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the architecture for distributed automation
In a striking affirmation of the ubiqui-
tous Modbus protocol, the Standardiza-
tion Administration of China (SAC) has
formally launched the following three
standards for industrial automation in
the People’s Republic of China:
GB/Z 19582.1-2004 Modbus Industrial
Automation Network Specification Part
1 – Modbus Application Protocol,
GB/Z 19582.2-2004 Modbus Industrial
Automation Network Specification Part
2 – Modbus Protocol Implementation
Guide over Serial Link, and
GB/Z 19582.3-2004 Modbus Industrial
Automation Network Specification Part
3 – Modbus Protocol Implementation
Guide over TCP/IP.
ARC Forum 2005 • Special Issue
have been working closely with
Schneider Electric as they develop and
propagate the use of standards in their
industrial infrastructure. In conjunction
with launching the Modbus standard, a
conformance test laboratory is antici-
pated to open in China within the next
few months.
Ken Crater, president of Modbus-IDA,
noted, “It makes sense that the Chinese
industrial market adopt Modbus as its
fieldbus standard. Simplicity and ease
of integration make it a natural choice.
The fact that it is already widely used in
China was also an important factor in
the standardization decision.”
Modbus Accepted as Chinese
Mr. Ouyang Jinsong, Director of
Standardization, Department of
Instrumentation Technology &
Economy Institute, People’s Republic
of China, and General Secretary of the
National Standardization Technical
Committee on Industrial Process
Measurement and Control, confirmed
the standards were effective October
26, 2004.
The news was welcomed by Modbus-
IDA, the international organization for
suppliers, implementers and users of
Modbus and its companion protocols.
The Chinese standardization authorities
Modbus-IDA at ISA EXPO 2004
Modbus-IDA opened for
the third year in a row at
ISA EXPO with an
expanded presence in more
ways than one. In a 400-
square-foot booth,
ACT’L, Control
Technology Corporation,
Harting USA, HMS
Industrial Networks,
Niobrara R&D, and
Schneider Electric met with
customers, prospective
clients, and fellow exhibitors
to showcase their Modbus-
based products.
With more than twice the
number of members as this
time last year, Modbus-IDA
promoted its own activities
and its members. The booth
featured a 20-foot-high multi-colored
banner display and a presentation that
illustrated the organization’s growth and
its members’ logos on an 8 x 8 foot rear
projection screen suspended over the
main aisle.
Members Precision Digital Corporation
and Scadaware participated through the
Show Visibility
Program, which allowed Precision
Digital to augment its presence in its
own booth elsewhere in the show.
Scadaware, one of Modbus-IDA’s
systems integrator members, took
advantage of this program to meet with
customers and chat with visitors
looking for expert assistance solving
challenging applications problems.
inside back cover)
News about the World’s Most Popular Industrial Protocol
Standards Activities • Standards Activities
Modbus-IDA Technical Committee News
Modbus-IDA’s technical committees are
now actively working in several areas.
IT Infrastructure
group had its
first meeting in November 2004.
Chaired by Dominic Iadonisi of
Hirschmann Electronics USA, the
group began to lay out issues on which
it plans to focus. These include:
Types of identified networks (e.g.,
standalone, non-interlocked);
Network topologies (daisy-chain,
tree, mesh, ring, star);
Required switch support (port
autonegotiation, auto cross, 10/
Desired switch requirements (e.g.,
SNMP, traffic thresholding, port
Standard LED requirements;
Alarm signal contact requirement;
Redundant power inputs; and
Specifications for cable types and
connector types, for both copper
and fiber cables
The group plans to meet again during
Q1 2005. Please contact Modbus-IDA
by e-mail if you wish to join
Conformance Test Policy
has been meeting monthly to further
define and develop Modbus-IDA’s
conformance testing program. In
recent discussions, the group has
elected to suspend the Conformance
Test Program and redesign it, adding
some physical layer testing and consid-
ering more options for interoperability
testing. Additional work on conform-
ance testing for Modbus over serial line
is also underway.
The group is considering a self-test
option, which would allow companies
to purchase the test suite and certify
devices under a strict set of require-
ments. Devices certified under the self-
test option would be identified by a
separate certification seal. An outside
test laboratory option will still be
available for those companies desiring
third-party certification and to verify
the validity of self-test program
Committees Work
on Modbus’ Future
Join one of
technical groups:
IT Infrastructure
Conformance Test
Device Description
Safety Layer
Real Time
Contact Ken Crater
for more information
or to join a Modbus-
IDA committee or
technical group.
The Modbus-IDA Newsletter
This is the newsletter of Modbus-IDA, the international nonprofit organiza-
tion devoted to the evolution and support of the Modbus and IDA protocols.
For more information about membership and other services of Modbus-IDA,
please refer to our website:
Newsletter Editor: Lenore Tracey
Copyright 2005 by the Modbus Organization, Inc.
