ModbusNews_March2010.pdf

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Newsletter
Spring 2010
Modbus Organization
Releases Toolkit v3.0
In February, the Modbus Organization released version 3.0
of the Modbus TCP Toolkit. This new version replaces
version 2.1 and includes some new resources for
developers seeking to implement Modbus. These include
an updated test tool, Modbus Conformance Test Tool
v3.0 (and accompanying updated test specification), and
various other resources for implementation and device
testing. Use of these resources speeds development and
allows developers to prepare for conformance testing at a
certified Modbus Organization Conformance Test Lab.
The upgraded conformance test tool provides broader
test coverage with more flexible configuration to test
nearly any memory architecture. Developers can test
specific function codes or run the full test suite. Each test
sequence is logged for post-test evaluation.
The new test tool is now being used in the Modbus
Organization’s Device Certification Program, available to
all suppliers. Modbus member companies participating in
the Modbus Organization’s self-testing program have also
switched to the new test tool.
Special thanks for contributed projects and committee
work go to Control Solutions, Eaton Corporation, and
Triangle MicroWorks.
Are You Going to
Hannover Fair?
Looking for suppliers of Modbus
devices or Modbus expertise at the fair?
Many Modbus Organization member
companies are exhibiting at Hannover
this year. For your convenience, here are
the locations of those members that will
be there:
•
connectBlue:
Hall 7, Stand A46
•
Control Techniques:
Hall 15, Stand H35
•
Danfoss:
Hall 9, Stand D68
•
Eaton:
Hall 11, Stand C69
•
EWON:
Hall 7, Stand D46
•
Fieldbus International:
Hall 7, Stand B09
•
Hilscher:
Hall 11, Stand A62
•
Hirschmann Automation & Control:
Hall 7, Stand A46
•
HMS Industrial Networks:
Hall 9, Stand D18
•
Klaxon Signals:
Hall 8, Stand B15
•
Phoenix Contact:
Hall 9, Stand F40
•
Softing:
Hall 7, Stand B09
Shouldn’t your
•
Schneider Electric:
Hall 11, Stand B39
•
Wachendorff Prozesstechnik:
Hall 7, Stand A46
Two new open-source applications have been listed on
the Modbus Technical Resources page
(www.modbus.org/tech.php):
•
Modbus4J — a high-performance and easy-to-use
implementation of the Modbus protocol written in Java
by Serotonin Software. This implementation supports
Modbus ASCII, RTU, TCP, and UDP transport as slave
or master, and supports automatic request partitioning,
response data type parsing, and node scanning.
•
Mango M2M — This browser-based, Ajax-enabled
M2M software enables users to access and control
electronic sensors, devices, and machines over multiple
protocols (including Modbus) simultaneously. Diverse
data sources can be created and configured while provid-
ing downstream management of user access, alerts, data
logging, and automation.
New Open-Source Modbus
Implementations
More Choices for Wireless
Modbus Communication
With increasing demand for wireless communications in
industrial settings, suppliers are responding by equipping
their devices with wireless capabilities. Three Modbus
member companies announced the release of new devices
to meet this demand over the past couple of months.
Read about the Hach Company’s sc1000 Multi-Parameter
Controller with built-in GPRS modem, ProSoft’s
Industrial Hotspot™ radios, and Phoenix Contact’s
busable 900-MHz radio on pages 5 and 6.
Many more wireless devices are listed in
the Modbus Device Directory
(www.modbus.org/devices.php). Simply
filter on “Wireless Devices Only” to
locate the right Modbus wireless device
for your project.
News about the World’s Most Popular Industrial Protocol
Member News • MemberNews
Meet Some of Our Members...
Headquartered in Birkerød, Denmark,
JVL Industri Elektronik A/S
focuses on its core business — motor
control systems.
JVL first gained its reputation in the
1980s for the
development of
modern,
compact step-
motor drivers
and controllers.
In the 1990s this was followed by
development of a series of second-to-
none AC and DC servo controllers,
which led to marketing of the unique
integrated AC servo motor, the MAC
motor, (up to 734W), and the
QuickStep, integrated stepper motor
(up to 2.9Nm) which both include the
entire controller, encoder, etc. in one
compact unit.
