Modbus TCP server/slave implementation

Does anybody have experience to implement a modbus server or slave application? My computer needs to communicate with a PLC. The configuration is: computer is TCP server or slave, PLC is TCP client or master. Computer to write input registers for PLC to read. Computer read holding registers from PLC. Cannot find any labwindows examples.
Thanks a lot.

Thank you all for the replies. Most the links are for modbus tcp client (master) instead of modbus server (slave).
Now I am trying to use a modbus slave activeX control (trial version) from Automated Solutions Inc: http://www.automatedsolutions.com/products/modbusslave.asp.
I created an instrument driver using Create ActiveX Controller wizard. I can get a correct object (modbusHandle) with the first line below (status = 0). But the second function (set a property) gives negative status return (status = -2147418113).
status = ASMBSLVLib_New_DASMBSLV (NULL, 1, LOCALE_NEUTRAL, 0, &modbusHandle);
status = ASMBSLVLib__DASMBSLVSetDataMode (modbusHandle, NULL, ASMBSLVLibConst_DATA_MODE_MB_TCP);
Did I create the activeX control correctly? Automated Solutions told me that the ActiveX control is a windowless control. It's a runtime control. Is it a problem?
Thanks in advance.
jzhu0000.

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TCP server implementation: thead issue

Hi,
I'm not actually doing a TCP server, but I need to implement a similar threading model
in my app, so this will serve as a good example. I have done a TCP server few times
with different programming languages and here's a pseudocode how I do it
do
    connection = waitForConnection(port, -1)   // -1 means wait forever
    if(connection != NULL)
       // Here, a new thread is created for each client and data transfer is started
       // The original thread continues to wait for connections
while(running)
I don't understand how could this be done with LabView. With the use of local variables,
I can think of a solution that could process the clients, if the number of clients is known
at development time.
Can I somehow create threads with LabView at run-time?
This whole thread issue in LabView is quite confusing to me, so if someone knows a good
solution to that TCP server problem, I would really appreciate it. I think that would help me
understand this matter.
Thanks.

The LabVIEW terms you need to understand are VI Server and reentrant execution. There are some great TCP examples that ship with LabVIEW that should help you along...
Take a look at the vi called examples\viserver\runvi.llb\DateServerUsingReentrantRun.vi
This examples folder is in your LabVIEW folder (LabVIEW x.x)
Message Edited by Phillip Brooks on 07-25-2007 08:57 AM
Now is the right time to use %^<%Y-%m-%dT%H:%M:%S%3uZ>T
If you don't hate time zones, you're not a real programmer.
"You are what you don't automate"
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Using Modbus TCP I/O Server with new DSC Shared Variables in LabVIEW 8.6

Hello,
I'm using LabVIEW 8.6 and want to communicate with a Beckhoff BK9000 Ethernet TCP/IP Bus Coupler via Modbus TCP. Instead of using the NI Modbus Library, I've tried the new LabVIEW 8.6 feature "DSC Shared Variables" as described at the bottom of this page: Latest NI LabVIEW DSC Module Features and Demos. Reading of analog input bus terminals works fine. However, I haven't figured out yet how to write on an anolog output bus terminal with these shared variables.
It's about a 16 bit analog output and I need to write to the registers 0x1121 and 0x0801. It works with the NI Modbus Library (just using function code 6 and choosing the registers), but on the other hand I don't know which shared variables I have to choose for these registers. I've tried several data items (e.g. 400001 upwards as well as 402049 for 0x0801) but none of them worked. I would be glad for a short explanation - thanks in advance for your support!
Regards
utechle

The Beckhoff documentation says, that holding registers start with 0x0800. I've checked this by using the NI Modbus Library. I used the "MB Ethernet Master Query.vi" togehter with the function code 6 for "Write Single Registers", changed the settings of the starting address to hexadecimal view and entered 801 (since it starts with a control byte in 0x0800 and the data out word follows in 0x0801). Furthermore, I had to address register 0x1121 in the same way for resetting the watchdog. As I've mentioned in my first post, this method works fine. However, I haven't found out yet which shared variables i have to use for accessing these registers.
On the other hand, it's no problem to read data from analog input bus terminals using shared variables. They start with 0x0000 (status byte) and 0x0001 (data in word) and I can read data with the shared variable and data item 300001, respectively.
Message Edited by utechle on 01-27-2009 11:12 PM

Nimodbus.llb vs modbus IO Server in project?

I am plan on creating 20+ Modbus TCP Masters on a CRIO and wanted to know if there is any real benefit to using the nimodbus.llb versus creating a modbus master through the project? I want to poll 20+ devices and consolidate the data on a modbus tcp slave on the CRIO. I plan on polling the devices every 500 or 1000 ms. Will there be noticeable performance boost if I use the nimodbus.llb? I personally would like to create the Modbus masters through the project cause its cleaner and easier but I was not sure if performance would be an issue with that many masters running on a CRIO.