PO Box 628, Hopkinton, MA 01748 USA
ph +1-508-435-7170 • fax +1-508-435-6929
Your Website
Ideas are needed!
In an effort to make more Modbus
information available to you more
easily, we are redesigning the
Modbus-IDA website. We are
adding more product announcements,
more device listings, more news.
E-mail any ideas you have for
features, functionality, or other
useful information for Modbus users
and developers to info@modbus- We look forward to making
the website work for you.
The Modbus-IDA Mission
Modbus-IDA is a group of independent users and suppliers of automa-
tion devices that seeks to drive the adoption of the Modbus communica-
tion protocol suite and the evolution to address architectures for distrib-
uted automation systems across multiple market segments. Modbus-IDA
will also provide the infrastructure to obtain and share information about
the protocols, their application and certification to simplify implementa-
tion by users resulting in reduced costs.
Standards Activities • Standards Activities
From the Modbus
User Forum…
Modbus frames: serial vs. TCP...
Sentekin Can wrote: How does serial
Modbus message, which is variable
length, form into fixed length Ethernet
Jiri Baum replied, “The TCP part of
TCP/IP simulates a continuous
connection. Just ignore the whole frame
thing and send your data. The TCP/IP
stack will break it into packets, send
them, retransmit them if they get lost
or corrupted, etc.”
Another reader suggested, “Modbus
TCP is Modbus RTU that has been
enveloped into a TCP data header.
Layer 1-6 is all TCP/IP data header.
Layer 7 is the application layer, which
holds the Modbus RTU data frame.
The only difference is that NODE
ADDRESS=1 as default.
“Function code is Modbus function
code to read or write same as RTU;
target register is the same as RTU;
number of registers is same as RTU;
data out is same as RTU and data in is
same as RTU; and the CRC check is the
same as RTU. The Modbus message is
not fixed. Many people will send a
Modbus TCP message over Ethernet to
serial converter, which strips out the
TCP/IP header and sends the Modbus
RTU message to the remote serial
device. By changing the Node address
to the remote node address this makes
it possible to communicate to serial
devices over Ethernet.”
Lynn Linse added, “I am not sure what
you mean by ‘fixed length Ethernet
frame.’ Ethernet has a minimum size to
enable collision detection (I think it’s 64
bytes?). However the link, IP, and TCP
headers occupy most of this. Any
Modbus/TCP message will be larger
than this min size.
Max Modbus serial message is 255 bytes
(3 + 250 + 2); Max Modbus/TCP
message turns out to be 259 bytes (6 +
3 + 250); Max Ethernet frame is 1500
bytes (given no fragmentation). So
Modbus creates no size problems under
“Or perhaps you mean since Modbus
RTU’s length is ‘felt out’ by watching
for time gap; how is Modbus/TCP
length detected? The Modbus/TCP
header includes a length byte, so it is
explicitly defined within the first six
“In other words, the Master in your
case does not have the slightest idea of
where the ‘reply’ came from (the store
& forward machine in your case) – it
simply assumes it is from the slave and
processes it as such.
“As a practical solution, you may want
to insert a delay, so your Master is
‘deaf ’ for the amount of time the store
& forward machine ‘talks’.”
Gregoire Belliveau wrote to the Forum:
“We are looking for information about
running Modbus TCP/IP over a
satellite network. Our network typically
has a 590-ms round trip delay time. We
are looking at running GE’s Multilin
software communicating to various GE
MultiNet (or similar) devices at the far
end. As for the MultiNet, we believe
that a device with remote IP
programming ability (similar to
Lantronix) would be a better fit. Any
suggestions here?î
Rainer Lehrig answered, “The Modbus
protocol works with request/response.
Thus you will have a big delay because
of your round trip time.
“It would be better to implement a
more intelligent server(s) at the local
places. These server(s) could talk to
Modbus. e.g., read your variables
Ask your question or
help out another user
on the Modbus-IDA
Community Forums.
For users:
For developers:
Modbus TCP/IP over Satellite Nework...
cyclically and send them to your remote
site without request/response. Only
send data across the satellite network in
one direction!
John Rinaldi responded,
Have you
tried it yet? I can’t think of any reason
why it wouldn’t work. There are two
timeouts to consider. One, the TCP
connection timeout. This is typically in
the neighborhood of two seconds. The
other is of course the Client timeout.
What kind of trip time will the satellite
give you? Can you modify these
timeouts to be 20-50 percent more than
the trip time?î
Finally, Micahel Weaver suggested, “You
should go to industrial standard TCP/
IP configuration and you should [have]
no problem with communication. It
would be transparent to the processor.î
Modbus Products and Innovations
Prosoft Technology Helps Take
Out the Ash
A thermal power station in Mexico needed Modbus communication between
Allen-Bradley PLC5 processors and the pneumatic transport system for light ash.
