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Special Issue
Modbus Organization celebrates its fourth birthday this
our eighth tradeshow, ISA EXPO 2006, and we are
year. In October 2002, the organization was formally
delighted to remind you that:
incorporated and began its support of the ubiquitous
Modbus is the World’s No. 1 industrial protocol,
Modbus protocol with one member (guess who?) and the
with the largest installed base.
following mission:
to drive the adoption of the Modbus
Modbus TCP/IP is the No. 1 protocol for industrial
communication protocol suite and the evolution to address architectures
for distributed automation systems across multiple market segments.
Modbus is open and royalty free.
As the industrial communications protocol with the largest
Modbus is easy to deploy, scalable, and requires a
number of installed nodes worldwide, we pride ourselves
very small footprint.
in being a service-oriented organization.
Modbus is media independent, with implementations
Our membership fees are modest; our certification fees
over Ethernet, RS-232/485 serial links, wireless, fiber
are modest; and we make the Modbus protocols and
optic, radio, cellular...
Implementation Guides freely available for download on
our web site,
Hundreds of devices that use the Modbus protocol
are available on the market.
Paul Gaugin’s famous painting titled,
Where Do We Come
From? What Are We? Where Are We Going?
is a great
At you will find:
reminder that organizations are always changing and
Free discussion forums for users and developers
Technical resources such as the Modbus protocol
Birthdays are a good time to reflect. What have we
standard documents, and links to tutorials and free
accomplished? Where do we hope to go from here? Since
Modbus drivers
October 2002, the organization has grown to over 50
Hundreds of Modbus serial line and TCP/IP devices
members, including companies from around the world.
catalogued for easy identification of potential devices/
We continue to build our membership, inviting companies
software for your application
that supply Modbus devices and software to join and take
Modbus System Integrator’s Directory, and more...
advantage of our growing device directory and systems
integrator database.
If you happen to be at ISA EXPO 2006 October 17-
19, stop by and wish us Happy Birthday!
Users looking for particular devices for their applications
frequently contact us after finding potential
components for their systems on our website, and
we put them in contact with members that supply
the types of devices they need.
In 2004, we released the first version of the
Modbus TCP/IP Toolkit, which is a free benefit to
members, and available for a fee to all others. This
year we released an updated version.
We continue to offer certification services, and list
certified devices preferentially in our device
Where Do We Come From? What Are We? Where Are We Going?
directory. We plan to offer local services in addition
Paul Gauguin, 1897–98
to the laboratories in the United States and in China
before the year ends.
oil on canvas, Museum of Fine Arts, Boston
image © 2006 Museum of Fine Arts, Boston
This October, as we turn four, we are exhibiting in
Where Do We Come From?
What Are We? Where Are We Going?
News about the World’s Most Popular Industrial Protocol
Organization News • Organization News
Meet Some of Our Members...
Modius, Inc.
is a technology company
aimed at solving the challenges of
industrial device interoperability,
integration of devices with enterprise
business applications, and access to
these devices using open standards over
IT networks. Modius’ OpenData™
solve the
challenges of
across the
enterprise by enabling businesses to
collect important data over the web
with a single, consolidated view of all
critical infrastructure devices, regardless
of location, device type, or
manufacturer. OpenData technology
provides secure network routing of
health and performance data for
business-critical power and
infrastructure devices, including:
uninterruptible power systems,
generators, computer room air
conditioning, power distribution units,
switch gear, automatic transfer
switches, fire detection and suppression
systems, static switches, building
automation systems, and other
equipment types.
Comtrol Corporation
is a worldwide
leader of device connectivity for
industrial applications. In 1982, Comtrol
pioneered device integration by
inventing, HOSTESS®, the industry’s
first multi-port serial controller card for
personal computers. The company then
released RocketPort® - the first multi-
port serial controller offering high
performance at a low cost.
Comtrol now provides serial device
control technology with its
DeviceMaster® device servers that
provide Ethernet-attached serial ports.
Devicemaster is programmable so that
it can run applications that enable it to
eliminate the need for standard PCs on
the factory floor.
