Modbus RTU - Reading Holding registers problem - Novus myPCLab

Hello,
I'm trying for a few days to read Holding registers in Novus myPCLab (it uses Modbus RTU protocol). I used both DSC module and Modbus library with no success (it appears to connect correctly but cant read).
I need to read the given current.
Does anyone have any idea about what am I doing wrong?
Here are my vi and the modbus address.
Via DSC
I used the PLC address at "HR Inicial" and 1 at "numero de registros"
Via library
I used the PLC address in "starting address"
Solved!
Go to Solution.

My problem now is that my VI doesn't stops after I stop the while loop, it is necessary for a flat sequence implementation.
Or my other best option is to read and write without closing the Modbus connection. Is that possible with a global variable?
I guess I do have to execute this "Modbus VI" to get my variables at a Main VI if they are in the same project. So, can I make my "Modbus VI" works in the background while the Main VI (who reads and write the addresses from the global variable) works, and when I finish running it, they stop together?
ModBUS VI

Similar Messages

Modbus tcp read holding registers not returning requested quantity

Background: I have a customer using an ELAU motion system - they are recording data to Holding Registers that they want to be able to read on a cRIO to match up with some analog data from FPGA (I have digital handshaking happening for that).
LabVIEW 2010 SP1
cRIO 9074
Using the NI MOdbus.llb VI Library to communicate with the other system. I can open the TCP communication no problem, and actually get SOME registers, just not ALL of the register data I want to read.
What I want to do is read 330 individual holding registers U16 values. I know how the data is being split to represent various lengths (i.e. most data items are using 2 registers to represent one 32 bit number). I just want to read all the individual registers and parse the data in another VI to convert it to the other data types.
I provide the MB Ethernet Master Query (poly).vi with the starting address for the first register and then the quantity of 330. The polymorphic instance selected the is "Read Holding Registers". The array returned from this VI via "Holding Registers" is only 74 elements and not 330 requested. I get no exception codes and no LV errors. Is there some inherent limit on the number of Holding Registers that can be read?
I am not using the MBAP Header input (not sure if that is needed).
Thank you.
Ryan Vallieu
Automation System Architect
Solved!
Go to Solution.

Simple solution once I dug into the MODBUS Serial protocol/TCP protocol documentation available out there via Google.
Serial based modbus history is the limitation that carried over to TCP - the maximum amount of bytes in one data pack can only be 256 bytes. Thus I was limited to around ~125 registers at once.
256 bytes = 2048 bits. Using 16 bit registers that gives 128 registers maximum. I went with 125 to make keeping track of totals easier.
Ryan Vallieu
Automation System Architect

Reading Holding Registers Float Modbus RTU

I'm having an issue with ready any floating point number correctly off of a Shark 100 Power Meter.
I can sucessifully read a block of registers containing the meter name and serial number as 16 bit characters.
For example when I read registers 1-16 on the map and flattening them to a string, I get a fully working string returned.
When I try and read holding registers 1012-1013 (Amps A) I get really funny numbers when I convert to a float.
Here is my code so far
With my front pannel settings:
Expected Value I'm looking to have returned: 11.11 Amps
but
When I use an indicator to read the registers: 19265, 0
Same value when I do a conversion
When I do a type casting to a float: 1.26484E+7
I've tried some different combinations such as reading only one register and joining the numbers, both in big-endian and little-endian order, still no luck.
Please Help!
Here is the modbus float definition as defined the owners manual:
Solved!
Go to Solution.

The float format you posted is the same as LV uses, so if you put example values (Register 1 = C4E1, Register 2 = 1DB9) in the code below it will give you right result.
What is the full value of your "Output Registers"? You should have two values in numeric array for this to work.

