Omega A2400 User Manual

User’s Guide

MADE IN

Shop online at

omega.com

e-mail: [e-mail]

For latest product manuals:

omegamanual.info

A2400 Series

Radio Modem Module

Servicing North America:

U.S.A.: One Omega Drive, Box 4047

ISO 9001 Certified Stamford, CT 06907-0047

Tel: [phone] FAX: [phone]
e-mail: [e-mail]

Canada: 976 Bergar

Laval (Quebec) H7L 5A1, Canada
Tel: [phone] FAX: [phone]
e-mail: [e-mail]

For immediate technical or application assistance:

U.S.A. and Canada: Sales Service: 1-800-826-6342 / 1-800-TC-OMEGA

®

Customer Service: 1-800-622-2378 / 1-800-622-BEST

®

Engineering Service: 1-800-872-9436 / 1-800-USA-WHEN

®

Mexico: En Espan˜ol: [phone]-7803 e-mail: [e-mail]

FAX: [phone] [e-mail]

Servicing Europe:

Benelux: Postbus 8034, 1180 LA Amstelveen, The Netherlands

Tel: [phone] FAX: [phone]
Toll Free in Benelux: 0800 0993344
e-mail: [e-mail]

Czech Republic: Frystatska 184, 733 01 Karvina´, Czech Republic

Tel: [phone] FAX: [phone]
Toll Free: 0800-1-66342 e-mail: [e-mail]

France: 11, rue Jacques Cartier, 78280 Guyancourt, France

Tel: [phone] FAX: [phone]
Toll Free in France: 0800 466 342
e-mail: [e-mail]

Germany/Austria: Daimlerstrasse 26, D-75392 Deckenpfronn, Germany

Tel: [phone] FAX: [phone]
Toll Free in Germany: 0800 639 7678
e-mail: [e-mail]

United Kingdom: One Omega Drive, River Bend Technology Centre

ISO 9002 Certified Northbank, Irlam, Manchester

M44 5BD United Kingdom
Tel: [phone] FAX: [phone]
Toll Free in United Kingdom: 0800-488-488
e-mail: [e-mail]

OMEGAnet®Online Service Internet e-mail

omega.com [e-mail]

It is the policy of OMEGA Engineering, Inc. to comply with all worldwide safety and EMC/EMI
regulations that apply. OMEGA is constantly pursuing certification of its products to the European New
Approach Directives. OMEGA will add the CE mark to every appropriate device upon certification.

The information contained in this document is believed to be correct, but OMEGA accepts no liability for any
errors it contains, and reserves the right to alter specifications without notice.
WARNING: These products are not designed for use in, and should not be used for, human applications.

A2400 USERS MANUAL

REVISED: 4/17/95

OMEGA ENGINEERING
ONE OMEGA DRIVE
P. O. BOX 4047
STAMFORD, CT 06907

PHONE: 1-800-DAS-IEEE
FAX: [phone]
e-mail: [e-mail]
www.omega.com

The information in this publication has been carefully checked and
is believed to be accurate; however, no responsibility is assumed for
possible inaccuracies or omissions. Applications information in this
manual is intended as suggestions for possible use of the products
and not as explicit performance in a specific application. Specifica­tions may be subject to change without notice.

The A2400 series are not intrinsically safe devices and should not
be used in an explosive environment unless enclosed in approved
explosion-proof housings

TABLE OF CONTENTS

CHAPTER 1 Getting Started

Quick Hook-Up 1-3
Default Mode 1-4

CHAPTER 2 Functional Description

Block Diagram 2-1

CHAPTER 3 Communications

RS-485 3-2
Multi-party Connection 3-3
RS-485 Multidrop System 3-4

CHAPTER 4 Command Set

Table of Commands 4-6
User Commands 4-6
Error Messages 4-13

CHAPTER 5 Setup Information and Command

Command Syntax 5-1
Setup Hints 5-8

CHAPTER 6 Delay Time Programming

CHAPTER 7 Power Supply

CHAPTER 8 Troubleshooting

CHAPTER 9 Extended Addressing

CHAPTER 10 Transparent Mode

Chapter 1

Getting Started

Introduction

This manual describes the function and application of the Radio Modem
Interface Module (A2400). The A2400 provides an intelligent interface
between radio modems available from many manufacturers and devices
designed to operate on a bi-directional RS-485 serial bus. Although the
A2400 has been designed specifically for our family of industrial I/O
modules, it may also be used with other RS-485 devices.