Comtrol’s technology is involved in
many postal automation systems,
baggage handling systems, power utility
automation, automotive assembly
facilities, as well as other machine
control and data acquisition
Comtrol manages communications for
PLC or PC platforms, between plant
floor devices and information
management systems. Comtrol’s
platforms support protocols for serial
devices including RFID readers, weight
indicators, bar code scanner and label
printer/applicators. Comtrol partners
with peripheral manufacturers, OEMs,
software developers, industrial
distributors and other industrially
focused organizations to provide
systems integrators and OEMs the right
tools for data communications and
device connectivity.
Precision Digital Corporation
been in business for 30 years. The
company’s product line consists of a
wide variety of digital display
devices, many with Modbus interface
capabilities. These products include
digital panel meters, PID
temperature controllers, and multi-
input controllers. Standard process
(4-20 mA) and temperature (T/C &
RTD) inputs are available, as well as
relay and 4-20 mA output options.
Become a
see back cover for
Modbus-IDA Discussion Forums
Modbus Slave Behavior...
On September 8th an anonymous
poster wrote:
I’m new to Modbus, can someone help
to answer the question below?
If a Modbus slave receives the second
request before it sends a reply for the
first request, what will Modbus the
slave do? Will it send two replies
(reply1, reply2) or will it only send a
reply for request 2?
Jerry Miille replied:
Excellent question! Going to be
interesting to see the responses to this
In theory this should not happen but in
the real world, it can, and it does all the
The answer for you is that the slave
should respond to all VALID messages
in the order that they arrive. I am
defining VALID here as a message that
has a good CRC or BCH AND is
addressed to the remote (broadcast
messages do not get a response
anyway). The Slave should never throw
a valid messages away, ever. Just take
the Master messages in one at a time,
process them and return the answer.
This is not a slave problem. A slave
should always answer a valid message in
the order it was received. The slave has
no control over anything. It can only
respond when spoken to and that is
This a Master problem and it can lead
to all sorts of problems.
CH added:
The main thing you should know is that
the Modbus master should only issue a
second request either after a timeout, or
after a response (be it successful or not).
So, it sounds like your master device
should up its response timeout.
No telling what the slave device will do,
as it all depends on how the slave
From the Modbus Discussion Forums…
manufacturer coded its Modbus driver,
whether it can buffer incoming
commands, etc. I would suspect it will
choke on the second request.
Chris suggested:
With the Modbus Network, there
should be one master, and up to 247
slaves. The master will initiate a query to
a slave and wait for a response. If no
response from the slave is received
within a timout period, the master will
abort the current query and get the next
message query to transmit.
A master should not transmit another
query until it has either received a
response from the slave, or the message
has timed out. If the slave cannot
respond within the timeout period, it
should not send a response.
Francois said:
On a Modbus TCP network there can
be several masters...
Johnson added:
I think the previous question was
regarding the capacity of Master to
send/initiate several queries at the same
time to a slave (without waiting for
previous response) by using Modbus
Plus or TCP and not about the number
of Masters allowed.
Is it right or not?
An anonymous additional question:
Just a curiosity: Does this behavior
happen/apply to Modbus RTU or is
the same for Modbus Plus and
Modbus TCP/IP?
A response from Modbus-IDA:
This behavior is defined for Modbus/
TCP, where several queries can be sent
and be pending on the same
connection or on different connections.
The transaction identifier (that must be
unique on a connection) is used to pair
the queries with the responses. There is
no guarantee that the responses will
respect the order of query submission;
this is implementation dependent and
not required.
The number of concurrently open
connections is also implementation
dependent, as is the maximum number
of pending queries per connection.
For Modbus ASCII or RTU the
protocol works with alternating query/
response, with time-out (excluding
broadcast, that has no response).
The client should not send the next
query before deciding on the
whereabouts of the previous query.
The time-out duration is
implementation dependent. By the time
the client sends the next query there is
no longer any expectation about the
response to the previous query. Such
response has either been received
(successful or not as far as the
application is concerned) or it timed
The application reaction to a time-out
is application/implementation
If a response arrives and it is not
expected (e.g., after a time-out), then it
is discarded, like if the query for it had
never been sent.
If a response arrives late and there is an
expectation from a subsequent query,
then it is expected to be the response to
that subsequent query, and an error will
be issued if it is not (based on the
possible recognition).
Most ASCII/RTU servers in the field,
while processing a query, silently drop
any query after the first, until after the
response to the first query is sent.