Hi Andy,
I would use the Modbus I/O server built into DSC. I believe that you will see a noticable performance difference, but the only way to know for sure is to benchmark it. I don't see any issues with having that many modbus master but have not tried it to know for sure. Please post back and let us know how it goes.
Also the following links my prove usefull for you:
Do LabVIEW and LabVIEW Real-Time Support the Modbus Protocol? (this talks about the differences between the I/O server and nimodbus.llb)
Modbus Library for LabVIEW to an FPGA-based implementation for use with the cRIO serial modules. (really cool)
Please let us know if we can be of any help
Joe Daily
National Instruments
Applications Engineer
may the G be with you ....

Modbus/TCP Queries

Hi,
I am trying to use Modbus TCP to set the time / date on my cFP-2120
running Labview Real-Time 5.0.1. For this purpose I have set up four holding
registers that hold two single precision floats, one for the time (format:
hhmmss) and one for the date (mmddyy). The idea is that each time the Modbus Master writes to the holding
registers, the time / date of the cFP2120 gets set.
In my application, I have implemented Modbus communication
similar to the MB Ethernet Example Slave.vi, which came with the NI Modbus library
for Labview 8.2. This is also similar to the Modbus tutorial here. Essentially,
I am reading the appropriate Modbus holding registers using the MB Register
Manager.vi (also part of the Modbus library) and use their values to set the
controller.
The problem arising is that I set the controller time even
if the Master has not written to the holding registers, which results in
wrong times/dates. Is there a way to determine whether these holding registers
have been written to by the Master, so that I can react on it as soon as this happens?
Thanks,
Volker

Devin,
thanks for the response.
Your suggestion are a possibility, especially the first one.
However, as these registers are holding registers (read/write), I was hoping to also post to them the current time by the slave to be read by the master.
In case the time needs to be synched, the master posts the correct time to the same registers upon which the slave will update its time and again start posting its current time to these registers.
I realize that the easiest way to accomplish this would be to just use different registers for posting the time by the slave and have the holding registers reserved to be written to by the Master. In a previous implementation in C ( ... please forgive me) I used to listen to the traffic and parsed each message coming in in order to determine whether (a) my device was queried and (b) which function code was used for which registers. Based upon the info I decided whether to just send back the current time of the slave to teh master or take the incoming time value and set the time of the slave.
How would I implement something like this in Labview? Or am I better off just having different registers for reading from and writing to the slave?
Volker

Modbus TCP/IP configuration steps

Hi everybody.
I have an application where I have to connect a robot with a NI CVS1455. Both, robot controller and NI CVS support Modbus TCP/IP communication protocol, so, I want to configure robot like “Modbus master” node and compact vision system like “Modbus slave” node.
I have downloaded the NI Modbus library (I’m using LabView 8.2) and I’m trying to understand how Modbus communication works with NI products using my computer (instead the robot controller) and CVS. To do that:
- First, I have configured in “VI Server:configuration” the right TCP/IP address and, then, the port as 502
- I have enabled the access for both, computer and CVS, in “VI Server:access list”
- I have deployed the Modbus slave program (VI) in CVS
At the moment when I start the Modbus master program (VI) in my computer I receive first “error 66” and then “error 62” .
I have already tryied to use “TCP Communicator Active and Passive Vis” and the communication is ok.
I think, I’m making some mistake in Modbus configuration. I didn’t find NI Manuals that explain step by step how to configure the different panels for Modbus TCP/IP communication, moreover, the Modbus functions don’t have the context help.
So, my questions are:
a) Can someone, experienced in Modbus, give some advice about my problem?
b) There is a manual or an “application note” about the “configuration steps”?
c) Can I use “shared variables” in a “NI Modbus slave” device even if the “Modbus master” isn’t a NI device?
Thank you in advance.
Michele
AUROS-Ing.Michele Rossi
Software e Tecnologie per Automazione

Modbus is used to read and write registers in slave devices. The master in your simulation should be the device reading and writing the registers, so if the CVS is supposed to read data from the robot, you need to reverse what you're doing.
In general, Modbus has nothing to do with the VI server. You can change the VI server settings back to their original values (port 3363).
The NI Modbus library has two example VIs - one for the slave device and one for the master. If you want to simulate this, you need to run those examples in their respective locations and give the master example the IP address of the slave.
Of course, in any case you need to know which registers you're going to read and write, but those are device specific.
Try to take over the world!

