ModbusNews_Dec2009.pdf

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Newsletter

December 2009

Modbus Device Directory Wastewater Treatment

Approaches 1000 Devices Plant Upgrade

Since launching the Modbus Device Directory in 2003, the

options for identifying appropriate devices for projects

involving Modbus networking have grown almost four-

fold.

Users can search for appropriate devices using one filter

for device type and another to limit devices to wired or

wireless (or include both). Device types include AC/DC

drive controls, controllers, HMI hardware or software,

I/O interfaces or modules, sensors, monitoring hardware,

network gateways, and various software product types

such as OPC, SCADA and test and simulation software.

Users can also limit their search to devices that have been

testing for conformance to the Modbus protocol.

Once identifying a selection of products for a project, all

the information necessary to contact the supplier is right at

hand. If the supplier is a Modbus member, a spec. sheet

can be downloaded directly from the device listing.

Check out the directory at www.modbus.org/devices.php.

Contact info@modbus.org to list your device or for

assistance identifying the right device for your application.

Two Modbus Organization member companies,

ProSoft Technology, Inc.

and

Schneider Electric

worked with local systems integrators to bring a

wastewater renewal project in Finland to satisfactory

completion.

The original automation system from 1992 of the central

wastewater treatment plant of Lapuan Jätevesi Oy was

brought up to date in 2008. The automation renewal

project included the controllers, the SCADA software

and the communication network where nine RadioLinx

industrial Ethernet radios are involved for safe and

reliable transfer of process and control data.

Wastewater treatment is a basic public service that affects

all of us: It is vital to keep our living environment

hygienic and healthy and our watercourses clean. Behind

the scenes, the wastewater treatment process combines

microbiology and chemistry with mechanical

engineering, instrumentation, and automation techniques

that offer high performance in a progressive way.

The central wastewater treatment plant of Lapuan

Shouldn’t your

Jätevesi Oy, a Finnish company located in Lapua, west

of Finland, receives household wastewater and industrial

wastewater from different locations: Atria, Lapuan

Nahka, Kation and Metso Power,

among others. The Lapuan

Jätevesi Oy treats the wastewater

from three locations: Lapua,

Nurmo and Kuortane. It also

maintains municipal wastewater transfer pipelines from

Nurmo and Kuortane to Lapua and the central

wastewater treatment plant in Lapua. This is where the

automation system had to be renewed.

The wastewater treatment

process starts with the primary

treatment where the influent

sewage water is strained to

remove all large objects, and

the oxygen level of the water is increased to facilitate

microbial activity; microbes clean the water by feeding

on its impurities.

(continued on page 5)

Wishing you all peace

and good fortune

in the new year!

News about the World’s Most Popular Industrial Protocol

Member News • MemberNews

Meet Some of Our Members...

Korenix

(www.korenix.com) has

developed a reputation for suppling

hardened, cost-effective, Ethernet

networking products for industrial and

rugged applications. Its award-winning

JetPoE and JetBox, flagship products

are widely used for industrial

automation, and outdoor markets such

as IP surveillance, and intelligent

transportation.

The JetBox series of industrial

networking computers consolidates an

industrial computer, router, managed

switch integrated with the interfaces of

a serial server and digital I/O to make

the network simple. In addition, it is

equipped with value-added software

and a Modbus gateway to adapt to

SCADA systems and make industrial

control easy.

Korenix recently released its new

JetBox 5300-w embedded Linux

computer for front-end industrial

control applications

(profiled on page 6).

Control Solutions

(www.csimn.com),

founded in 1995, was a spin-off from

a contract design and product

development company founded in the

late 1980s. The company has been

developing networked control

products since then, specializing in

Since 1984,

ACKSYS

Communications & Systems

(www.acksys.com)has specialized in

designing and supplying data

communication equipment, including

wireless network solutions, serial

Modbus-to-Modbus TCP data

gateways, serial device servers

(Ethernet, USB, and WiFi), media

converters, serial communication

controllers, MIL-STD-1553 and

HDLC for avionics test and simulation.

Shouldn’t your company

be a member?

building automation, facility

management, and commercial

automation. Control Solutions offers a

line of off-the-shelf embedded control

products with BACnet, LonWorks,

Modbus, and SNMP connectivity.

Control Solutions also provides

custom programming for standard

hardware and customized hardware

solutions. The company’s engineers

each have over 20 years of experience

in embedded controls technology. All

products are designed and made in the

USA, manufactured under ISO-

9000:2000.

The company

recently released a

new resource for graphical

programming for Modbus, BACnet,

LonWorks, and SNMP I/O devices,

which has its own web site:

www.graphical-programming.com.

ACKSYS products are available

through an extensive distributor

network, spanning 20 countries across

the globe.

ACKSYS’s committment to helping its

customers maximize their capital

investments is impressive. The company

bases its work on three major factors:

•

Choosing components with

guaranteed sustainability and/or

second sources

•

Checking the obsolescence potential

of strategic products every

six months

•

Monitoring availability of crucial

parts for at least five years after a

product ceases to be manufactured.

