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<== Date ==> <== Thread ==>
Subject: Re: 2D waveform records
From: "Johnson, Andrew N. via Tech-talk" <[email protected]>
To: "Sobhani, Bayan" <[email protected]>, "[email protected]" <[email protected]>
Date: Thu, 14 Feb 2019 17:48:32 +0000
Hi Alex,

On 2/13/19 11:13 AM, Sobhani, Bayan via Tech-talk wrote:

Is it possible to have 2D waveform in EPICS? I have a waveform PV called ScanResults with type “DBR_DOUBLE[4]”. Would it be possible for it to have a type “DBR_DOUBLE[2][2]”? I notice the asynPortDriver::readFloat64Array function has a field for nElements but does not seem to have a field for dimensions.

 

Alex

The IOC can only handle 1-D arrays, but it doesn't really know/care about the layout of the double values within the 1-D array, so provided everything that accesses the waveform's VAL field agrees how what the array dimensions are meant to be you can treat the array as 2-D or higher if you want. Mark Rivers' AreaDetector modules do this and can support any number of array dimensions, although I'm not implying that you have to use AreaDetector.

The C language spec defines the memory layout for multi-dimensional arrays, e.g. see
    https://docs.microsoft.com/en-us/cpp/c-language/multidimensional-arrays-c?view=vs-2017
Note that this works even between different CPU architectures as long as you're always reading/storing values of the correct type (in your case double). Channel Access will perform any necessary byte-order changes when transporting the 1-D array values if your client and server have different endianness.

HTH,

- Andrew
-- 
Arguing for surveillance because you have nothing to hide is no
different than making the claim, "I don't care about freedom of
speech because I have nothing to say." -- Edward Snowdon
References:
2D waveform records Sobhani, Bayan via Tech-talk
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