Figure 1.1 depicts a typical application that incorporates A2400’s. In many
data acquisition situations, the sensor data is inaccessible to the host
computer due to large distances or the lack of telephone facilities to
incorporate conventional dial-up modems. In some cases, sensor data may
have to be monitored full time and the cost of telephone service can be
prohibitive. For these and a multitude of other reasons the use of a radio link
can be the best solution.

Unfortunately, radio modems are designed for computer-to-computer com­munications and require a certain amount of intelligence at each radio site
in order to construct useful systems. The cost of a local computer at each
radio can easily make the concept impractical. The Radio Modem Interface
Module (A2400) fills the need for a low-cost intelligent interface between the
radio modem and the RS-485 data acquisition devices.

In a typical system as shown in Figure 1.1, there is one host or master
computer and any number of slave sites. The master radio transmitter and
the slave receivers communicate on the same radio frequency, and of
course, the slave transmitters and the master receiver are tuned to the same
frequency. While it is common to use two frequencies for simultaneous
transmitting and receiving, it is possible to use one frequency for all
communications. In an idle condition all slave transmitters are turned off.
Each slave site is assigned a unique address so that the master may direct
commands to a particular site. To initiate a communications sequence, the
master will transmit a command by radio which is received by all the slaves.
The transmitted command contains an address which directs the command
to a particular slave site. The slave site that matches the address will
respond to the command. At this time, the addressed slave site will turn on
its radio transmitter and communicate back to the master in response to the
command. Once the response is complete the slave will turn off the
transmitter and wait for a new command. To avoid interference, only one
slave transmitter can be on at any given time. The primary function of the

Getting Started 1-2

A2400 is to control the slave transmitter to allow multiple slave sites.

Figure 1.1 System Overview.

Leased Lines

This manual has been written with emphasis on radio modems. However,
the A2400’s may be used just as effectively with leased telephone lines.
Typically, leased lines do not have dial-up capability and some means of
addressing and multiplexing must be employed if multiple stations are used.
A2400’s may be used with leased line modems in an identical manner as
with radio modems.

Getting Started 1-3

Getting Started

To get your A2400 up and running for an initial check-out, connect the unit
to a power supply and terminal as shown in Figure 1.2. The power supply
can be any dc source from 10 to 30 volts, capable of 1 Watt of power. The
terminal can be any RS-232 dumb terminal set for 300 baud. A computer
configured as a terminal can also be used. Be sure to ground the DEFAULT*
pin.

Figure 1.2 A2400 Quick Hookup.
After checking the connections, power up the A2400. Type the following

command on the terminal:

$1RD

Make sure to use upper case characters for the ‘RD’ (Read Data) command
and terminate the command with a carriage return. The A2400 should reply
with the message:

*+99999.99

Getting Started 1-4

This message is terminated with a carriage return. If the response message
cannot be obtained, re-check all the wiring, making sure that the proper
power is on the A2400 connector and that the DEFAULT* line is shorted to
the GND pin. The terminal must be set to 300 baud.

If, after several attempts, the response message does not appear, refer to
Chapter 8 Troubleshooting in this manual.

If you have an IBM PC or compatible computer, running the S1000 setup
software will ease the task of setting up the A2400 for your application.

After establishing communications with the A2400, read the manual and feel
free to experiment with the various commands and setups available. If
communications is lost due to an improper setup, returning back to the hook­up of Figure 1.2 will restore communications to the A2400.

Default Mode

The A2400 contains an EEPROM (Electrically Erasable Programmable
Read Only Memory) to store setup information. The EEPROM replaces the
usual array of switches necessary to specify baud rate, address, parity, etc.
The memory is nonvolatile which means that the information is retained
even if power is removed. No batteries are used so it is never necessary to
open the module case.

The EEPROM provides tremendous system flexibility since all of the setup
parameters may be configured remotely through the communications port
without having to physically change switch settings. There is one one minor
drawback in using EEPROM instead of switches; there is no visual indication
of the setup information in the module. It is impossible to tell just by looking
at the A2400 what the baud rate, address, parity and other settings are. It
is difficult to establish communications with a A2400 whose address and
baud rate are unknown. To overcome this, the A2400 has a pin labeled
DEFAULT*. By connecting this pin to ground (GND) the A2400 is forced to
a known communications setup called Default Mode.