The latest firmware for the integrated
servo motors MAC400 and MAC800
now supports Modbus RTU.
The implementation supports the Read
Holding Registers and Write Multiple
Registers commands, gives R/W access
to all motor registers at speeds up to
one Mbps.
The firmware supports two-wire RS-
485, as well as both four-wire RS-485
and RS-422 standards.
(www.jvl.dk)
Hach Company
provides the widest
digital communication capability in the
world of continuous sensors and
controllers for water and waste water
analysis. These sensors help with secure,
efficient operation of treatment plants
as well as other industrial applications.
They can be integrated easily into
common PLC, OPC, and SCADA
systems using Modbus RTU and
Modbus TCP
—
even for wireless
applications, using a built-in GPRS
module as the transport layer for global
communication.
Hach’s offerings include its MOD I/O
Modbus Interface, a cost-effective
solution for analog-to-digital
communication. The MOD I/O comes
with a power supply, RS-232 cable, and
a manual. Using Modbus ensures
compatability with vitually all PLC/
SCADA systems. MOD I/O can be
used with all Hach AquaTrend water
analysis instruments.
With annual production of more than
16 million pump units,
Grundfos
is
one of the world’s leading pump
manufacturers. Circulator pumps for
heating and air-conditioning and other
centrifugal pumps for industry, water
supply, waste water, and dosing are the
company’s major products. Today,
Grundfos has approximately 50
percent of the world market for
circulator pumps.
In addition to pumps, Grundfos
manufactures norm and submersible
motors for its pumps (the motors are
also available as separate merchandise),
as well as electronics for monitoring,
controls for pumps and other systems,
and the products BioBooster and
NoNox, which are part of the
company’s new business activities.
Established in 1945, Grundfos Group
Management is located in Bjerringbro,
Denmark.
(www.grundfos.com)
Shouldn’t your company
be a member?
(www.hach.com)
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 communica-
tion 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.
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This is the newsletter of the Modbus
Organization, the international non-
profit organization devoted to the
evolution and support of the Modbus
protocols. For more information about
membership and other services, please
refer to our website: www.modbus.org
Newsletter Editor:
Lenore Tracey (lenore@modbus.org)
Copyright 2009 by the Modbus
Organization, Inc.
PO Box 628
Hopkinton, MA 01748 USA
ph +1-508-435-7170
fax +1-508-435-7172
info@modbus.org
Modbus Discussion
Q&A
Modbus/TCP and inetd?...
In December, Ian Goldby wrote:
I’m wondering if it is sensible to write
a Modbus TCP server as an inetd child
process. In many ways this simplifies
the server, because it has to deal only
with one client at a time and all I/O is
piped through stdin and stdout (no
sockets programming).
But I wonder if many Modbus clients
open and close a connection each time
they make a request. With inetd there is
something of a performance penalty
when a new connection is opened,
because inetd has to fork a new
instance of the server process each
time.
A Google search for “Modbus inetd”
returns nothing that seems relevant.
Does anyone have any relevant
experience they can offer?
M. Griffin explained:
It’s normal for a client to hold the
socket open unless some major error
forces it to close. The reason of course
is to avoid overhead when polling at a
fast rate.
The exception to this is when the client
is polling very slowly, in which case it
may close the socket as soon as it is
done. In some applications the client
might only poll once per hour or even
once per day, in which case there is no
point in leaving the socket open. In that
case, the extra overhead of opening the
From the Modbus Discussion Forum…
connection isn’t typically going to be a
problem.
The server needs to deal with multiple
simultaneous connections, but they all
need to go back to a common data
table. A forking server would probably
work just fine. I don’t know if it has
been done before. If it hasn’t, it’s
probably because in most designs the
server is part of the application
program.
If you made the Modbus server a
separate server and the application
program (the thing that is going to
actually do something interesting) could
talk to it via some other fairly simple
means (e.g., via a UNIX socket) then
that would probably be useful. The
Modbus server could run as a
privileged program on port 502, while
the application program could talk to it
without having to worry about setuid.
I’ve written several Python programs
which have Modbus TCP servers
integrated into them. MS Windows, has
no security protection for “well
known” server ports, so they could use
port 502 directly there. For Linux, the
solution that I came up with was to run
the program on a higher port and use
iptables to redirect 502 to that port.