Modbus - Issues with Reading Holding Registers

I am trying to read the holding registers on a RS485 2 wire modbus power monitor (Veris Industries E50C2).I am receiving bytes from the device so there appears to be communication. However, the response string is the exact same string as my command string. For instance, I am sending the hexadecimal string 0103 0000 0005 in order to read the first 5 holding registers, but I am receiving this exact same string back via the read VISA read function. I have attached an image of my VI. I would expect to see the response as "0103 (the device ID and command) followed by the number of bytes to follow and the contents of the holding registers.
The VI is running on a cRIO 9012 with 9111 chassis. I am using the 9871 c-series RS485/RS422 interface module.
Any ideas what the issue may be?
Kind Regards
Adam
Attachments:
Modbus VI.png ‏15 KB

I indeed manage to fix the issue for myself.
I used the same method you are starting right now, i conencted the NI9871 to the rs485 adapter given by the supplier and tried sending and reading my own signals.
After much hassle and some changes to your (and my) initial vi setup as shown in your first post, everything worked out fine:
(see the attached picture)
1. I used the NI modbus VIs, but it worked as well with the normal visa commands and building the modbus command on your own, but way easier with the NI VIs.
2. i added the function to resize the buffer to its maximum size (not sure if thats actually helping with anything, but better be safe then sorry).
3. Change the wire mode for the 9871 to rs485/wire2-auto with a property node.
4. remove the large delay between write and read
5. Last but not least, the one thing that changed the fact that i only received my own echo:
I changed the connection of the Rx+/Tx+ and Rx-/Tx- around, basically connecting the 2wire sensor the other way around the manufacturer said it should be connected to the master device.
(6. Recheck the amount of stop bits, all modbus device i use in my lab are using 2 stop bits, but that could be a coincidence)
Maybe something of that list helps you
Attachments:
Modbustest01.jpg ‏95 KB

Modbus RTU over TCP (Simex Transmitter and Advantech Server)

Hey Guys,
somehow i am not able to get a connection over TCP IP using RTU Mode (reading values) from a transmitter which is hooked up to an advantech EKI 1542 Server. The Port is configured as an RS 485.
i am using the modbus library and i tried to adjust the MB serial read holding registers vi to accept a TCP IP Socket input instead of an INSTR. Do you have any suggestions?
Thanks a lot for the forum support!

Hi Christo and welcome to NI Forums!
Is this the ModBus library you are using? If so, I'd recomend to check out the newer, object based library available here. It has a more straightforward setup, especially for using TCP connections. You'll find an example VI for using a master, as well as a slave device, and these you'll be able to use right away without much modification. The code'll look like:
Please test your device communication using this library.
Kind regards:
Andrew Valko
NI Hungary
Andrew Valko
National Instruments Hungary

How do I read ModBus counter holding registers in Lookout 5

We are using Lookout v5 with Zetron M1700 and M1716 RTUs. The M1700 is Modbus1 and the M1716 is Modbus2.
We need to be able to count the number of times a digital input is turned on and off over a period of time.
We are using digital input #9 on the M1716 RTU. According to the Zetron literature, the counter holding registers for digital input #9 are 42091-42093 for the value and 42094-42096 for the period.
I tried to calculate the the value stored in the registers by using this formula which was suggested by Zetron in an expression:
(((Modbus2.42093*10000)+Modbus2.42092)*10000)+Modbus2.42091
However, this always returns zero.
I read through your example on reading the BCD registers and doing calculation
s on them but it hasn't helped me with my problem.
Is there some command I need to use or a special type of an object I need to use to read the counter holding registers?
I know we are communicating with Modbus2 properly because I can see when the digital input is on or off.
Any help is greatly appreciated.

Hi,
Is it possible to share or point us to the Zetron literature explaining this calculation?
If you just read the registers 42091-42093 individually from Modbus2, what do you get?
Alternatively, you can maybe count the digital line directly in Lookout. For this, you would create a Counter object and specify the digital input as the "Count=" input. Not sure if this is a feasible option for you.
Again, if you can share the Zetron documentation, I can take a look at it to see why it may not be working.
Regards,
Khalid

Failure of LabVIEW DSC 8.0.1 to yield values of Modbus RTU holding registers

So, I know what I'm doing--or used to. Have done this before in LV 6 and 7. Previously used Lookout OPC drivers and the "Tag Engine".
Recently with 8.0.1, I first set up the Industrial Automation OPC servers (AKA Lookout) to communicate with the holding registers (40,001-49,999) of my modbus rtu Watlow 96 and MLS316 temperature controllers. Subsequently, with the Server Explorer all of my 133 parameters on our 2 rs485 serial ports are communicating bidirectionally.
So now Duh, why can I not bind a shared variable to register, say 40,334, and eliminate the OPC? The data is always reported as bad. Neither can I link shared variables to these addresses through the OPC I/O server.
Am I missing something? Are there any examples of code available where individual modbus holding registers are communicated with bidirectionally?