Modbus Newsletter

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)

Organization, Inc.

PO Box 628

Hopkinton, MA 01748 USA

ph +1-508-435-7170

fax +1-508-435-7172

info@modbus.org

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.

Modbus Discussion

Q&A

On December 1, Arun wrote:

I am implementing the Modbus

protocol for TCP/IP. Per the TCP/IP

implementation guidelines, I

understood that a slave can handle up

to 16 connections simultaneously.

I believe this can be implemented using

multi-threading concept. But can a slave

device (for example, a PLC or

embedded controller) handle 16 such

threads in parallel without any issues?

And another question, is there any other

way to implement this without multi-

threading?

Curt Wuollet replied:

Some can, some can’t, some can’t even

handle one connection decently. Back-

to-back packets will upset some. But

TCP/IP itself handles a lot of the

issues. For the threading, etc. issues, I

suggest you look at any number of

OSS tools and implementations to see

how this is handled in a more or less

standard fashion. It’s pretty much

cookbook stuff and it’s a bad idea to

go rogue. Many, many examples to

look at.

Russ commented:

If by “implementing” you mean rolling

your own, I would first ask “why?”

All slave devices are not created equal,

so do not count on 16 channels on

every one that you end up with. A lot

of slave devices have limited

processing power and will choke, stall,

From the Modbus Discussion Forum…

or die if you attempt to read/write too

fast.

Yes, if it supports 16, that means 16

simultaneous, but I would recommend

that you set up your Master device to

use fewer. Normally one for reads and

one for writes, one for data-grams, if

needed.

Only look at multi-threading if you

MUST have the extra speed and then

use it on demand if the Slave can

handle it.

M. Griffin suggested:

There is nothing to stop a Modbus

TCP server (slave) from handling as

many simultaneous connections as it

wants. The limitations of any particular

PLC will be due to the internal

software and resource limits. Some

PLCs can only handle four connections

at a time. If the Modbus TCP server is

a program running on a PC, there is no

reason why it couldn’t handle as many

connections as the operating system will

allow.

The thing to keep in mind is that as far

as communications are concerned, the

PLC is handling “connections,” not

“threads.” Each client (master) would

make one connection to the PLC. The

client would send a command, wait for

a response, send the next command,

wait for a response, etc. It would then

normally wait some period of time

(the polling rate) before repeating the

sequence of commands. As far as the

communications are concerned, there

should only be one request or response

in the pipeline between each client and

the server.

As far as implementing the software is

concerned, you can do it

synchronously, threaded, or

asynchronously. However, keep in

mind this is an internal software

implementation and does not affect

how the messages go out on the wire

(other than how many simultaneous

conversations you can keep going).

Synchronous software is for where you

need the simplest possible

implementation and don’t need

multiple simultaneous conversations

(e.g., between a client and a couple of

servers).

As far as threaded versus asynchronous

is concerned, asynchronous has less

overhead but threaded software is

more common, because that is what

most programmers are familiar with.

All you have said about your

application is that you are

implementing the protocol. It is

difficult therefore to go into much

more detail. If you want to say what

you are doing (client or server,

embedded or on a computer, how

many connections you want to be able

to handle, etc.) then we can talk about

it more if you wish.

Read the rest of this discussion and

add your commects to this thread

at modbus.control.com/thread/

1259677525

Modbus TCP/IP Implementation...

Modbus-IDA Discussion Forums

Modbus Discussion

Modbus Exception on Read

Only Registers...

Enrico posted the following query:

I’m writing source code for a slave

module to connect in Modbus, but

I’m in a little confusion about

exception codes.

What exception code does the unit

transmit if the master attempts to

write a read-only register?

Mark replied:

AFIK, Modbus does not define any

registers as “read only.”

Having said that, I would expect an

exception code 3: illegal data value.

I would be sure to document it for

users.

HTH,

Mark (www.peakhmi.com)

Isaac disagreed:

I beg to differ. The Modbus standard

does define read/write for the 4 basic

data registers in which “input” registers

are read-only. I cite the Modbus

Application Protocol Specification

V1.1b (www.modbus.org), p. 6, 4.3

Modbus Data Model.

Modbus bases its data model on a

series of tables that have distinguishing

characteristics.

[The

four primary tables are shown at the top

right.]

“Input Registers: 16-bit word; Read-

Only; This type of data can be

provided by an I/O system.”

I would expect Modbus Exception

code 01, Illegal Function, in response

to a write command to a read-only

input register.. For reference, see pg 49,

Modbus Application Protocol

Specification V1.1b.

Modbus Exception Code 01

Name: ILLEGAL FUNCTION

Meaning: The function code received

in the query is not an allowable action

for the server (or slave). This may be

because the function code is only

applicable to newer devices, and was

Q&A

More from the Modbus Discussion Forum…

Primary

Tables

Discretes Input

Object

Type

Single bit

Read/Write

Read-Only

Comments

This type of data can

be provided by an I/O

system

This type of data can

be alterable by an

application program.