That meant I didn’t have to deal with
making sure that I was handling all the
security implications correctly, since the
program runs at a normal user level at
all times.
http://sourceforge.net/projects/
mblogic/
A Modbus TCP server that ran as a
separate component wouldn’t have to
deal with that, and so would be a
useful way of adding server
capabilities to programs.
Tallak Tveide added:
From what I’ve seen I would expect
close to all clients doing periodic
requests on a single TCP/IP
connection. The overhead in question
should be no big deal :-)
You could see for yourself with
wireshark
—
each connection will be
shown clearly.
Read the rest of this discussion and
add your commects to this thread
at modbus.control.com/thread/
1260976781.
Need a Modbus
Device for Your
Project?
Search the database at
www.modbus.org/devices.php
to find the right Modbus device for
your application.
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Modbus-IDA Discussion Forums
Modbus Discussion
Modbus Master to
Master...
Q&A
More from the Modbus Discussion Forum…
In February, Santi posed the
following question:
If an intelligent device (PLC) is only
configurable like a Modbus master,
can I query this from a PC application
(e.g., SCADA)?
Russ Bartels answered:
Maybe. Some Modbus master devices
will reply to standard “slave” requests
from another master on the same bus.
Just remember that they need different
node addresses.
You should check with the PLC
manufacturer..
From M. Griffin:
Like most protocols, Modbus is a
client/server (or master/slave)
protocol. The client (master) sends a
query, and the server (slave) responds
to it. It’s just like the way your web
browser (client) communicates with
Control.com (server).
If the SCADA can be a server and the
PLC is a client, then the PLC can talk
to it. If the SCADA and the PLC both
insist on being clients (masters), then
the solution is to put a “proxy server”
in between them. The proxy acts as
server to both parties. The PLC would
write to the proxy, and the SCADA
would read from it (and vice versa).
If you are dealing with Modbus TCP
on Ethernet (as opposed to an RS-485
Modbus), then any station can be both
a client and a server simultaneously
provided
its software was written to
support this. This is not typical for
PLCs (they are usually one or the
other). For the SCADA you would
have to look into what its drivers are
capable of.
Ian Goldby did not agree:
No, you can’t. Not directly anyway.
The fundamental problem is that a
Modbus master does not respond to
unsolicited requests. There is no such
thing as this in the Modbus protocol.
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The only way a Modbus master can get
information is by asking for it.
Conversely, a slave deals only with
unsolicited requests. It cannot send a
message unbidden.
The second problem is that Modbus is
fundamentally a master/slave protocol.
There is no peer-to-peer in Modbus.
Patrick Lansdorf suggested the
following approach:
You will need a gateway that is a
Modbus RTU slave to Modbus RTU
slave, such as the Anybus X-gateway.
With it you will be able to share data
between two Modbus RTU masters,
pretty much like a DP/DP coupler for
Profibus.
Fred Loveless offered another
possible solution:
I have never seen it done yet, but that
does not mean it cannot be [done] if
the devices support it. If you need to
get data from the master device into a
SCADA and from the SCADA back to
master, use an OPC or DDE Server
that can be configured to be a Modbus
slave on the network. Then the master
PLC can Read and Write to the slave
device, and your SCADA is updated
from the slave server and can write
down to it so that the value can be
polled by the master.
Mike Lamond had another idea:
I know of one case where a Modbus
port can be both slave and master, but
not at the same time, of course. In
Modicon Quantum, Momentum, TSX
Compact and some Micro 984 models,
the XMIT 984 loadable and the
Concept IEC XMIT function take
control of a Modbus port to act as a
master device. When XMIT is not
active, the port reverts to its configured
slave setting.
We used this on a radio telemetry
project to make one site a data repeater
for another site that was blocked from
the master radio by a hill. There are
extra flags programmed so that the
repeater site waits as a Modbus slave
until it receives permission from the
master site, after which it becomes a
Modbus master and the master site acts
as a Modbus slave. Once finished, the
repeater returns permission to the
master site and the cycle continues.
I haven’t had any experience with the
newer Schneider PLCs and software,
so I don’t know if this technique has
become obsolete.
