B&B Electronics 485T User Manual

Not Recommended for New Installations.

Please contact Technical Support for more information.

RS-422/RS-485 Line Tester

Documentation Number 485T1995

Designed and Manufactured

B&B Electronics Mfg. Co. Inc.

707 Dayton Road -- P.O. Box 1040 -- Ottawa, IL 61350

PH [phone] -- FAX [phone]

 1992 B&B Electronics -- Revised August 1992

Model 485T

This product

In Ottawa, Illinois

USA

of domestic and imported parts by

Internet:

http://www.bb-elec.com

[e-mail]

[e-mail]

Documentation Number 485T1995 Manual Cover Page

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

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TABLE OF CONTENTS

INTRODUCTION ...............................................................................1

RS-422 AND RS-485 STANDARDS .................................................1

CONTROLS AND CONNECTIONS ON THE 485T ..........................6

A

AND B TERMINALS ........................................................................6

ROUND TERMINAL..........................................................................6

G

HRESHOLD POT..............................................................................7

T

OSITIVE THRESHOLD EXCEEDED LED.............................................7

P

EGATIVE THRESHOLD EXCEEDED LED............................................7

N
RS-422/RS-485 S

ERMINATION SWITCH ......................................................................8

T

OWER............................................................................................8

P
RS-232 D

ATA OUT CONNECTOR ......................................................8

RS-422/485 TEST .............................................................................9

WITCH.................................................................8

Documentation Number 485T1995 Manual Table of Contents i

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

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INTRODUCTION

The Model 485T Tester can be used to check the condition of
signals at any node on an RS-422 or RS-485 network. The tester
determines if the maximum permissible negative or positive voltages
are being exceeded. If these voltage levels are exceeded it is
possible to damage the RS-422/485 receivers and drivers. The
tester also checks to see if minimum required differential voltages
exist on the lines. The tester uses the normal data that is
transmitted on the line to check the value of the minimum differential
voltage. There is an RS-232 output that can be used to view the
network data.

RS-422 AND RS-485 STANDARDS

The RS-232 Standard (Interface Between Data Terminal
Equipment and Data Circuit-Terminating Equipment Employing
Serial Binary Data Interchange) was developed many years ago by
the Electronic Industries Association (EIA). It was used to connect
modems to terminals for dial-up computer systems. Since the
modem and terminal were usually located close to each other, wire
length was not much of a factor in the standard. RS-232 maximum
wire lengths were set at 50 feet. RS-232 lines can run farther with
special (low capacitance) wire but there is a practical limit of 300 or
so feet.

The EIA then developed two newer standards to allow serial
data communications to go past the 50 foot barrier. The first is RS­422, Electrical Characteristics of Balanced Voltage Digital Interface
Circuits. The second is RS-485, Standard for Electrical
Characteristics of Generators and Receivers for use in Balanced
Digital Multipoint Systems. Using these standards, wire lengths of
about 4000 feet are attainable. This is possible by using two wires
for each signal instead of the one wire used in RS-232.

A typical RS-232 system can have eight wires, seven signals
and one ground, not all of which are used in every system. As each
signal goes high or low (typically +12 and -12 volts) it uses the one
ground wire as a reference. Ground currents can cause confusion.
Also, the RS-232 drivers are not capable of driving much more than
2500 picofarads before the signal on the line is badly distorted.

Both RS-422 and RS-485 use two wires for each signal. These
two wires are driven differentially. When one wire is low the other is

Documentation Number 485T1995 Manual 1

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

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high and vice-versa. Also, the drivers are capable of driving a wire
with as much as 25 times more capacitance before the signal is
badly distorted. Most system designers try to minimize the number
of signals used because these standards take two wires per signal.
This is one reason most RS-422 and RS-485 systems are either
two-wire with ground or four-wire with ground.

There are two major differences between RS-422 and RS-485.
The first is that with RS-422, one driver can only drive 10 receivers.
With RS-485, one driver can drive 32 receivers. Secondly, RS-485
drivers can be “tri-stated” or turned off. This allows you to put all 32
drivers and receivers on one two-wire line. When a device needs to
output data, it first turns on its driver to seize the line and then sends
its data. The other 31 devices will be listening. RS-422 requires at
least two pairs of wires, one pair to send data in one direction and
the other for return data.

Typically, the RS-422 or RS-485 driver will switch between +5
volts and ground. Since devices can be long distances apart, they
can be powered on different power lines or transformers that can
force their “reference grounds” to be at different voltages. The RS­422 Standard allows the signal lines to go as high as +7 volts and as
low as -7 volts. The RS-485 Standard allows +12 volts and -7 volts.

Another requirement is that the differential input voltage at the
receiver must be more than 0.2 volts. The differential voltage starts
out from the driver at about 2 volts but the resistance, inductance,
and capacitance of the wire combine to attenuate this signal. As the
wire gets longer, this voltage will get smaller until it falls below the

0.2 volt level and then communications can no longer be
guaranteed.

Both specifications discuss termination of the two-wire line. A
typical two-wire twisted-pair line looks like a 100 ohm transmission
line. In general, the line does not need to be terminated for slow
data rates or for short-wire lengths.

More information and the RS-232, RS-422, and RS-485 Standards
are available from:

ELECTRONICS INDUSTRIES ASSOCIATION

Engineering Department

2001 Eye Street N. W.

Washington, DC 20006

Phone: [phone] --- FAX: [phone]

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Documentation Number 485T1995 Manual 3

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH [phone] -- FAX [phone]