Timeout when using Modbus vi

[zener3]

Hi,

I am developing a .vi that polls data from two seperate flowmeters and a D-A converter via Modbus.  The interface on the flowmeters are set at address 111 and cannot be changed so I cannot use the same master to get both flowmeters data.

Ive attached a picture of the code for reference. Ive only included the Modbus portion so as not to confuse the issue.  

The problem is, when I change the timing of the loop to 5 seconds, I get timeout error 61 from the modbus reads and the re-connect loops try but cannot re-connect them. When the timing is 3 seconds or quicker, they work perfectly.  At 4 seconds, I get intermittent errors and the re-connect case works fine.

3 seconds or less = works fine

4 seconds = occasional errors

5 seconds = no work.

I'm not an expert at Modbus for sure.  So please excuse my messy code.  I intend to clean it once I get it working correctly.

My flowmeters are RS-485, 38,400 baud, Modbus RTU.  They connect to ADAM 4017 modules that I use Modbus TCP to connect to.  The A/D converter is also Modbus RTU and its on the same circuit as the MMin meter and its address is 3.

Any help and/or guidance would be appreciated. I can attach the entire .vi if necessary.

[diabeticdaniel · Accepted solution]

Firstly, I wanted to note that it looks like you're using a different API from the NI-supported Modbus API, so take my answers with a grain of salt.

As a complete shot in the dark, it's possible that the device you are communicating with requires some sort of communication within a timeout period otherwise it goes into some idle state after connecting. For example, if you make a Modbus TCP connection and then no communication happens for over 4 seconds, the slave goes into some idle state. This does not seem like a likely problem, but it's something to look in to. Due to the fact that I'm not familiar with the API you are using, I don't think there's much more I can recommend.

[zener3 · Accepted solution]

Found the issue!

The ADAM 4017 modules had a "Host Idle timeout" set at 4 seconds.  I increased this timeout to 10 and now I can poll at 5 second intervals just fine.

Your answer got me looking at the settings again on the Adam modules and thats where I discovered the solution.

Thanks!