This type of data can

be provided by an I/O

system

This type of data can

be alterable by an

application program.

Coils

Single bit

Read-Write

Input Registers

16-bit word

Read-Only

Holding Registers

16-bit word

Read-Write

not implemented in the unit selected. It

could also indicate that the server (or

slave) is in the wrong state to process a

request of this type, for example

because it is unconfigured and is being

asked to return register values.

Mark got back into the discussion:

Differing allowed. I knew when I

posted the reply someone would come

up and say but but but.

It only made sense to me that he was

referring to registers that COULD be

written to marked as “read only” for

his device. This is very common for

slave devices that are not PLCs and use

Modbus as a means to publish status

information of the device.

And that he was using a WRITE

function code.

If he was not using a legal function

code then the reply would be illegal

function and that would be that.

None of the registers that support a

write function are “read only” per the

spec.

Mark (www.peakhmi.com)

Fred Loveless responded:

Actually Input registers 0x04 and input

coils (discrete inputs) 0x02 are essentialy

read only. You cannot write to them

from a master as there is no function

code to do so. For registers that can be

written to there is no way in the

protocol specification to designate

them as read only.

However, in my job I have seen many

devices that have implemented read-

only data in holding registers. They

simply return an exception code of

0x01 (Illegal Function) if you try to

write to them.

Keep in mind that this is a design

feature that is set by the hardware

manufacturer only.

Fred Loveless

Kepware Technologies

Michael Griffin added:

Read-only registers are called “Input

Registers.” Read-write registers are

called “Holding Registers.” The master

can’t even attempt to write to an input

function code to do that. The protocol

avoids the issue altogether by design,

therefore the problem simply doesn’t

arise. That is why there are two types

of registers.

Have a look at the list of Modbus

functions. There are functions for

reading both holding and input

registers, and there are functions for

writing to holding registers. There are

no functions for writing to input

registers.

Read the whole thread & add your

thoughts at modbus.control.com/

thread/1026250536

Wastewater Treatment Plant Renewal

(continued from page 1)

During this biological treatment phase,

the microbes in the wastewater are

given suitable growing conditions in

terms of temperature, oxygen level,

and nutrition.

The next phase includes chemical

secondary sedimentation, where

aluminum-based chemicals are added

to the water from the biological

treatment to prompt flocculation of

slowly-degrading organic and other

materials.

In the last phase, the remaining sludge is

treated by removing water from it. The

water separated from the sludge is

taken back to the beginning of the

treatment process, and the solid sludge

is taken to the Lakeuden Etappi biogas

plant.

High-tech automation system

The central wastewater treatment plant

is fully automated. The automation

system from 1992 had reached the end

of the road and it was time to renew

the wastewater treatment plant:

Requirements included.

• Ease of installation and maintenance

• Flexibility to adapt to fast-changing

regulation

• Improvement of the overall solution

• Compliance with existing

applications and methodologies

The whole system was renewed in

collaboration with Schneider Electric

and Seinäjoen Teollisuussähkö Ky. “The

old automation system had served its

time. We have been using it every day

for the past 16 years,” Operating Chief

of the plant, Vesa Hahtokari, explains.

The original automation system

involved six controllers from the TSX7

series, with Monitor 77/2 software

environment, and used MAPWAY

communication protocol.

In the new solution, six Modicon

Premium controllers are implemented

with Monitor Pro v7.6 and are using

Modbus TCP/IP

over wireless, using

nine of ProSoft Technology’s

RadioLinx industrial Ethernet radios.

Modbus Application Story

About 2000 process variables are

transiting over the wireless network,

which is also used for programming

and maintenance purposes.

Why wireless?

A total of seven locations had to be

integrated into one single tight network.

The six Modicon controllers are

located in different buildings on the

wastewater treatment plant, and the

plant has two control rooms.

From the user point of view,

the first

advantage of the wireless networking

option was the cost and time savings

for the installation: no need to dig

trenches, no need to clean up existing

cable paths.

From the integrator point of view,

RadioLinx network was “the easiest

part of the implementation. We didn’t

have any problem. These radios are

very easy to configure and mounting

recommendations given by ProSoft

Technology were very clear. Schneider

Electric made some tests in their office

and then explained us how to

implement the wireless network on the

field.”

From Schneider Electric’s point of

view,

the engineering of the network

was reduced to a minimum. “When we

started the project, we did not locally

have any specific RF expertise,”

explains Jouni Aarnu, Application Sales

and Key Account Manager, Wood and

BioEnergy, at Schneider Electric

Finland. “We talked to ProSoft

Technology Technical Support

Engineers and provided them with the

basic engineering and layout of the

network. They made some calculations

that were necessary for this type of

application and they provided us with

the recommended lists of accessories

for each radio location: cables, antenna,

lightning protector, etc.”

(continued on page 6)

ProSoft Technology RadioLinx industrial Ethernet radio &

Schneider Electric ConneXium 499 NES 25100

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