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Modbus Organization Newsletter, December 2010
Happy Holidays and All the
Best in 2011!
2011 marks the beginning of the Modbus Or-
ganization's ninth year. We'd like to take a
moment to thank our members and the many
suppliers and users who contribute to our ac-
tive community.
The Modbus Device Directory has grown to
include over 1000 devices, and is searched
daily by users seeking the right Modbus device
for their application. The Modbus.org website
and Modbus discussion forum site combine to
serve the needs of over 40,000 unique visitors
each month, looking for advice on Modbus im-
plementations, downloading the protocol speci-
fication and implementation guides, seeking
links to free, open-source Modbus resources.
Together, the community supports the imple-
mentation of Modbus throughout the world in
the most diverse markets.
We are happy to be there for you and with you.
Modbus Wireless Access to
HART Instruments
Phoenix Contact recently launched a HART7-
compliant WirelessHART® gateway that al-
lows Modbus TCP users to access their HART-
compatible instruments
remotely. The Highway
Addressable Remote
Transducer (HART) pro-
tocol has been used in
process, measurement
and control applications
for the past two decades,
and more than 30 million
installed instruments
already have built-in
HART functionality.
Historically, a Distributed Control System
(DCS) has been used with HART devices, but
the HART functionality has not always been
used. The HART signal is superimposed over a
standard 4-20mA signal, and in many cases,
the price differential between an Analog I/O
card with HART and one without has led cus-
tomers to choose the latter.
According to Garrett Schmidt at Phoenix Con-
tact, the WirelessHART gateway allows cus-
tomers to network up to 250 WirelessHART
field devices and use a second control platform
to monitor the behavior and calibration of each
device. Phoenix Contact uses the entire suite of
Modbus TCP commands. Schmidt says that
even a PLC with TCP capability can be used in
less critical non-process applications.
The 45 mm-wide RAD-WHG/WLAN-XD fea-
tures an integrated 802.11b/g WLAN trans-
ceiver, is rail-mounted and can increase the
reliability and flexibility of an existing HART
network. The integrated WLAN can serve as
the backhaul connection, allowing the gateway
to be installed in the field closer to the moni-
tored devices.
continued on page 5
Best wishes for a healthy and
prosperous New Year!
News about the World’s Most Popular Protocol
Member News
Member News
Contemporary Controls
has a long history of
developing products for diverse
automation industries. The
company's communications
hardware designs include hubs,
switches, routers and gateways.
Member News
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Secure Crossing's
team of
security professionals develops
and implements
powerful, protocol
filtering tools
unlike any other,
to support
critical infrastructure and remote
connectivity at an affordable cost.
With the growing need for
broadband and wireless access to
perform remote maintenance and
to access distant equipment,
Secure Crossing's products are
well suited for SCADA and
remote location usage in power/
smart grid, freshwater/
wastewater, oil and gas, life
sciences, machine builders and
other remote plant-floor
Member News
Member News
Contemporary Controls' BAS
Remote product line provides a
convenient way to expand
building automation systems in
the field when using Ethernet for
network communication. In
addition to BACnet/IP or Modbus
TCP universal I/O points, the
unit can map Modbus variables
Andover, Mass.-based
is a global provider
of instruments, subsystems and
process control solutions that
measure, control, power, monitor
and analyze critical parameters
of advanced manufacturing
processes to improve process
performance and productivity.
into BACnet objects. With the
BAS Remote, users have a
flexible building block for
integrating diverse building
automation protocols or
expanding the number of points
in a building automation system.
MKS Instruments' control
products contain high density
mixed I/O to afford the lowest
possible price per point in the
market. High-resolution and
high-accuracy analog and digital
I/O, plus dry contact relays and
temperature I/O provide a
flexible environment to build
controllers to the specifications
customers require.