Well, this may seem rather silly but I viewed the video demo that NI has on binding modbus registers to shared variables before I even made my attempt.
Now, differing from the demo video, I am using modbus RTU on a serial port. Also differing from the video, when I create my shared variables, they are not placed in the same lvlib as the modbus server just created--LabVIEW prompts for a new library to be created.
Could the NI product service staff please test with a serial modbus RTU device (a real living creature and not just a simulation please) the bidirectional communications and prove functionality with the Shared Variable Monitor of your product? I suggest that you attempt communicating with holding registers 40,001 through 49,837.
By the way, my com is set up correctly for I can view and or change all of the desired modbus RTU registers using the OPC Lookout Protocol Drivers through the Server Explorer.
Please do not send me any more links to videos that test with simulations. A real solution is required.
Lloyd Ploense

Need help with read/write to MODBUS RTU registers in Labview 8.0

I’m trying to write and read the registers in G3800xx016. controlling temperatures and lightning. Please look at the attached pdf file for this unit..
I have used Lookout for about three years, and it works fine.. Now I want to expand my system and try to use Labview 8.0
My temperature register start adress is 100 Hex (256 dec) I read from Register with Function code 03.
This register have values from 0-255 witch mean in labview for me as unsigned byte U8
I read my temperature from the register like this: eks.: 40257 lets say value=5
Slave    FC       Start Adress     Byte     Value               CRC
01        03        0100                02        0005                7847
This is the status received from the Modbus tester when I use null-modem connection..
I receive the same value if I use Modscan32
I have used null-modem cable to read out the values.
I have also PIR detectors that control movements and light of and on, and I read these starts at 40017.1
01   03     0010     02
The register to switch on and of heating and lights starts at 404097.1 FC16
Values mention here is read from Lookout 5.0
I have tried examples and read for weeks about this, but everything stops when I try to use Labview 8.0    I can’t read any values from my Master unit, neither write to it, or read any values in Modbus testers when I use null-modem cable.. I have tried different baud rates in my serial connection and it works perfect in Lookout, but Not Labview..
I have even bought DSC module, but I can’t write to either com ports or Modbus testers..
Please anybody… is there anybody that can give me help with this??
Start address for for PIR detectors is 40017.1 (FC03) and for writing bits for on of to light and heatsystem is 404097.1 (FC16)
I would be very happy for any idea that can give me some positive results… Look at the attached pdf file.
Thanks.. and very best regards from
Benjii…
Attachments:
G3800x016_02-2005_eng.pdf ‏70 KB

Hi... and thanks for your answer..
Yes, I have downloaded the NI Modbus drivers.. I have downloaded that one + a lot of other things about Modbus..
I have also tried communication between lokout and labview true null-modem cable.. and it works..
I have tried several Modbus examples and I communicate with most of them with null-modem cable.
I write to the right registers as well, cause I use Modbus scanner to read the information..
I have used the same settings for serial communication as my Master device use, bust still I can't get any response..
There was an error message in one of my example from the Modbus INIT module.. Maybe I should try to set up a new connection using VISA ??
Maybe the problem is to INIT my serial port? I have tried restarting my computer after closing my system.. But still, I have the same problem..
I hope you can give me any idea...
Thanks again.. by the way.. is it possible to write and read directly to serial port with the DSC module?? If so.. how??
Very best regards..
Benjii..