The Default Mode setup is: 300 baud, no parity, any address is recognized.
Grounding the DEFAULT* pin does not change any of the setups stored in

EEPROM. The setup may be read back with the Read Setup (RS) command
to determine the communications parameters stored in the A2400.

An A2400 in Default Mode will respond to any address except the six illegal

Getting Started 1-5

values (NULL, CR, $, #, {, }). A dummy address must be included in every
command for proper responses.

Setup information in an A2400 may be changed at will with the SetUp (SU)
command. Baud rates and parity setups may be changed without affecting
the Default values of 300 baud and no parity. When the
DEFAULT* pin is released, the module automatically performs a program
reset and configures itself to the baud rate and parity stored in the setup
information.

The Default Mode is intended to be used with a single module connected to
a terminal or computer for the purpose of identifying and modifying setup
values. During normal operation, the DEFAULT* pin should be left open.

Chapter 2

Functional Description

Block Diagram

The A2400 is an RS-232/RS-485 converter specifically designed to inter­face D series RS-485 modules to radio modems. To this end the A2400
provides three functions:

1) Perform the RS-232 to RS-485 electrical conversion.

2) Control the data direction of the RS-485 bus.

3) Create hand-shaking signals to control the modem.

A simplified block diagram of the A2400 is illustrated in Figure 2.1. This
shows the RS-485 and RS-232 drivers connected to their respective
interface pins. The A2400 contains two UARTs (Universal Asynchronous
Receiver / Transmitters), one dedicated to each port. The RS-485 port is
connected to a UART that is integral to the supervisory microprocessor. The
RS-232 port connects to a second UART external to the microprocessor. All
data communicated to and from the A2400 on either port must pass through
the microprocessor. The micro controls the data flow depending on the
content of the data and setup information specified by the user.

The A2400 contains an Electrically Erasable Programmable Read-Only
Memory (EEPROM) which is used to store operating parameters specified
by the user. The EEPROM will retain the setup data even if power is removed
from the A2400. The EEPROM requires no battery and is guaranteed to
retain data for at least ten years. The setup data stored in the EEPROM
includes the baud rate, address, parity, timing data, etc. When power is
applied to the A2400, the internal microprocessor reads the setup data from
the EEPROM and automatically configures itself. The setup data may be
downloaded with a terminal or computer connected to the RS-232 port.

+5V

Functional Description 2-2

RX

TX

MICRO­PRCESSOR

UART

+5

DEFAULT

RTS

CTS

RX

TX

EEPROM

Figure 2.1 A2400 Block Diagram.

5.6K

DO0/RTS

Functional Description 2-3

Pinout

1) TRANSMIT This is the RS-232 Transmit output from the A2400. This
pin is normally connected to the Receive input of a modem. This output is
also used to connect to a terminal or computer to configure the A2400

2) RECEIVE This is the RS-232 Receive input of the A2400. This pin is
normally connected to the Data Output of a radio modem. This input is also
used to connect to a terminal or computer to configure the A2400

3) RTS RS-232 Request To Send output. This output is used to
control the transmitter of the modem which allows multiple transmitters to
exist on the same system. The RTS output is typically connected to the RTS
input of the modem. The timing of the RTS signal is user-configurable with
the T1, T2, T3 commands. The polarity of the RTS signal may be configured
with the Setup (SU) command.

4) CTS This is the RS-232 Clear To Send input. Some modems
provide a signal to indicate that the transmitter is ready after the RTS line has
been asserted. This ready signal may be connected to the CTS input to
provide a hardware handshake to provide a fast turn-around time. If the CTS
line is not used, it may be left open and delay time T2 will function as a
software handshake.

5) DO0/ARTS This is a digital output that can be configured to perform two
functions. The function configuration is set by a bit in the Setup (SU)
command. Normally this pin is configured as a general-purpose digital
output. It may be turned on and off by commands from the host computer.
The exact circuit schematic of the output is shown in Fig. 2.1. This circuit
provides an output that is TTL and CMOS compatible. The diode in the circuit
allows the collector to be pulled up to 30 volts to interface to relays and other
higher-voltage devices. This output may also be configured as an Alternate
Request To Send signal. With this setup, the ARTS signal exactly mimics the
RTS output. This output may be used when a TTL signal is desired or a high­current open-collector signal is necessary.