The hot light ash is retained from flue products and discharged into special
containers, which are
air into a larger bin. The system also consists of
emptied by
compressors, fans, and air filters. Once the ash is placed in the bin it is
transported to either an ash disposal area or to a cement works for cement
The system is very sensitive to control system outages. If a failure occurs, pipes
can be filled with hot ash, which will harden. To avoid outages, the system was
equipped with a redundant processor.
The control system was a redundant solution of PLC5s communicating over a
ControlNet bus with RIO modules.
System Integrator, AUTEL knew that since Modbus was the communication
protocol between the PLC and the DCS supervisory system, a Modbus
Communication Module was needed. Autel selected the Prosoft 3100-MCM. A
3100-MCM, functioning as a Slave, was placed in each PLC. The 3100-MCM
modules contain two ports.
HMS Releases FIPIO and
ModbusRTU Slave Versions of
Anybus Communicator
The addition of FIPIO and ModbusRTU on the fieldbus side of the Anybus
Communicator extends the gateway concept to a total of nine different
The ModbusRTU slave side is available for both RS232 and RS485.
The AnyBus Communicator Gateway is an external DIN-rail mounted gateway
that provides fieldbus network connectivity on one side and a configurable serial
interface on the lower side, e.g., Modbus RTU master. The gateway can therefore
work as a data concentrator on a Modbus network being Modbus RTU master on
one side and a Modbus RTU slave on the other side.
In the sub network side it has serial protocol support for Modbus RTU Master
Mode and Generic Data Mode, which is fully configurable through the ABC
Config Tool for Windows and the
configuration port on the AnyBus
Communicator. Other protocols can be
implemented by HMS on request.
...the listings of Modbus
compatible products at
Modbus Products and Innovations
Sealevel Systems I/O
Device Certified
Sealevel Systems, Inc.
(, recently submitted
for Modbus conform-
ance testing. The South Carolina-based
company has manufactured quality
Serial and Digital I/O products since
The SeaI/O is a modular I/O system
that offers
and a wide
variety of I/O types for distributed
control and data acquisition require-
ments. An array of configurations is
available, each designed for maximum
flexibility and easy field wiring. Order-
ing options allow connection to the
host device via Ethernet (Modbus/
TCP), RS-485 (Modbus/RTU), USB, or
RS-232 (Modbus/RTU). SeaI/ O
modules are designed for a wide variety
of applications and environments
including process control, data acquisi-
tion, broadcast automation, security,
and facility management. I/O models
offer a choice of optically isolated
inputs, Reed relay outputs, Form C
CTC Releases Enhanced
Web-Enabled Controller
Technology Corporation
(www.ctc- has added the
to its
Blue Fusion™
line of web
enabled automation controllers. The
Model 5200 features a built-in web
server, enabling users to securely
monitor, control and upgrade processes
via Internet access. The unit also
includes multi-unit expansion racks to
allow users to control larger
applications with more I/O with a
single CPU and control strategy. A
single Blue Fusion system can now
Hopkinton, Mass.-based
Acksys Introduces OEM
Serial to Wireless
Ethernet Module
ACKSYS Communications
( widens its range of
wireless solutions by introducing a serial
to Ethernet WiFi
module for the OEM
The product relies on
the IEEE 802.11b
standard (2.4 GHz)
and is IEEE 802.11g-
compatible. It features a high-speed
serial interface (up to 250 Kbps)
available on a SUBD 9 connector for
RS232 model or on a 16-pin HE10
connector for the TTL model.
It can be powered from the DC power
source (+3.3VDC or +5VDC); con-
sumption doesn’t exceed 3W.
This module supports TCP Client/
Server, TELNET RFC2217, COM port
redirection, TELNET server, raw,
point-to-point and the serial Modbus-
to-MODBUS/TCP gateway.
The device is administered over a web
browser or with TELNET.
Blue Fusion Model 5200
relay outputs, and TTL interface to
industry standard solid state relay racks.
Field removable terminal blocks are
standard, facilitating fast, flexible field
wiring. SeaI/O modules operate from
9-30VDC, and power can be input via
terminal block or DC jack. Both table
mount and DIN rail mounting options
are available, and configuration is made
easy using Sealevel’s software tools.
control up to 192 I/O points and up to
six axes of motion.
Bi-directional e-mail support lets the
Blue Fusion controller send alerts or
production data in an e-mail message.
Users can also send an e-mail to the
Model 5200 with customized
instructions to modify the controller’s
The Model 5200 supports UDP, TCP/
IP, HTTP, and Modbus TCP Master/
Slave. Through DHCP support, the
controller can be named and appear like
any other node on the network. The
Model 5200 can additionally be set up
as an FTP server for upgrades, support
or for propagating control strategies
from one control unit to others.
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