Dsc modbus - Serial communication input/holding registers

Hello,
I am trying to create a labview application to act as a Modbus master and control multiple stepper motors. I have the DSC module in labview and am using a serial communication with an RS-485 converter. I have followed this guide: http://www.ni.com/white-paper/13911/en/ However, the driver I am using does not support coils, it only uses input and holding registers and I have not found a guide or tutorial that shows me how to create a labview application that can read and write to a specific address. For now all I want to do is turn the motor on and off using labview. I have used a modbus simulator to communicate with the driver and can do everything I want to do through the simulator. I can not get labview to do anything for me. I would be very happy if someone could direct me to a tutorial that can help me understand labview dsc module for reading and writing to addresses using modbus.
Meesh

Ok, so this is really strange. I solved my problem in a round about sort of way. I'm using a modbus simulator (mdbus) to help me communicate with the driver/stepper motor so I know which address to use. But after I use the simulator I get a communication port error when I try to use Labview. It's like the port is locked in with the simulator and I can't end the connection and let Labview use the same communication port. I can't find anyway to fix the problem except going to this NI site: http://www.ni.com/download/labview-run-time-engine-6.1/746/en/ downloading this software and using the "repair" function. Then I can do a system restart and cycle the power on the driver/motor and then Labview with work again through the correct port. But if I need to use the simulator again then I have to go through this whole process again. Does anyone know what the heck is going on? Or if there is a better way to clear the communication port??

Lookout 6.0.2 modbus rtu problem?

One of our customers has Lookout 6.0.2 communicating with several CMI Scada Packs over wireless modems using Modbus RTU protocol. Suddenly last Friday night he got comm fail alarm pages on all but one site. The Modbus statistics window revealed that Lookout had stopped polling all sites except the one site that stayed in communications. The only activity was the one site.
He watched it poll this way for 2 hours, never returning to the normal polling cycle.
I had him exit and re-launch Lookout and polling returned to normal and has been that way since late Friday night.
This system has been in service for several years with very near 100% good communications sice we deployed it, up until now.
It was obviously a Lookout failure, since the only action taken was to exit and re-launch Lookout.
Isn't there a watchdog timer for this sort of error?
Is this a failure of the Modbus cbx object? (We are using the plain vanilla Modbus driver)
Roger Foote
Foote Control Systems
Solved!
Go to Solution.

Hey Mr Domer
If it's the one I think it is, I designed and built the RTU system and repeater system after the "Packaged" system became obsolete.
That was back when 1200 baud was fast! I guess it was compared to the 300 baud limit on the old stuff. Probably been re-built a couple of times since.
Thanks for the sequencer suggestion. I will have to think on this since we are pushing the limits for polling... Some ScadaPacks in this syatem have to do up to 25 modbus transactions per site... We have serious I/O in this sytem lots of which is in floats. Haven't tried DNP yet but I am getting pressure from some distributors to go Clear SCADA and use DNP to thin out redundant polling. Thing is this system has worked flawlessly for years and now suddenly we have this issue, so I am not ready to throw in the towel yet and I doubt the board would go for the moneys to implement it. We are adding another 8 pump site this October after the canals are off line but before they are drained for winter. This is so we can test the system before the frost protection season for the almond crop.
If NI can't fix this I will probably just insist that my clients reboot every week. I am fortunate that these are not unmanned districts. Only problem is that each of them are at least a 2 hour drive from any one point so I need to keep callbacks to a minimum which would be zero callbacks if it weren't for software screwups.
Best regards
Roger