6) DEFAULT* By grounding this pin, the A2400 is placed in a known
communication setup. This is essential if the baud rate and address of the
A2400 are not known. The default communications setup is: 300 baud, no
parity, any address is recognized. The DEFAULT* pin should be grounded
only if the A2400 is being setup or configured. In normal operation this pin
is left open.

7) (Y)DATA+ This is the positive polarity signal of the differential RS-485

Functional Description 2-4

bus. This bus connects to multidrop RS-485 devices such as D series
modules.

8) (G)DATA- This is the negative polarity of the differential RS-485 bus.

9) (R)V+ A2400 power connection. The A2400 operates on 10 to 30
volts dc.

10) (B)GND This is the ground connection common to all circuits. The
A2400 does not have isolation between power and the two communications
ports.

Note that pins 7 through 10 are designated Y, G, R, and B respectively. This
corresponds to the Yellow, Green, Red, and Black colors normally found in
common telephone cable. All D series RS-485 devices carry this nomencla­ture. If all yellow connections are connected together, and green to green,
etc., the RS-485 wiring will be correct.

RS-485 Termination

The RS-485 port lines are terminated as shown in Fig. 2.1. The 220 ohm
resistor is used as a transmission line terminator to improve signal fidelity.
A similar 220 ohm resistor should be installed at the far end of the RS-485
bus. The 1 K ohm resistors are used to bias the signal lines to a Mark
condition. This is necessary when all transmitters of the RS-485 bus are off,
which is the most prevalent condition. The resistor bias helps to prevent
noise pickup.

The RS-485 port is protected from potentially destructive voltages with
positive temperature coefficient thermistors and transient suppressors.

A2400 Operation

A typical installation is shown in Figure 3.1(see Chapter 3). The A2400 is
used to provide an interface between a modem and a string of standard
modules. Each module and the A2400 has a unique address. In a typical
communications sequence the host computer and radio (not shown) will
transmit a module command over the air. The modem receiver picks up the
message data and presents it to the A2400 on the RS-232 port.

Character Filter

Due to the nature of radio data transmission, noisy data is almost always
present at the output of the modem. This is due to inadequate squelch or
noise generated when radio transmitters are turned on and off. The A2400
uses noise reduction techniques to reduce the possibility of bad characters

Functional Description 2-5

reaching the RS-485 bus.
The first operation performed on the modem data is to check for noise and

framing errors. If either condition exists, the bad character is re-formatted as
a null character (ASCII $00). Since the null is not a legal character for use
as an address in the modules, transmitting a null is preferable to aborting the
character when an error is detected. This cuts down on the possibility of a
module being incorrectly addressed.

After noise and framing errors are checked, the data must be qualified with
a character filter. This filter will flush any data until a valid prompt character
is detected. These characters are: ‘$’, ‘#’, ‘{‘, and ‘}’. Parity is ignored. Once
a prompt character is received, The A2400 assumes that a valid command
is being transmitted. The A2400 will transfer the data to the RS-485 bus or
hold it in an internal buffer depending on the address data. At this time the
character filter checks for a carriage return character which terminates the
command. If a carriage return is detected, the character filter is reset to flush
characters until another valid prompt is found. If 32 characters are received
after a prompt without a carriage return found, the data is considered to be
noisy and the character filter is reset for prompt detection.

The character filter may be disabled for non-D series systems.
The qualified data may take one of two paths depending on the address

data. Commands addressed to the A2400 itself are not transmitted on the
RS-485 bus. Therefore the prompt character is saved in buffer memory until
the address character can be examined. If the A2400 detects its own
address, the subsequent command data is processed internally to the
A2400. No data will appear on the RS-485 bus.

If the address does not match the A2400, the prompt and address charac­ters are transmitted to the RS-485 port, along with any subsequent data until
a carriage return or character over-run occurs.

The A2400 ignores parity on all data except for commands addressed to
itself.

The RS-485 port is normally in receive mode, and when the A2400 places
data on the bus it enables the RS-485 transmitter on a per-character basis.
This means that the port is returned to receive mode immediately after a
command has been transmitted.