Ask your question or help
out a fellow engineer on the
Modbus Discussion Forums:
Modbus Products and Innovations
New 802.11b/g Connectivity for
SeaI/O Data Acquisition Modules
Sealevel Systems, Inc.
expanding its popular SeaI/O data acquisition line with the introduction of new
wireless SeaI/O modules at the ISA 2006 exhibition in Houston. Choose from a
variety of I/O options including Reed and Form C relays, optically isolated inputs,
TTL interface to industry standard solid-state relay racks, A/D and D/A
functionality. Connect to your SeaI/O network from any 802.11b/g host. All
wireless SeaI/O modules offer WEP and WPA encryption for secure, reliable
For maximum configurability, multiple units of any I/O type can be daisy chained
together easily using convenient pass-through connectors. Removable terminal
blocks are standard, enabling fast, versatile field wiring. SeaI/O modules operate
from 9-30VDC, and power can be input via terminal block or DC jack. For easy
Anybus X Gateway
software integration, application programs or 3rd party software can use the Sealevel
SeaMAX library or industry-standard Modbus protocol. Both table mount and DIN
rail mounting options are available, and installation is easy using Sealevel’s software configuration tools.
For more information, visit
NetBiter® webSCADA Gateway
IntelliCom Innovation AB, is proud to announce a new web gateway with built-in alarm manager and data logger. The
NetBiter webSCADA Gateway is an industrial serial to Ethernet gateway that gives remote access to serial devices over
Ethernet, Internet, LANs, telephone modems, GSM and GPRS. The gateway contains built-in features for alarm handling
(SMS, e-mail, SNMP), data logging and web-based data access.
The NetBiter® webSCADA Gateway’s built-in web server is
used for all configuration and data presentation through a
standard web browser. Users can access devices easily and
securely from anywhere at anytime.
The device’s main features include:
GSM, GPRS and modem support
Built-in I/Os
Built-in web server for data access using standard web
Alarm / Status management by email, SMS and SNMP
Data logging into built-in memory with historical trend graphs
No Windows tools or HTML editors are needed
No licenses or royalties
OPC Server available
Transparent Modbus RTU/TCP Gateway
Multi language support
webSCADA Design Kit available for designing custom web pages
For more information, visit
NetBiter webSCADA Gateway
Modbus Products and Innovations
TMW Announces: Modbus .Net Component
Triangle MicroWorks new .NET component for Modbus can be used with any .NET language (e.g., VB.NET, C#,
Managed C++, J#), and is an ideal way to add Modbus support to your .NET applications. The component supports
Modbus serial, Modbus RTU, Modbus TCP, and (with the proper hardware)
Modbus Plus.
Purchase includes one year of Maintenance and Enhancement Plan (additional
years may be purchased), which provides technical support and free updates.
Support includes assistance integrating the .NET Component into your product as
well as assistance troubleshooting any communications problems that may occur in the field.
Key features include:
Conformance to Modbus Application Protocol Specification V1.1 (including all data types and function codes
required to pass the Modbus-IDA Conformance Tests)
No royalty fees per unit sold
Supports any physical communication network including RS232/485 (for Modbus RTU and Modbus ASCII),
Modbus Plus, and TCP.
Simple configuration for byte order: most-significant-first (Motorola) or least-significant-first (Intel)
Extensive built-in diagnostics including a protocol analyzer to visually display and decode protocol messages.
Records communication protocol errors such as “Unsupported function code,” “Database errors,” “Address range
errors,” “Exception response, FC=xxx, Exception Code = xxx”
Both runtime and source code versions are available.
Based on .NET 2.0.
For more information, visit
Advertise Your Modbus Products
on the Modbus-IDA Website
With our growing number of site visitors and the increasing popularity of our device directory, what
better place to advertise your Modbus devices and software than at
All banner ads must conform to the following specifications:
File size: 20k maximum
Maximum dimensions: 468 pixels x 68 pixels
File type: Graphics Interchange Format (GIF) or animated GIF
Resolution: 72 dpi
Color depth: 8 bit (256 colors)
Every banner must include a working URL.
Please note that we will not post ads. with cycle rates or animation modes that are irritants to our
visitors. We would be happy to help you design an effective Web ad with acceptable cycle rates and
types of animation.
Contact for a rate sheet.
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