to retrieve 2 CH data from DSA72004B simultaneousl

[djsg]

Hi

I managed to use one of the sample code: CurveQueryWinforms to retrieve data from a DSA72004B scope.
In the data source command, I can specify two channels: CH1,CH2.
But
scope.FormattedIO.ReadLineBinaryBlockOfInt16();
only gives me the data of CH1.

I wonder

• I guess the scope should be able to pass me the data of the two channels, isn't it? Since it can show two channels' data at the same time.
• what API function shall I use to get the data? This may have something to do with NI.VISA library.
• My code works when I use GPIB cable to link the scope to my computer. When I connect them with LAN cable, the TCPIP address method never works. What is the trick to use LAN connection?

Thanks.

[dwyban]

Hi @djsg,

The method call
scope.FormattedIO.ReadLineBinaryBlockOfInt16();
is a call to part of the IVI Visa.NET library that is expecting an IEEE formatted binary block of data, which is reads and parses, returning an array of 16 bit values. This method cannot tell the difference of whether the binary block of data contains the data for just one channel or two channels. It just reads the binary block header and the data following it and outputs a data array in accordance with the block header and the data type (which is Int16 in the case of this call).

I gave this a try on a 70K scope and I think I see further into what you are saying the problem is. Let's say my record length is 50 points, my channels are CH1 and CH3, and my data is being transferred as two bytes per point (Int16). Below are the commands I sent the instrument and the response back.

data:width 2
data:sour CH1
curve?
#3100x00 xAE x00 xAE x00 xAE x00 xAF x00 xAF x00 xB0 x00 xB0 x00 xAF x00 xAE x00 xAE x00 xAE x00 xAF x00 xB1 x00 xB2 x00 xB3 x00 xB2 x00 xB0 x00 xAD x00 xAB x00 xAC x00 xB2 x00 xBF x00 xD1 x00 xE8 x00 x01 x00 x18 x00 x2C x00 x3A x00 x41 x00 x43 x00 x41 x00 x3E x00 x3C x00 x3B x00 x3C x00 x3F x00 x41 x00 x44 x00 x45 x00 x45 x00 x44 x00 x43 x00 x43 x00 x43 x00 x44 x00 x44 x00 x45 x00 x45 x00 x44 x00 x44 x0A
data:sour CH1,CH3
curve?
#3100x00 xAE x00 xAE x00 xAE x00 xAF x00 xAF x00 xB0 x00 xB0 x00 xAF x00 xAE x00 xAE x00 xAE x00 xAF x00 xB1 x00 xB2 x00 xB3 x00 xB2 x00 xB0 x00 xAD x00 xAB x00 xAC x00 xB2 x00 xBF x00 xD1 x00 xE8 x00 x01 x00 x18 x00 x2C x00 x3A x00 x41 x00 x43 x00 x41 x00 x3E x00 x3C x00 x3B x00 x3C x00 x3F x00 x41 x00 x44 x00 x45 x00 x45 x00 x44 x00 x43 x00 x43 x00 x43 x00 x44 x00 x44 x00 x45 x00 x45 x00 x44 x00 x44 x23 x33 x31 x30 x30 x00 x46 x00 x47 x00 x46 x00 x47 x00 x4A x00 x48 x00 x45 x00 x47 x00 x48 x00 x47 x00 x46 x00 x46 x00 x45 x00 x47 x00 x49 x00 x47 x00 x46 x00 x46 x00 x46 x00 x46 x00 x47 x00 x47 x00 x3F x00 x27 x00 x04 x00 xE3 x00 xC9 x00 xBA x00 xBA x00 xBE x00 xBE x00 xBD x00 xBD x00 xBC x00 xBB x00 xB9 x00 xB6 x00 xB5 x00 xB8 x00 xB9 x00 xB8 x00 xB8 x00 xB8 x00 xB6 x00 xB3 x00 xB5 x00 xB9 x00 xB8 x00 xB5 x00 xB5 x0A

Notice the binary block header after the first curve? query and how it says #3100... indicating that 100 bytes of data follow (50 points X 2 bytes per point). This makes sense as we are fetching only CH1 (data:sour CH1).
Then notice in the binary block header of the second curve? query, after we set data:sourc CH1,CH3, that the block header still says #3100... even though there is clearly twice as much data sent back. Since the block header says there is only 100 bytes of data, the method
scope.FormattedIO.ReadLineBinaryBlockOfInt16();
is only returning a data array made from the data of the first 100 bytes. The second 100 bytes is ignored and may even be available in a subsequent read.

OK so we see the problem. How do we fix it? Well there is really only a couple of choices.

1. Read each channel's waveform individually. data:sour CH1 -> curve? -> data:sour CH3 -> curve?
2. Don't use the scope.FormattedIO.ReadLineBinaryBlockOfInt16();. Instead read the data back as a raw array of bytes and then parse out the data in your own code.

Option 1 would be the simplest to implement. However, if you are trying to curve? the instrument while it is in continuous free-running acq mode, you cannot guarantee that both channel's waveforms will be from the same acquisition so you will want to perform a single sequence between acquisitions.
Option 2 requires a lot more coding effort on your part. However, it will make the transfer quicker as there will be less command overhead to transfer both waveforms. It also will ensure that both channel's waveforms are from the same acq, if the scope is in continuous free-running acq mode.

If absolute maximum speed is not critical, I'd stick with option 1.

[djsg]

Hi @dwyban

Thank you very much for your advice. It is of great help.

Option 1, I think, is good enough for me, because we just want to check if two signals are synchronized, if not, what is gap. As long as that gap is within a certain range. We are happy.

1. Can I double check with you: I am OK to read the data of 2 channels in sequence, and we don't need continuous capturing; one shot will do. However, the data must captured simultaneously; Does Option 1 guarantee that?
2. What the function to read back data as a raw array of bytes? Sorry, I am really new to this IVI Visa.NET library. Is there any good reference manual for it?

[dwyban]

Hi @djsg,

The scope always captures the data for all channels at the same time so as long as the scope is stopped before fetching the waveforms, you can be sure that all waveforms fetched are from the same acquisition. The only way you would get waveforms from different acquisitions on each channel is because you left the scope in free-run and by the time you finished querying one channel, the scope has already made another acquisition. Just perform Single Sequence acquisitions and then fetch the data after each acq completes.

As for how to read the data back from the scope as an array of bytes (so you can parse through it yourself) is to use one of the IMessageBasedRawIO.Read() methods. This would look like
response = scope.RawIO.Read();
in the context of this program.

As for documentation, if you are using NI-VISA as your VISA library, look in your Start menu -> National Instrument -> NI-VISA Documentation. This will open a folder with documentation links. You want to use the one labeled NI-VISA .NET xx.x Help were xx.x is the version number. This will be your easiest to use documentation. Otherwise, you can look at the IVI VISA.NET specification directly which you can find here:
https://www.ivifoundation.org/downloads/VISA/vpp436_2025-01-13.pdf
This is the document that all vendors must write their VISA.NET implementation in accordance to.

[djsg]

I have used scope.RawIO.Read() to get a few data samples, each of which has a few k data points. Yet to do analysis.
Thanks. @dwyban