Assume that a module on the RS-485 bus has received a correctly

Functional Description 2-6

addressed command and it responds back with information on the bus. The
A2400 receives this information and places it in a buffer that can hold up to
96 characters. The parity of received characters is ignored. As soon as a
character is received, the A2400 starts a timing sequence to control the
modem transmitter. Three user-programmable timers, T1, T2, and T3
control the data flow. See Figure 2.2

Figure 2.2 Programmable Delay Times.

.

Functional Description 2-7

T1

As soon as the A2400 detects a character in the RS-485 receive buffer, time
delay T1 is activated. This is a dead time to allow the host to prepare for the
receipt of a message. This is particularly important when a simplex connec­tion is used, where the send and receive data is transmitted on the same
frequency. During this time the A2400 creates no control output, but any
data received on the RS-485 port is stored in the receive buffer. At the end
of time T1, the A2400 asserts the RTS control signal to ‘key’ or turn on the
transmitter of the radio.

T2

Once the RTS signal has been asserted, the T2 delay is activated. This is
a delay time to allow the transmitter to power up and settle in anticipation of
a transmission back to the host. The settling time required is specified by the
modem manufacturer. When the T2 time period is over, the A2400 will start
to transmit the data held in the receive buffer, and will continue to transmit
until the buffer is empty.

Some radio modems provide a CTS (Clear To Send) signal that indicates
that the transmitter has settled and is ready for data. This signal may be
connected to the CTS input of the A2400 to provide hardware handshaking.

The delay period T2 ends when either the CTS signal is detected or the T2
timer ends, whichever comes first.

T3

The A2400 will transmit data to the modem until the receive buffer is empty.
When the receive buffer is empty, and the last character has been transmit­ted, time delay T3 is activated. T3 provides two functions: it provides a clean
break between the transmitted data and the turn-off of the radio transmitter,
and it allows the host to poll more data without keying the transmitter on and
off. During time T3, the host may transmit another command which would
pass through the A2400 to the RS-485 bus. Typically, this command would
generate a response from a device on the RS-485 bus. If the response data
is received by the A2400 before T3 is complete, T3 is canceled and the
received data is immediately transferred to the modem. When the receive
buffer is empty, T3 is activated again, and the cycle repeats itself. This allows
the host to establish communications with the remote radio and talk back
and forth without wasting time re-keying the transmitter for each response.

If T3 times out and the receive buffer is empty, the radio connection is
terminated by turning off the RTS signal.

Chapter 3

Communications

Introduction

The A2400 modules have been carefully designed to be easy to interface to
all radio modems and many leased-line modems. All communications to and
from the modules are performed with printable ASCII characters. This allows
the information to be processed with string functions common to most high­level languages such as BASIC. The ASCII format makes system debugging
easy with a dumb terminal.

This system allows multiple modules to be connected through the A2400 to
a modem with a single 4-wire cable. Up to 32 RS-485 modules may be strung
together on one cable; 121 with repeaters. The modules communicate with
the host on a polling system; that is, each module responds to its own unique
address and must be interrogated by the host. A module can never initiate
a communications sequence. A simple command/response protocol must
be strictly observed to avoid communications collisions and data errors.

Communication to the A2400 modules is performed with two- or three­character ASCII command codes such as RD for Read Data. A complete
description of all commands is given in the Chapter 4. A typical command/
response sequence would look like this:

Command: $1RD
Response: *+99999.99

A command/response sequence is not complete until a valid response is
received. The host may not initiate a new command until the response from
a previous command is complete. Failure to observe this rule will result in
communications collisions. A valid response can be in one of three forms:

1) a normal response indicated by a ‘ * ‘ prompt

2) an error message indicated by a ‘ ? ‘ prompt

3) a communications time-out error

When a module receives a valid command, it must interpret the command,
perform the desired function, and then communicate the response back to
the host. Each command has an associated delay time in which the module
is busy calculating the response. If the host does not receive a response in
an appropriate amount of time specified in Table 3.1, a communications
time-out error has occurred. After the communications time-out it is assumed
that no response data is forthcoming. This error usually results when an

Communication 3-2

improper command prompt or address is transmitted. The table below lists
the timeout specification for each command assuming that delay times T1,
T2, T3 = 0:

Table 3.1 Response Timeout Specifications.
Mnemonic Timeout
DO, OC, CC, RD, REA, RID, RLP, RS, RSP, RSU, ≤ 10 ms
RT1, RT2, RT3, WE
EA, ID, LP, RID, RR, SP, SU, T1, T2, T3 ≤ 100 ms

The timeout specification is the turn-around time from the receipt of a
command to when the module starts to transmit a response.