Modbus Newsletter
The Modbus Organization Mission
The Modbus Organization, Inc. is a group of
independent users and suppliers of automation
devices that seeks to drive the adoption of the
Modbus communication protocol suite and the
evolution to address architectures for distributed
automation systems across multiple market
segments. Modbus Organization also provides the
infrastructure to obtain and share information about
the protocols, their application, and certification to
simplify implementation by users resulting in
reduced costs.
This is the newsletter of the
Modbus Organization, the
international nonprofit
organization devoted to the
evolution and support of the
Modbus protocols.
For more information about
membership and other services,
please refer to our website:
Newsletter Editor:
Lenore Tracey
Member News
Member News
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Colway Solutions Makes It Easy to Integrate Modbus
Colway Solutions
is building
its reputation by helping develop-
ers easily integrate Modbus into
their applications and devices.
In the typical development pro-
ject, engineers are pressed for
time as they develop and test the
core functionalities of their prod-
uct. In addition, they may be re-
quired to build a communications
infrastructure, including pro-
gramming the communications
protocols. Today’s engineers need
to save time, money, and device
memory on this part of the pro-
ject and concentrate on their pro-
prietary design to achieve com-
petitive differentiation.
As a lightweight Modbus Slave
Protocol stack in C source code
for Modbus TCP, Colway Solu-
tions' first product addresses this
problem directly. Colway chose
the eight key functions from the
Modbus protocol specification
and made it available as a li-
brary. Customers buy only the
function codes they need —single
function codes or all eight func-
tion codes bundled together. The
library takes up approximately
5 kB of code space and about 512
bytes of RAM.
Deploying this source code offers
a huge savings in development
costs. Typical time saved is up to
six or eight weeks per project.
Customers may need to impart
Modbus functionality to either a
single product or multiple prod-
ucts in their portfolio. Colway
Solutions’ royalty-free licensing
model supports unlimited embed-
ded copies for either scenario.
This also benefits systems inte-
grators who may wish to deploy
Modbus on specific devices for
network consolidation. A single
Modbus function can be deployed
on a single product for $50.00.
In addition to packaging and li-
censing, Colway Solutions offers
other ease-of-deployment fea-
Free downloadable Modbus
Protocol Tester.
After the
source code is integrated with the
application and embedded in the
target device, the Colway Tester
is a handy tool to check compli-
ance, verify the robustness of the
communication interface, and
gather and analyze data on com-
munications performance.
Free “semi-ports”.
Colway So-
lutions has tested and character-
ized its source code on popular
micro-controller hardware includ-
ing ARM, 8051 and Coldfire. The
microcontroller-specific porting
steps, such as setting variables
and macros, is already done and
available free to Colway custom-
ers. Colway plans to add semi-
ports for additional micro-
controller hardware.
Specific task-oriented on-line
Engineers do
not need to plough through
reams of documentation. Col-
way’s on-line documentation is
readily available in an easy to
understand “how-to” format for
each step in the porting process.
Colway Solutions
more information.
Write Mul-
Customers buy only the Modbus Functions Codes they need.
Member News
Modbus Discussion
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Modbus Discussion
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Modbus Discussion
Q&A from the Modbus Discussion Forum...
Accessing a Memory Address...
Ax wrote to the forum:
I am new to Modbus and looking
for some enlightenment. I have
read a few good tutorials, but I
still have some questions.
I am assigned to connect to a
Modbus slave that uses six-bit
addressing and access memory
So when I am given address
308195 to access, does that mean
I need to use Function Code 3
and address 8195?
Mark answered:
I would suggest reading the pro-
tocol specification.
The function code for reading an
input register 3XXXXX is 4. 3 is
for reading a holding register
M Griffin offered:
First I wish to clarify something
important. So far as the actual
Modbus protocol is concerned,
there is no such thing as 4-digit
addressing, 6-digit addressing, or
anything else like that. All Mod-
bus addresses are 16-bit binary
numbers starting at 0 and going
up to 65535.
Any other way of expressing ad-
dresses is simply some manufac-
turer's idea of how to write their
manuals or software user inter-
face. It has nothing to do with
what actually gets sent through
the wires.
