-down resistor to
ground attached to the other. In
conjunction with an end-of-line
terminator, a voltage divider is created
which impresses a bias across the linethat exceeds 200 mv. Therefore, thereceivers are biased in the “mark”(off, logic 1) state when the network
is idle or when the transmitter sends a
logic 1. When a transmitter sends a
logic 0, the line will revert to “space”(on, logic 0).
Bias can be applied at any point on
the bus segment but it is not necessary
to lump the bias at only one point. The
bias can be distributed throughout thesegment with each node providing a
portion of the bias. The advantage ofthis approach is that there is no needto provide an external bias networkand power supplies. The problem withthis approach is that the amount ofbias developed depends upon thenumber of nodes on the bus. If too
few nodes are connected, insufficientbias may result. Too much bias canresult if too many nodes are connectedcausing excessive loading. If it isdesired to supply lumped bias, asource for +5 volt power needs to befound which may be awkward to
arrange. The other approach is toprovide the complete bias requirement
within each node while providing
disconnecting jumpers on the node. In
this way, only one node needs to be
strapped for bias so record keeping
must be good to ensure that thelocation of this node is known when areplacement is necessary.
Profibus nodes source +5 volt power
to the connector allowing for the bias
resistors in the shell of the mating
connectors. DIP switches within theshell disable the bias. Profibus uses
390 ohms for the pull-up and pulldown resistors and 150 ohms fortermination. This provides about 800
mv of bias and a Thevenin equivalenttermination resistance of 125 ohms. In
TSB89 the resistance values are 620
and 130 ohms respectively whichyields 475 mv of bias and a Thevenin
equivalent termination resistance of
118 ohms. Either approach is adequate
in terminating 120 ohm cable.
However, it must be remembered thata termination resistor exists at theother end of the cable. The distantterminator will load the bias network
thereby reducing the bias voltage by afactor of two. So instead of having 475
mv of bias, the resulting bias will be
only 240 mv which is still above the
200 mv limit. This analysis assumes
there is no resistance in the cable.
If the required bias is applied at two
points, then cabling rules are simplified.
Apply bias and termination only at theend two nodes by using a combination
bias/termination resistor network.
There is another benefit to applying
bias at end nodes. If a lumped bias
was applied to one end of a longcable with termination at each end, a
voltage divider is formed with the DC
resistance of the cable. For 24 AWG
cable, the resistance is 24 ohms/1000
ft. Since there are two wires in thecable, the cable resistance is effectively
48 ohms/1000 ft. An 2500 foot cablewould have the same DC resistance as
the end terminator thereby reducing
the effective bias at the end node by afactor of two. This could put the
distant nodes in an unreliable statecompared to devices closer to thesource of bias. A way of correcting thisis to increase the cable wire gauge toreduce resistance or apply an equalamount of bias at the distant end of
the cable effectively eliminating thevoltage drop due to cable resistance.

CONNECTORSWith coaxial and fiber optic cabling,
specifying connectors is easy since
there is common practice in theindustry. However, with twisted-paircabling there are many options. Since
the EIA-485 standard does not addressconnectors, trade associations or
manufacturers must do so. Frompractice there seems to be three
popular approaches. The traditional
approach is to use a four-pin, six
position RJ-11 or eight position RJ-45
providing plenty of pins for signal andground reference. Some RJ styleconnectors are shielded.
Another approach to connectorization
is to use removable open style screw
connectors. DH-485 uses a six-position
connector providing all the necessaryconnections including termination.
A popular connector is the DB9
connector that is used with the Profibus
standard. With nine pins, it is easy toaccommodate signals, logic ground,shield connection and power pins. DB9
housings are also available with metal
shrouds for better EMC performanceand the housing has built-in bias andtermination provisions. This connector,
however, tends to be pricey.

GROUNDINGIs EIA-485 a two wire or a three-wiresystem? It is most definitely a threewire system. The standard clearly statesthat generators and receivers require areturn path between circuit grounds ateach end of a connection. This return
path could be an actual wire in thecable connecting each of the logic
grounds together or earth can provide
the return path by having each logic
ground returned to earth. Using thelatter approach, a single pair twisted
cable can be used. If the third wire is
to be used, the standard states that theconnection between logic ground andthe third wire contain some resistanceto limit circulating currents when otherground connections are provided forsafety. This resistor could be between
logic ground and frame (frame is tied
to earth) or it can be between the logic
ground and third connection. The
standard uses 100 ohms as an example
for both situations.

If a third wire connection is used,
resistors must be used to limit
circulating ground current.
There is much confusion andmisunderstanding of the third wire
requirement and difficulty in evenfinding a third wire. If the logic
grounds of the transceivers are tied toearth and a third wire is used, there isalmost a guarantee of a ground loop
current which may or may not induce
excessive noise that could disrupt datatransmissions. The third wire will also
be the path for fault currents which
could be significant when the two
ground potentials are different due to asignificant electrical event. Still thethird wire helps to ensure that thecommon mode requirements (-7 to+12 volts) of the transceivers aremaintained. Excessive common mode
voltage is the most common reason fortransceiver failure.
Protection Circuitry
To protect EIA-485 transceivers fromexcessive common mode voltages,
diode protection circuits are used which
are referenced to earth or logic ground.
Usually protection is provided from eachdata line to earth and it is necessary toprotect against either a positive or
negative occurrence which doubles theprotection circuitry. The more robust theprotection, the more the capacitancewhich limits the data rate.

