NI LabVIEW Modbus API Discussion

[ShivamSaxena]

Hello Porter, 

I have an application where the Modbus slave has an IP address, but using the Create TCP Slave VI, I don't see an input where I can specify the IP address. How would I go about doing that - using property nodes?

[ShivamSaxena]

Sorry everyone - there was an error in documentation on the manufacturer side. All addresses are in just one unit, so I can do it the conventional way that I described. All good!

[Tomwell17]

Hello, 

I am new to the data communication protocol for Modbus RTU and have been looking around for some help encountering an error. I am currently using the NI (unsupported) Modbus package to collect data from a Vaisala WTX536 weather station. From the weather station using RS485, it is connected to a RS485 to Ethernet converter, and then into my laptop. I am using LabVIEW 2021 and only reading 6 different input registers in a loop, every 5 minutes. Every so often, I get "Error 56: The network operation exceeded the user-specified or system time limit." So far, I have tried to increase the timeout up to 1 whole minute but with no luck. I have also read around that maybe I need to specify the data length that I am expecting but I cannot find how to do that for the input registers, so I am unsure where to go from here. Next, I will try using a different Ethernet cable to see if that will do anything but I am reaching out to see if anyone can help as I am new to this. I have attached a screenshot of the LabVIEW block diagram that I am using and the error reported while running.

I am confused mainly because I can get data, sometime the error will show, and I can continue to collect data after I clear the error. So it is not a consistent problem. Any help is greatly appreciated.

Thanks,

Thomas

[ShivamSaxena]

Hey Tomwell17, 

While you should definitely experiment with another Ethernet converter, there are some steps you can take to improve the robustness of your code.

1) Read the registers in one shot instead of 6 back to back reads with no wait in between. I've noticed that on certain older devices, too many consecutive reads can cause Error 56 - although I'm not sure of the reason why (perhaps the connection or device is getting overwhelmed). So set the starting register at 10, and the registers to read as 32, meaning it will give you all of the registers from 10-42 in an array. Then index or iterate through the array to scale the raw data to real-world values.

2) I like to maintain a shift register in the loop to count the number of consecutive failures (or errors). If there are, say, 5 consecutive errors, then I move into a state that resets the device connection. You might want to put a queue machine inside your loop to reflect this. See here: https://en.wikipedia.org/wiki/State_machine_(LabVIEW_programming)

3) I wouldn't "wait" for 60 seconds, then you won't be able to stop the loop before waiting for that amount of time. Instead, you can use a time elapsed VI with a custom input and put that in the loop to check every 250 ms or so. If the time has elapsed, go into a read state and get your data. If not, just idle. Here's the time elapsed VI: https://zone.ni.com/reference/en-XX/help/371361R-01/lvexpress/elapsed_time/#:~:text=The%20Express%20....

[Tomwell17]

ShivamSaxena,

Thanks for your advice on best practice methods. I have implemented 1 into my program and it has significantly decreased the amount of Error 56 that have resulted. I am still working on getting the concepts of 2 and 3 into my program. For 3, would this time elapsed VI be included in the state machine from 2? If I want the data to be collected at 5 minute intervals, how could I specify this in the time elapsed VI to move into the read? I am guessing that this will be through stages in the state machine:

1. Begin connection to Modbus device
2. Read input registers
3. Wait for XX seconds/minutes then move back to state 2
4. If X amount of errors occur, reconnect 

Thanks,

Thomas Firsich

[ShivamSaxena]

You have the right idea.

INIT - connect to device, hold connection in the shift register. GOTO IDLE
IDLE - keep checking the time elapsed VI, if false, return to IDLE, if true, GOTO GET DATA
GET DATA - read registers. If error, increment number of Errors. If errors exceeded GOTO RESET
RESET - close connection, wait for some time, maybe 30 seconds, go to INIT.

Also, you can disable the Error box dialog from showing following this link

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019SFfSAM&l=en-CA

[constructionworker]

Hi everyone,

I want to ask a question that I could not find here.
I'm trying to create an application using Modbus API. While doing tests, something caught my attention while reading and writing data, while loop speed drops for the address I read. When I try to read data from 3 addresses. I reach 100ms loop refresh. If I increase the read address, the refresh delay increases. Is there a fix or an alternative? ?

Only 100ms is fine, but if I do the same thing from 3 devices at the same time, the While loop refresh delay reaches up to 200ms.
Is there a way to read and write information faster.
Note: I am running these tests on NI-myrio.I want to read data from 3 addresses at the same time.

Every opinion given is worthy of respect.

Sample code is below.

[carlos_camargo]

Hi!

I don't have a solution for you but I have some comments and questions.

1. Your error tunnels should be shift registers, on the while loop and for loops

2. You might want to, in addition to your control, want to exit on error

    a. This might show you too if an error is occurring during reading or writing.

3. Do you have to write and read at the same time?

    a. Could you write in one loop when you need to?

    b. You'd then read all the time at the maximum possible rate.

4. Even if you need to write and read at every loop, you might want to investigate which operation slows you down.

5. Is the slowdown linear as you add addresses?

6. Is there something in the library functions slowing things down?  Something like an excessive wait?

[constructionworker]

---

@carlos_camargo wrote:

Hi!

I don't have a solution for you but I have some comments and questions.

1. Your error tunnels should be shift registers, on the while loop and for loops

2. You might want to, in addition to your control, want to exit on error

    a. This might show you too if an error is occurring during reading or writing.

3. Do you have to write and read at the same time?

    a. Could you write in one loop when you need to?

    b. You'd then read all the time at the maximum possible rate.

4. Even if you need to write and read at every loop, you might want to investigate which operation slows you down.

5. Is the slowdown linear as you add addresses?

6. Is there something in the library functions slowing things down?  Something like an excessive wait?

---

1. This is not the problem causing the slowdown. But I'm considering your suggestion.
2. There is no error condition at this time.
3. Case Structure When I activate the read state, I do not have to read and write at the same time. Meanwhile, the cycle rate is increased by half.
4. The slowdown in the loop is due to the register reading.
5.Yes, the deceleration is linear where I add each read address.
6. It may be due to the library. But that's exactly what I was wondering about. Is 50ms loop refresh normal when I read data from only 3 addresses?

[NathanLeung]

I got a same problem, Error 538193 occurred at IP Data Unit.lvclass:Read ADU Packet.vi:5030001. It just a very simple Modbus TCP, but it happens error 538193 and stop to read the data from my sensor. And we need to restart the program to read again. What the problem and any solution for fix this error. Any one help?