One documentation convention
that is sometimes used is to pre-
fix the addresses with a numeric
digit indicating the type of ad-
dress. These are typically as fol-
lows: "0" = coils, "1" = discrete
inputs, "3" = input registers, and
"4" = holding registers (note there
is no "2"). This is analogous to
how IEC PLC addresses are pre-
fixed by "I", "Q", "M", etc. There
is nothing about this in the Mod-
bus protocol specification because
it isn't part of the official Modbus
protocol. It's just a unofficial con-
vention that some people use.
Another thing to note is that
some manufacturers write their
addresses starting at "1", while
others start at "0". However, that
is also a documentation issue, the
actual Modbus addresses always
start at "0".
I have a web page at the follow-
ing URL which may provide some
additional details:
Read more or add your com-
to this thread.
How to assign coil
address in PIC?
Iman asked the forum:
I am doing a project of controlling
a relay that will control another
motor using Modbus. A computer
will be the master and a
PIC16F877 connected using
RS232 the slave. The circuit is a
lot similar to that of Modbus.pl
by Andrei (memory max 232)
My question is how to assign
Modbus address of coils to a spe-
cific port in Microchip PIC? I am
using PIC16F877 with the com-
piler PIC C.
When I used function 05 which is
to force a single coil to close or
open, it will actually affect the
relay. Any ideas?
Curt Wuollet suggested:
Look at how the reference code
on Modbus.org works to receive a
request. You will need to imple-
ment something like it to receive
requests and sort out which coil
needs actuation. Then you will
figure out how that is done on the
PIC and when the request comes
in, wiggle the pin.
continued on page 5
The Modbus Community is where it's happening:
Active technical
Knowledge aggregation
Contact with other
Modbus users
Discussion supported by...
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Modbus Wireless Access to
HART Instruments
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cont’d from pg 4
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cont’d from pg 1
Assign coil address in PIC...
You will probably need to fake
just enough to do what you want
and ignore other requests as the
whole ball of wax might be kinda
large for a PIC depending which
you have. And yes, it's non-
Iman replied:
Thanks for the reply. It always
bugs me when I see in other fo-
rums that they only touch the
memory part. They send signal
from and to the PIC and read
whether they are correct or not.
But actually, the pins control any
input that you want to capture or
control any output. But little ex-
planation is done on this part.
I guess I have just to do trial and
error to assign specific address
functions only to the specific
Bruce Durdle's advice:
If you are writing the software
for a Modbus slave, you can do
what you like to allocate I/O pins
to internal memory bit or byte
This allows the user to create a
“clustered” network topology,
effectively creating several
smaller WirelessHART networks
with a wireless connection to the
host system.
To maintain a secure connection,
the gateway uses the 802.11i
(WPA2) standard with 128-bit
AES encryption to protect the
WLAN data. The WLAN
transceiver can also be disabled,
and the host connection made via
the wired Ethernet port. All
programming and diagnostics
can be accessed via an embedded
web server. The RAD-WHG/
WLAN-XD can also be
programmed using a HART
handheld programmer.
The WirelessHART gateway,
designed and manufactured in
the U.S., is uniquely positioned
to allow Modbus system
integrators to build both new and
refurbished networks using a
PLC and HART-compatible
instruments. This is ideal in
infrastructure projects using the
funds from the American
Recovery and Reinvestment Act
of 2009 (ARRA).
To learn more, visit
You can also do what you like
to allocate Modbus addresses
to internal memory or I/O.
In my Modbus slave routine,
I assign an array of internal
memory addresses as
"holding registers," "input
registers," (loaded from A/D
likewise for internal bit
strings and input bit strings
(derived from input ports).
It is not always a good idea to
allocate Modbus addresses
directly to I/O addresses as
some processing allows isola-
tion. Analogue values read in
from the A/D converter need
to be buffered into memory
Read more or add your
to this thread.
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