Re : Modbus read

[MogaRaghu]

I am trying to read three holding registers of a Modbus slave unit from inside of a timed loop. The timed loop is configured to run at 250ms interval in the original application and hence I will not be able to wait till the read is completed. Hence I am reading each  register one by one allowing a 250ms gap in between each read.

I just want to know if the VI as I have coded will do the job ?? ( Reading one register in a  VI with Timed loop is working fine )  I am particularly keen to know if the way I have opened the VISA reference and using it is correct.

Thanks 

[chilly_charly]

No need to chain three case structures containing exactly the same code. Instead, you could create a short array containing the adresses to be read, and index it accordingly, using the result of a single Q&R operation.

Also, I usually use a different re-order code (see picture). However, I did some timing and although more compact, my solution is surprisingly 2 times slower 😞

[MogaRaghu]

Hi CC,

Agreed there was NO need to repeat structures as I did. But when you are struggling with a code snippet and do not know why it does not behave as it should I guess it is good to KISS 😉

The conversion as you did is simple but it does run a tad bit slow. I am already fighting for time and hence chose the fastest means.

But since you did not remark anytrhing about the way I handled VISA,  I guess it is oK ?

Thanks

[chilly_charly]

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Raghunathan a écrit :

The conversion as you did is simple but it does run a tad bit slow. I am already fighting for time and hence chose the fastest means.

But since you did not remark anytrhing about the way I handled VISA,  I guess it is oK ?

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the decoding step takes less than 10 microseconds, so I don't believe it will have any impact on the overall performance 😉

Waiting for the bytes transmission is orders of magnitude longer, leaving time for parallel processing.

Nothing wrong with VISA as far as I can see.

[MogaRaghu]

Hi CC,

Thanks for the input confirming that the way the VISA is handled is OK.

With regard to the "slower " execution of the conversion with Split and Join number functions, the remark was to point the relative merits.

Thanks

[RavensFan]

Why not use the Swap Words function?

[chilly_charly]

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Ravens Fan a écrit :

Why not use the Swap Words function?

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because it's two times slover than char reordering. I did some timing.

[RavensFan]

My calculations show that the swap words doing it in about 2/3 the time.  About 14 msec per million as opposed to 21 msec per million for the split/join method.  Sometimes about the same.

Though either way the calculation is very fast.

[chilly_charly]

Of course, you are right. Sorry, I was still refering to the string manipulation.

[MogaRaghu · Accepted solution]

Hi Ravens and CC,

Thanks for your interesting alternatives in handling the data conversion. Of course in this case the maximum time is taken by the write - read to  the Modbus and  thus this conversion timing is not too critical.

But of course compared to the original way of stripping - reorganizing and concaneting the alternatives are very elegant.

thanks again.