Write values to PLC addresses via MODBUS/TCP

I am trying to write values to some addresses on a PLC connected to my computer with MODBUS/TCP. I've mapped the addresses that I would like to write to on my project's MODBUS library as shared variables. However, I seem to neither be able to read from nor write to them these addresses. I've tried viewing the variables in LabVIEW's Distributed Systems Manager and the variables, when configured as read-only or read/write, report an unknown value, and when configured as write only, they do not display a value.
I've verified that the PLC is connected properly, and I am able to read from other addresses on the PLC that I do not wish to write to. There also does not appear to be a communication error either. I have also successfully read from and written to the addresses using a third-party application, but for some reason, Labview will not do the same.
Any suggestions on what might be wrong?
Thanks!

Hello Griffon,
How did you set up your Modbus I/O Server? Did you follow the steps in this document?
Connect LabVIEW to Any PLC With Modbus
Make sure when setting up your server that you put in the IP address of your PLC (not localhost, or 127.0.0.1).
Also make sure that you're adding variables using the "create bound variables" dialog.
If you did set everything up in that way, and it still not working, can you clarify what you mean with this sentence?
"I've verified that the PLC is connected properly, and I am able to read from other addresses on the PLC that I do not wish to write to"
Does this mean that you can read values out of the PLC using modbus, just not the values you want?
Colden

Using modbus/TCP with USB ethernet adapters

I have a test stand where I have a handful of USB instruments and a single modbus/TCP slave. I'm considering adding a USB ethernet adapter to the stand so that there is a single USB cable connecting the computer to the hardware rather than both a USB and an ethernet connection. Does anybody out there have experience (good or bad) using USB ethernet adapters with modbus/TCP slaves?
The existing USB instruments all use ASCII/SCPI-style command sets. The Modbus/TCP slave is an Automation Direct EBC-100. Nothing high-bandwidth or low-latency. Basically, the computer issues no more than a handful or command or request (ether SCPI or Modbus/TCP) per minute.
Mark Moss
Electrical Validation Engineer
GHSP

Adam G wrote:
Hello Mark,
I was wondering what NI hardware you are going to be using in your application? Also are you going to be using LabVIEW for the programming? Have a great day!
Best Regards,
Adam G
The test software is written in LabVIEW, and the test stand uses M-Series cards for data acquisition.
Mark Moss

MODBUS/TCP very slow execution

Dear all
We are new user to Labview and to LabVIEW forum.
We are designing a LabVIEW application that interacts with PLCs.
We are trying to exchange information between a PC/labview and a PLC. Both support the MODBUS/TCP protocole. We have designed the 2 programs easily, the PC/LabVIEW one being easily made with VI Library.
We first the PC/LabVIEW as a cleint and the PLC as server and it works perfect. It is quite fast as it takes less than 1 second.
We test the reverse configuration ie PLC as client and PC/LabVIEW as server. Of course we use different programs there and monitor the MODBUS/TCP frames with WireShark.
It works also but the execution on the PC/LabVIEW is very long (in the debug mode) : it takes 5 secondes to execute the Write Multiple Register function ( 2 data to write) to prepare the response frame and to send it back. It is very nice to see the animation on the diagram but why is it so long ? This long time frame is a very big problem for our application.
We re not sure to know where to dig in !
Thank you in advance for your help.
Regards
Thierry

It sounds like you need to learn more about the LabVIEW development environment if you were confused about the slow speed of highlight execution.
I would recommend looking at the online LabVIEW tutorials
LabVIEW Introduction Course - Three Hours
LabVIEW Introduction Course - Six Hours

Modbus TCP with Beckhoff (Wago)

Hi,
I'm looking for examples/how to/lib/dll to build a communication to a Beckhoff BK9000(similar to Wago 750) which have a Ethernet - Modbus TCP Option.
I'm able to write via LabVIEW-TCP-Functions but only once after powering on and the result/answer is more confusing.
For the lookout OPC variant i heared there is still some additional software needed - true? or is it possible to set up the communication with lookout OPC without third party driver?
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Hans

Hi JAMR,
There are plenty of examples that setup up the tcp server communications. If you open up the NI Example Finder and double click on the Networking Folder (under the Browse tab). You can then select the TCP & UDP folder and find several examples that use TCP. Also, there is a great knowledge-base article that provides information about network communication with TCP/IP messaging protocol. Essentially, what you are going to want to do is establish basic connectivity between the computer and the device and then assign the device an IP address. Before you do anything with the driver for the device, you might try to ping the device first. Once you can get this working, then you can set up the TCP/IP communication. I hope this helps!
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Modbus tcp on c-rio

Hello everyone. I already wrote an application to control via modbus tcp the plc of my brushless motor. It works fine. Now i'd like to do the same thing using 2nd ethernet port of c-rio (just set like ni guide). It's possible this application?
thanks a lot
Francesco