Data Format
All A2400 modules communicate in standard NRZ asynchronous data
format. This format provides one start bit, seven data bits, one parity
bit and one stop bit for each character.

RS-485

RS-485 is a recently developed communications standard to satisfy the
need for multidropped systems that can communicate at high data rates
over long distances. RS-485 is similar to RS-422 in that it uses a balanced
differential pair of wires switching from 0 to 5V to communicate data. RS-485
receivers can handle common mode voltages from -7V to +12V without loss
of data, making them ideal for transmission over great distances. RS-485
differs from RS-422 by using one balanced pair of wires for both transmitting
and receiving. Since an RS-485 system cannot transmit and receive at the
same time it is inherently a half-duplex system. RS-485 offers many
advantages over RS-232:

1) balanced line gives excellent noise immunity

2) can communicate with modules at 38400 baud

3) communications distances up to 10,000 feet.

4) true multidrop; modules are connected in parallel

5) individual modules may be disconnected without affecting
other modules

6) up to 32 modules on one line; 124 with repeaters

7) no communications delay due to multiple modules

8) simplified wiring using standard telephone cable

An RS-485 system usually requires an interface such as the A2400 to
convert RS-232 to RS-485.

Communication 3-3

Communication 3-4

RS-485 Multidrop System

Figure 3.1 illustrates the wiring required for multiple-module RS-485
system. Notice that every module has a direct connection to the A2400. Any
number of modules may be unplugged without affecting the remaining
modules. Each module must be setup with a unique address and the
addresses can be in any order. All RS-485 modules must be setup for no
echo to avoid bus conflicts (see Setup). Also note that the connector pins
on each module are labelled with notations (B), (R), (G), and (Y). This
designates the colors used on standard 4-wire telephone cable:

Label Color
(B) GND Black
(R) V+ Red
(G) DATA- Green
(Y) DATA+ Yellow

This color convention is used to simplify installation. If standard 4-wire
telephone cable is used, it is only necessary to match the labeled pins with
the wire color to guarantee correct installation.

To minimize unwanted reflections on the transmission line, the bus should
be arranged as a line going from one module to the next, starting with the
A2400. ‘Tree’ or random structures of the transmission line should be
avoided. For wire runs greater than 500 feet total, the end of the bus should
be terminated with a 220Ω resistor connected between DATA+ and DATA­. The A2400 has a resistor built in to terminate the start of the bus.

When using a bi-directional RS-485 system, there are unavoidable periods
of time when all stations on the line are in receive mode. During this time,
the communications lines are left floating and are very susceptible to noise.
To prevent the generation of random characters, the lines are biased in a
MARK condition as shown in Figure 2.1. The 1K resistors are used to keep
the DATA+ line more positive than the DATA- line when none of the RS-485
transmitters are on. When enabled, the low impedance of an RS-485 driver
easily overcomes the load presented by the resistors.

Special care must be taken with very long busses (greater than 1000 feet)
to ensure error-free operation. Long busses must be terminated as de­scribed above. The use of twisted cable for the DATA+ and DATA- lines will
greatly enhance signal fidelity. Use parity and checksums along with the ‘#’
form of all commands to detect transmission errors. In situations where
many modules are used on a long line, voltage drops in the power leads

Communication 3-5

becomes an important consideration. The GND wire is used both as a
power connection and the common reference for the transmission line
receivers in the modules. Voltage drops in the GND leads appear as a
common-mode voltage to the receivers. The receivers are rated for a
maximum of -7V. of common-mode voltage. For reliable operation, the
common mode voltage should be kept below -5V.

To avoid problems with voltage drops, modules may be powered locally
rather than transmitting the power from the host. Inexpensive ‘calculator’
type power supplies are useful in remote locations. When local supplies are
used, be sure to provide a ground reference with a third wire to the host or
through a good earth ground. With local supplies and an earth ground, only
two wires for the data connections are necessary.