MODBUS-RTU connection with aleatory VISA communication fails

Hi everyone,
I have a big problem that I'm trying to repair since a pair of months. I have a MODBUS gateway connected to my pc via RS-485 in RTU mode. The physical implementation is based on Ethernet cable, so I have two connection converters in both endings of the Ethernet cable that wire each line to its corresponding terminal in a RS-485 connector. On the side of the PC i also have an electronic card to convert RS-485 to USB for connect using a USB port for pc's connection. That card is the model USB-COM485-PLUS1 from the company FTDI chip. I have installed on my PC the drivers of the card that configure a virtual COM serial port for my RS-485 communication. On labview I have programmed my interface for communication using a MODBUS library that was installed previously on my PC (I am using my company PC, because i have this problem on my workplace).
The problem is that, usually, my system works correctly, and the communication is perfect. I read always the holding registers of my gateway, and i can write the control registers without problems, but, some times, the connection spend more time without communication that reading, and after some hours in that conditions, it fall in continuous fail and doesn't work any more since the next day. That problems persists even if i restart the PC or shut it down for a while. By the other side, I realized that when the communication falls it only fall on the reading wire, because even if the reading communication is with continuous fail, i can write on the corresponding registers of my gateway.
I'm sure at 80% that the gateway runs correctly, and I have other installation similar with the same gateway, cable, connections and card working correctly. I tried changing the Ethernet cable, changing the RS-485 to USB conversion card and only when i changed my PC, the problem disappears. So I thought it was a problem of the PC. But some time after I changed the PC, the problem returned. After this, I continue making probes and actually I have the theory that it may be a problem because i have other communications running at the same time in my labVIEW app. I think it may be a problem of incompatibility of drivers or a bridge over different communications that make it fail. Because the pc's where this error disappear has not installed NI RIO drivers.
My LabVIEW program also have a communication via Ethernet with a NI cRIO 9076 chassis. That chassis has a module for PROFIBUS communication, also has a module for thermocouple inputs, a module for voltage inputs and other for voltage outputs. All this modules work correctly always. My theory is that some of this other modules in NI cRIO 9076 chassis may cause a conflict that make my system works wrong. Other things that seems me annoying is that i bought recently a new conversion card for RS-485 to USB connection and it doesn't works on my system, I have other two identical cards and they work correctly, and i know that the new card works also in other systems i have.
The issue I don't understand is that this problem doesn't happend many times and it seems to be aleatory... The fail that the labVIEW return when it can not read the holding registers is a timeout error because of a frame error. It would be illuminating for me if that error where periodic or systematic, but occurs only a day every two or three weeks and the rest of time works properly. But on my company, when this installation will work continuously, it can't have any day of non-work.
So, please, I need help with this Issue, I'm lost int he way of resolving it. Thanks only for read this large post.
Regards,
Joaquín
PD: Please, don't damn my English, it is not my mother tongue, I'm spanish.

First of all, thanks for your quick reply.
I had the USB port save power mode enabled. I have disabled it. Actually my system works correctly so I can't know if the problem has been solved, but i have a new converter card for RS-485 to USB and it doesn't works, i have other two that work properly but the new only writes, just like when i have the problem commented. So, I think that if i solve the problem with the new card, i can be sure that I will have solved my connection problem. The new card is configured with the same options that the others, except by the port number, i configured COM 3, COM 4 and COM 5 respectively.
By the other side, when I said that I think cRIO interferes in MODBUS communication I mean that the only things that changes in PC's that correct the problem are that they have not installed the FPGA module or the RIO drivers. And once i changed my PC, the problem was solved since I installed this module and drivers, moment when the problem reappeared.
Thanks again

Modbus Ethernet read and write to a Eurotherm 6180XIO Modbus server using LV8.2 shared variables

I am having EXTREME difficulty trying to establish communications with a Modbus device using LV8.2 shared variables. The device is a Eurotherm 6180XIO Datalogger configured as a Modbus master. The PC and a cFP-1804 are slaves. All IP addresses are set correctly. This approach using shared variables would seem simple, but I can't find any examples or proper guidance on how to get it working. I am trying to avoid having to mess around with TCP/IP, OPC, or any other old-fashioned method.
I have read many threads on related topics but none directly apply to this situation. I have created a library containing a Modbus I/O server and shared variables bound to read and write holding registers. I have followed all recommended tips for creating such variables but I can neither read or write data. All data types are U16 due to Modbus protocol limitations. I have also applied the LV x10 factor in the most significant digit in the register offset (6 digits instead of 5).
I have a cFP-1804 on the same network which reads into the datalogger OK. The registers I use are 31000 (for CH0 on module 0, 31002 for CH1, etc) and the data can be read as FLOAT32. I have updated the firmwate on the 1804 to the latest level. I cannot even get shared variables to read SGL values. Using registers 301001 for CH0 and 301002 for CH1 I can only read U16 values, and not a 2-word SGL.
Third party Modbus simulation software is able to write to and read from registers very easily, but not LabVIEW.
Some questions are:
- do I use a Modbus master or slave as an I/O server in the library as a target for binding the shared variables?
- is there some other wierd translation in register offsets between LabVIEW and traditional Modbus?
- is this actually possible using shared variables or am I wasting my time?