Ok, I understand what you would like to do.
Yes you can do this, as soon as the shared variables hosted on the cRIO will not be served to both subnets at the same time. I found this information in the following link, under NI Dual Ethernet Technology.
Introduction to Dual Ethernet Architectures
http://zone.ni.com/devzone/cda/tut/p/id/7103
I think that also this example will help you:
Using Secondary Ethernet Port on a Remote Target for Data Transfer:
https://decibel.ni.com/content/docs/DOC-8744
Antonios

MODBUS TCP Error 56

Hello,
I am having trouble with MODBUS TCP communication. I am trying to read the input registers on a master.vi after a slave.vi has written to it. Attached you will find my slave and master vi's. I have made sure I had the correct IP address, same starting address for master and slave, and also the same port number. The input register is array size 4 and I've also wired that in on the master for the quantity to read. Could someone please give me some pointers?
Greatly appreciated!
Attachments:
Master.vi ‏15 KB
Slave.vi ‏13 KB

I found my problem. I was missing the TCP communication in the slave.vi.
Thanks

Watlow PM with Modbus TCP

Hi community,
I have a watlow pm temperature controller connected to my computer with an ethernet cable. I need to communicate with the device via modbus tcp. I know for sure that the unit is functioning as if I use a modbustcp test program (see the first attachement), then I can read the temperature of the K type thermocouple which is connected to the unit. (the IP is 192.168.10.3, and I read two words from address 360). The temperature on the screenshot is 23.61992C
I would like to do this from LabVIEW by using an IO server. My problem starts at the beginning. I right click on my computer >> new >> I/O server >> Modbus
then in the popup window I select "Modbus ethernet" in the top right corner. I have attached this popup window. What is the Address in the communication settings section. The IP need to be entered at the bottom, so I have no clue what is the Address field is referring to.
Then I try to create bound variables... I see a huge list of registers, but honestly I dont understand how the address 360 I use in the standalone program translates to this list. Which one should I select from the list? Whatever I select gives me either 0 or FALSE if its a boolean. According to the documentation 360 is the relative address and the absolute is 400361. Actually there are a register list from 400001-465535, but if I select 400361-400362 I still read zero.
Reading only zeros and FALSE values... thats suspicious. Something is wrong.
Please help out with your thought!
Attachments:
modbustcp_OK.png ‏132 KB
modbus_config.png ‏56 KB
modbus_reg_list.png ‏98 KB

OK, just a quick (and dirty) test. Can you try this VI.
Kees
Attachments:
MB TCP test.vi ‏20 KB

Is there a Modbus TCP driver built into Lookout ?

Is there a built in ModbusTCP driver or do I need to add-in a third party OPC server.
Thanks!
Greg.
Schneider Electric.

The Modbus object in Lookout supports Modbus RTU/ASCII, Modbus Plus, and Modbus TCP. I am using all three protocols with great success.

Problem communicating via Modbus TCP with Honeywell UDC3200

Hello:
I have to read the temperature of the Input 1 of this controllers. I've succesfully read the temperatures in the controllers. I'm using Modbus TCP/IP for the communication through the NI Modbus Library 1.1. I'm working on LabVIEW 8.5.1. However, sometimes when I open the connection, I get error 56 (timeout), despite that the controller responds to a ping. When I can initiate all the connections to the controllers, some hours after the monitoring has started, randomly, I get an error 66 (the connection was closed by the peer). This error interrupts the connection and the temperature readings stop. This is not acceptable.
Also, I've found out that if you don't request some data from the controllers in 5 seconds or more, also the error 66 appears. To solve this, I poll twice a second the controllers but the error 66 still shows up.
The controllers and the PC are interconnected via an Ethernet switch. The switch works well.
In brief, I can communicate with the controllers but the communication is not reliable. Sometimes when I start the communication I get an Error 56 and sometimes after the communication starts I get Error 66. It cannot be predicted when this will happen.
Why does the error 66 appears?
Has somebody communicated succesfully with this controllers?
I'd really appreciate your help, I'm running out of time.
Thanks in advance.
Robst.
Robst - CLD
Using LabVIEW since version 7.0

Hello:
Thanks for answering Pedro. I've read the document you told me to, but it refers to problems with VI Server, the application is not using VI server, so I guess the problem comes from somewhere else. I still haven't figured out why this is happening.
About the Error 56, well, I'll try to slow down the communication, I'm not doing any delays after sending the information. I wasn't aware that I needed to do this.
Any other ideas?
Thanks in advance.
Robst.
Robst - CLD
Using LabVIEW since version 7.0

Modbus TCP server/slave implementation

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