Shared Variable communication (network publiched )

[sbkr]

Hi all ,

The whole day I have been banging my head with Shared variable communication between RT Host and host computer :

I essentially attempted to pass an array from the RT host to the Host computer .For this purpose I used a Network published shared variable of type Array U64 . I utilized this link to help me understand usign shared variable communication 

http://www.ni.com/white-paper/4679/en .. 

I also enabled RT FIFO .Now in the mentioned link this the information given about using single element FIFO or Multi element FIFO

'' By enabling the real-time FIFO, you can select between two slightly different types of FIFO-enabled variables: the single-element and the multielement buffer. One distinction between these two types of buffers is that the single-element FIFO does not report warnings on overflow or underflow conditions. A second distinction is the value that LabVIEW returns when multiple readers read an empty buffer. Multiple readers of the single-element FIFO receive the same value, and the single-element FIFO returns the same value until a writer writes to that variable again. Multiple readers of an empty multielement FIFO each get the last value that they read from the buffer or the default value for the data type of the variable if they have not read from the variable before.'' 

I had read the above content a 100 times but still I am not able to get a clear picture of the difference and when to use which option .So I decided to throw away the text and attempted to try somethign out for myself .

I had a loop in the RT host that writes different 3 element arrays into the ''network published shared variable '' . Now when I use the multi element option with '' number of arrays as - 2 '' I am able to read the arrays seperates one by one in the host computer ( as we can read a general queue).

Now when I used the single element option for the RT FIFO with un editable 1 array option ,only the last written array into the ''network published shared variable '' gets read in the host computer .

This sounds absolutely silly as if this is the case then what is the purpose of setting the ''buffer''.For testing purposes I had set the buffer as 5 arrays .

In fact I have only one writer ( writing in loop in the RT Host ) and one reader ( again reading in loop in the Host computer ).So I am not even sure if I require The RT FIFO option for the network published shared variable .

I apologise for the long message but I have reached the limits of running in circles waiting to bang somewhere 😄

I know I am doing a silly mistake somewher or I am probably missing a simple obvious trick but dont know what it is ..

I request for some clarity .. In fact I had posted a little while earlier today as well 😞 .... link - http://forums.ni.com/t5/LabVIEW/RT-Host-to-Host-computer-communication/td-p/2348130 )

any help would be appreciated

Cheers

me  

[Aurelie · Accepted solution]

Hi,

I think the difference between both types is clearer in the LabVIEW help :

"

Enabling the Real-Time FIFO

You can enable the real-time FIFO of a shared variable from the Real-Time FIFO page of the Shared Variable Properties dialog box. Place a checkmark in the Enable Real-Time FIFO checkbox to deterministically share data using single element or multi-element FIFOs.

Single Element FIFO

A single-element FIFO shares the most recent data value. The shared variable overwrites the data value when it receives a new data value. Use this option when you need only the most recent value. Configure the size of the array elements or the size of the waveform for the FIFO buffer if you select an array or waveform data type.

Multi-Element FIFO

A multi-element FIFO buffers the values shared by the shared variable. You can configure the size and the elements of the FIFO buffer to match the settings from the Use Buffering section of the Variable page, or you can configure a custom size for the FIFO and the FIFO elements.

Note  For both single-element and multi-element FIFOs, if the variable contains array or waveform data, you must configure the size of the FIFO elements equal to the size of the data you want to share. If both the network buffer and the RT FIFO are enabled, the network buffer must be at least as large as one FIFO element. Sharing data smaller or larger than the length you specify causes a memory allocation that affects determinism.

Note  Waveforms contain variable-size variant elements that are not compatible with the Real-Time FIFO. Therefore, if you enable the Real-Time FIFO on a shared variable that contains waveform data, the variant element of the waveform data does not transfer.

"

http://zone.ni.com/reference/en-XX/help/370622K-01/lvrtconcepts/rt_projectvariable/#Single_Element_F...

'Hope it helps

Aurelie