Sending the whole 60-character string using a string or array would be the most efficient. I have tried both methods, and these only cause the datalogger to flag a message log but no text is displayed.
For a string variable, I have used the following binding "My Computer\Modbus Test.lvlib\ModbusServer6180\442305", where ModbusServer6180 is a Modbus I/O server configured with the logger IP address, and 42304 is the register offset at the start of the text block in the logger. I need to write to 30 consecutive registers starting with this one. I am not using buffering and have not enabled single writer.
Can anyone confirm whether this method should work in 8.2?
Does the string need a special termination character?

I can't write holding registers

hay My program mast visualize some data of a controller. This with an Ethernet connection. I can read everything and I can write coils bet I cant write the holding registers. I hope someone can help? Heir my VI: Rutte
Solved!
Go to Solution.
Attachments:
Master.zip ‏824 KB

Thanks for the Flat sequences tip. And for clean up my mess JI think the data must be 16 bit integer because I can read it with 16 bit integer.I also think that the values must fit into 2 consecutive modbus registers. Because when I read it fits. Now I have typecast the data ant I have convert it to I16. bet it didn’t work. What do you mien with split/join numbers?
Attachments:
holding registers.doc ‏40 KB

Need help with Modbus RTU over TCP communication

Hello!
I'm working with a project that involves communication between a LabView PC, a cRIO-9073 and a few Crouzet PLC's with XN05 modbus extentions. The issue I'm having is to get a stable and reliable connection between the units. The cRIO is communicating with the PLC's by Modbus RTU over TCP and I've created code that read and write to them in a sequential order to avoid possible ModBus conflicts. The LabView PC is sending a "ping" (boolean true) every second to the cRIO over UDP and the cRIO is then sending the "ping" to the PLC over Modbus/TCP. The PLC has a timer that waits for pings, if no ping has been recieved in 7 seconds the emergency brakes are applied and the system shuts down.
The main problems right now is:
* After a while I lose the connection between the LabView PC and the cRIO. "Connection with RealTime target cRIO-9073 has been lost".
- I also get either error code 56 or 66 on the TCP link.
* The second problem is that if I quickly pull a TP-cable from, let's say, a PLC and then directly plug it back in I want the TCP connection to quickly reestablish. However what happens is that the "pings" from the cRIO stops coming to the PLC for like 5-6 seconds (time to reestablish link?), which means the 7 second timer is almost full. One option would be to raise that timer value, but the drawback is that the system will take longer time to respond to an true error which could be bad.. My question is simply, does it really take 5-6 seconds to establish a TCP connection between a cRIO and a PLC? What affects the time?
I would greatly appreciate help or respons on this one. I attach a simplified version of the code I use (all variables and such has been replaced with constants).
Is this a good way to do a ModBus communication over TCP with 3 PLC's? Is there a better, safer, more reliable way? All suggestions on improvment is appreciated
Thanks!
/J
Attachments:
MB_TCP.vi ‏29 KB
MB_PLC.vi ‏28 KB

Dear J,
Check the state machine Architecture here.
1. In your acquisition if you got error in any system you close all TCP connections that is not right your other systems are working fine.
2. You go with parallel loop connection. For each PLC you run separate code with error handlers.
3. Check the attached VI for TCP Read. Same VI you can modify and use for your ModBUS operations.
4. In you Sub VI you are passing a constant cluster. I am not able to understand what is the use of case structure inside the subVI, In you code you are passing the constant so its always same.
As you done previously made 3 loops or VIs to read the data in a main VI. First try to made one VI with one PLC, remove your all bugs and errors and copy it 3-time with different-different configurations.
Thanks and Regards
Himanshu Goyal | LabVIEW Engineer- Power System Automation
Values that steer us ahead: Passion | Innovation | Ambition | Diligence | Teamwork
It Only gets BETTER!!!
Attachments:
Tcp Host.vi ‏28 KB

Converting modbus data from two registers into float

I am reading modbus data from a DUT from two holding registers- 40609 and 40610. The register content I get is an array of two elements (12458, 14019). I want to convert these numbers into a floating point value. Does modbus library has a VI that does it? If not, how do you convert it?
Thanks!
Solved!
Go to Solution.

Like this
That is join numbers and also typecast.
Attachments:
sample1.png ‏11 KB

Modbus RTU - Reading Holding registers problem - Novus myPCLab

Similar Messages

Maybe you are looking for