Protection circuitry is usuallyreferenced to earth.

It is quite possible that systems will
refuse to work at the desired data rate
due to the increased capacitance. Some
protection is afforded when theprotection circuit consists of a bulkytransient voltage suppressor in series
with a diode. The capacitive divider
created by the diode and suppressor inseries yields a capacitance that is lessthan the diode itself thereby lessening
the impact of protection on data rate.
Of course, protection is possibly
required at each node increasing the
likelihood that either data rate or
distance will be compromised byadding protection circuitry.
Optical Isolation
Optically isolated transceivers can betreated like DC coupled transceivers.
The termination and fail-safe bias
issues are the same, so what is
isolated? What is isolated are signals
TXD, RTS and RXD. Therefore, three
opto-isolators are required. The two
used for data should be high speed
while the transceiver enable isolator
can be slower. A DC-DC converter
needs to be provided and its
breakdown voltage will probably be
the limiting factor in terms of isolation.
The optically isolated transceiver
design is the most expensive approachand it does not guarantee that the EIA485 transceivers, which are connected
directly to the cable, will survive abusefrom severe electrical transients.
Chances are, however, that the damage
will stop at the isolators and not
involve the equipment attached to thenode. Optically-isolated EIA-485 forcesa third wire connection since thetransceivers must have a return path.
However, this time there is no groundloop since logic ground of thetransceivers is not connected to theearth. Where do we get the third
wire? Many times the shield is usedand not everyone is in agreement on
the wisdom of this approach. Sincesuch a small current is going to flow, it
seems a reasonable approach. DH-485
uses a two pair cable with one wire of
one pair dedicated as the common
ground. A shield covers the two pairs
and is only grounded at one point.

Other cabling issues include the wire
size of the conductors, the need forshielding, the presence of a third wireground and the type of insulation. It isbest to only use the manufacturer’s
recommended cable and not substitutewithout consulting with the
manufacturer first.

Sometimes the shield is used as thethird wire with an optically-isolated
interface.

CABLINGOne of the more critical decisions tomake is the selection of cable. There
are many choices of cable and people
incorrectly assume that any 24 AWG
telephone cable will do. Cable
selection depends on several factors
including data rate, signal encoding
and distance desired. Cables attenuate
the transmitted signal and introduce
distortion of the signal waveform itself.
Additional distortion occurs by the way
receivers are biased. Jitter can occur
when the receiver attempts to recover
the distorted data. Intersymbolinterference results when a new signal
arrives at the receiver before the last
signal reached its final value.
Therefore, the two successive symbols
interfere with one another resulting ina time shift in the data recovery whichis called jitter. National Semiconductor
discusses this phenomenon in AN-808.9
Some jitter is usually acceptable,
however, if it is excessive, the only
solution is to obtain better cable,
reduce the modulation rate or reducethe distance.

REFERENCES
1. Telecommunications Industry Association,
Electrical Characteristics ofGenerators and Receivers for Use inBalanced Digital Multipoint Systems,
TIA/EIA-485-A, March 1998.
2. Telecommunications Industry Association,
Application Guidelines for
TIA/EIA-485-A, TSB89, June 1998.
3. Rockwell Software, SLC500 Modular
Hardware Style Installation andOperation Manual, SLC500CD, July 1998.

www.ccontrols.com

4. Profibus Trade Organization, Profibus
Standard-Part 1, DIN 19245, 1993.

REPEATERS
EIA-485 segments can be extended
using active hubs or repeaters;
however, care needs to be exercised in
the selection of repeaters. Since only
two wires are used, the direction of
signal flow through the repeater mustchange dynamically. Usually adirection control line is provided to therepeater to control flow or the repeater
automatically senses traffic and adjusts
accordingly. Do not assume any lowcost EIA-485 repeater will work at all
speeds. Repeaters that sense line
activity could be fooled in believing
data flow from one direction has
stopped when in fact it was a seriesof logic “1”s or “0”s without statetransitions. This is especially true of
RZ or NRZ encoded data. Repeatersthat operate with a knowledge of thedata link protocol are much morereliable than off-the-shelf solutions.

SUMMARY
With some attention to detail,EIA-485 can be an effective physicallayer technology.

5. ASHRAE, A Data Communications
Protocol for Building Automation
and Control Networks,
ANSI/ASHRAE 135-1995, Dec. 1995.
6. FieldComms USA, ARCNET’s AlreadyFlexible Physical Layer Enhancedwith Several EIA-485 Variants,
George Thomas, June 1997.
7. National Semiconductor, A Comparisonof Differential Termination
Techniques, AN-903, 1993.
8. National Semiconductor, Fail-safe Biasingof Differential Buses,AN-847, July 1992.
9. National Semiconductor, Long
Transmission Lines and Data Signal
Quality, AN-808, March 1992.