Robert Willis confirmed this:
The XXMIT Function block would
allow the Modbus Port (Port 1) to be
used as either a master or a slave. Once
the XXMIT “master” message is
completed, the port would revert back
to the slave configuration. The
arbitration between master and slave
would need to be worked out by the
developer so that both the local and
remote can communicate.
You also might want to look at the
NR&D products that have a Modbus
Gate Mode setting for serial ports. This
allows the serial port to automatically
transition between a master or slave
configuration depending on the
message source.
Modbus users & suppliers
get together on the
Modbus Community for:
•
Interactivity
•
Knowledge aggregation
•
Contact with Modbus
users and suppliers
Discussion supported by...
Modbus Discussion
From Gustavo A. Valero P.:
Going back to the initial question
written by Santi, the answer is YES but
it depends on the SCADA’s driver.
Some drivers support something called
“Unsolicited Read,” and with this
feature you can get what you want, e.g.,
most FactoryLink drivers support
“Unsolicited Read” feature including its
Modbus TCP driver.
This driver opens three threads/
connections to the NOE module of
PLC (was developed to work with
Quantum PLCs originally). One
thread/connection is required for each
of the following operations: Reads,
Writes, and Unsolicited Reads. These
threads must be available even if these
operations are not configured in the
application.
At its beginning, if a PLC that claimed
to support Modbus TCP protocol was
not able to support these three
connections, the driver was not going
to work with the PLC, but after some
modifications and improvements, this
issue was eliminated.
Unfortunately, the driver only works
and waits for Unsolicited Reads based
on Holding Registers (HREG)...
nothing is perfect!
Your plan B could be to use an I/O
Gateway device able to work as
master/slave Modbus at the same time
and exchange data between the masters
using the Gateway’s internal/virtual
memory as a “bridge.” I have used this
solution before with [SIXNET]
equipment perfectly.
JMW :
This thread is very fortuitous for me as
I have been puzzling over a similar
problem.
I have two transmitters that normally
would both be slaves multi-dropped to
a master controller.. However, I have
an application where I need these two
transmitters to share information with
each other, perform similar
calculations, and then output the results.
So to share information one or the
other or both would have to be able to
solicit data from the other and both
respond to an external data call from
the master controller.
Effectively this means three masters...
or am I wrong?
It would be neat and tidy if the true
master were to poll data from both
slaves and then send data collected
from one slave to the other slave, but I
also have the possibility that in some
applications there will be just the two
transmitters and the “master” controller
would only accept 4-20mA data so the
two slaves will have to do it all
themselves.
I just need a heads up on feasibility for
now, thanks.
M Griffin replied:
A device could be a client (master) or
server (slave). In some cases it is
possible to be both at the same time. It
is up to the device designer to decide
what features and functions make sense
for that application and then to
implement those features. Modbus
specifies what the message format
looks like. It doesn’t (with a few
exceptions) specify how your device
works. As long as the messages which
go out on the wire conform to the
format in the Modbus specifications,
it’s still Modbus.
You will find the same is true for many
(if not most) other protocols. There are
a lot of features in many protocols that
are intended for certain narrow
applications, and which have no value
in other applications. The designers then
just implement subsets of the full
protocol. With proprietary protocols,
however, they very rarely tell you what
is really going on under the lid, so it
may not be as obvious as to what the
limitations are.
Read the whole thread & add your
thoughts at modbus.control.com/
thread/1265915114
Hach Liquid Analysis Portfolio Goes Modbus TCP
Wired and Wireless
The Hach Model sc1000 Multi-Parameter Controller is now available with
state-of-the-art Modbus TCP communication protocols and a built-in GPRS
modem for seamless integration into corporate internet and MES systems.
Using Modbus TCP, system data can now be transmitted over an Ethernet
physical layer or streamed wirelessly using GPRS around the globe.
System data that is stored in the sc1000 can also be accessed using an on-board
web server. The enhancements in the sc1000’s digital communications
capability enable customers to integrate the sc1000 controller easily into their
networks as well as save them money by reducing wiring and cabling costs. Up
to five Modbus masters are supported simultaneously while parameterization,
configuration, and diagnostics can be performed using any browser.
For more information, visit www.hach.com.
Hach Company’s
sc1000
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