VISA probe location

[bbean]

Does anyone know where the VISA Instr Probe (custom probe) source code is located?

 

I'm interested in how the "Is session valid?" and "Is Instr?" Booleans are determined

 

 

[Darren]

C:\Program Files\NI\LVAddons\nivisa\1\vi.lib\_probes\default\VisaProbes.llb\VisaProbeInstr.vi

[Rolf Kalbermatter]

On 4/24/2025 at 9:54 PM, bbean said:

Does anyone know where the VISA Instr Probe (custom probe) source code is located?

 

I'm interested in how the "Is session valid?" and "Is Instr?" Booleans are determined

 

 

A VISA Session is simply a LabVIEW refnum too, just a different flavor (sepcifically TagRefnum) which has an attached user readable string. Same about DAQmx sessions and any other purple wire. As such the "Is Not A Number/Path/Refnum" works on it too.

One difference is that unlike any other LabVIEW refnum, you can actually configure if the lifetime of the VISA refnums should be tied to the top level VI or just left lingering forever until explicitly closed. This is a global setting in the LabVIEW options.

Edited 11 hours ago by Rolf Kalbermatter

[bbean]

3 hours ago, Rolf Kalbermatter said:

A VISA Session is simply a LabVIEW refnum too, just a different flavor (sepcifically TagRefnum) which has an attached user readable string. Same about DAQmx sessions and any other purple wire. As such the "Is Not A Number/Path/Refnum" works on it too.

One difference is that unlike any other LabVIEW refnum, you can actually configure if the lifetime of the VISA refnums should be tied to the top level VI or just left lingering forever until explicitly closed. This is a global setting in the LabVIEW options.

Thanks for the detailed explanation.  I think the "Is Not a Number/Path/Refnum" is what I'm looking for based on what you mentioned and what the probe has under the hood.  I've never really had a need for using it before with VISA until now and I'm learning things even after many years of coding LabVIEW.

What brought up the question was a situation where two separate "instrument drivers", one an Keysight 973 multiplexer and the other a Pendulum CNT-91 Frequency counter were being called in separate loops.  The loop for the Keysight 973 was setup to periodically (every 10secs) sample its channel list, the loop for the Pendulum was setup in a similar fashion to sample every 500ms.  At some point the end user disconnected the Keysight 973, but wanted to continue measuring with the Pendulum CNT-91.  But with the Keysight disconnected, the VISA calls in its loop were locking up or reserving the USB bus and interfering with the measurements in Pendulum loop. 

Diving into the weeds of the code, I noticed:

• the Keysight 973 driver (actually an older version using Agilent 34970 code) had Synchronous VISA Write and Read calls. So I thought those must be running in the UI thread and blocking the execution of the Pendulum loop when the instrument was connected.  After switching them to Asynchronous VISA calls the problem persisted. 
• Both VIs that sampled the data were had VI properties for Execution set to "same as caller" and since they were both called from top level VIs that had property nodes, event structures etc they were probably both running in the UI thread.  I switched the Keysight to the "data acquisition" thread and the Pendulum to the "instrument driver" thread.  But the problem still persisted.
• in the Keysight loop I had made use of the VISA User Data property to store and track the number of channels were being queried.  This VI was taking ~2secs to execute when the Keysight was not connected.  This was a surprise to me because I thought it was just a passive property node that would return its value (quickly and without error) even if the instrument was disconnected.  But apparently, its doing much more under the hood, possibly even trying to reconnect (possibly VISA open under the hood?) to the Keysight instrument.  After temporarily disabling this code and all instrument query in the 10 sec sampling loop, the interference or blocking of the Pendulum went away.

Lessons learned:

• If an instrument is disconnected physically, make sure you disable any queries to it if the VISA session isn't valid or didn't' connect properly in your initialize state. I'm still not sure why a VISA query timeout or User Data call would block a separate VISA loop with a different address from executing if they are properly setup to use separate threads (maybe I'm still not doing that correctly).  I guess I could use NI I/O trace to get more details about what's going on under the hood when the User data property is read.  What's also weird to me is that the VISA timeout for that session was 10 seconds (not 2 seconds) but every call to the User Data property took almost exactly 2 secs.
• Avoid using the VISA user data property for passive storage because its not actually passive.
• the "Is Not A Number/Path/Refnum" works with VISA sessions.  Even though I disabled sampling now if the Keysight initialize state fails, I'm also using the "Is Not a Number/Path/Refnum" as a paranoid check before executing the sampling code now.  Looking at the VISA probe VI that Darren linked to, it appears like the code also checks the VISA property "Rsrc Class" to see if the VISA session "Is Instr" if the "Is Not a Number/Path/Refnum" returns a False. Does anyone know if this is a passive call under the hood?

Thanks for everyone's help.

Edited 8 hours ago by bbean

[Rolf Kalbermatter]

Synchronous is not about running in the UI thread. It is about something different. LabVIEW supports so called cooperative multitasking since long before it also supported preemptive multithreading using the underlaying platform threading support.

 

Basically that means that some nodes can be executed in Asynchronous operation. For instance any build in function with a timeout is usually asynchronous. Your Wait for Multiple ms timing node for instance is not hanging up the loop frame waiting for the expiration for that time. Instead it goes into an asynchronous operation setting up a callback (in reality it is based on occurrences internally) that gets triggered as soon as the timeout is expiring. The diagram is then completely free to use the CPU for anything else that may need attention, without even having to more or less frequently poll the timer to check if it has expired. For interfacing to external drivers this can be done too, if that driver has an asynchronous interface (The Windows API calls that Overlapping operation). This asynchronous interface can be based on callbacks, or on events or occurrences (the LabVIEW low level minimalistic form of events). NI VISA provides an asynchronous interface too, and what happens when you set a VISA node to be asynchronous is basically that it switches to call the asynchronous VISA API. In theory this allows for more efficient use of CPU resources since the node is not just locking up the LabVIEW thread. In practice, the asynchronous VISA API seems to be using internal polling to drive the asynchronous operation and that can actually cause more CPU usage rather than less.

It should not affect the lifetime of a VISA session and definitely not the lifetime of a different VISA session.

But disconnecting an instrument doesn't automatically make a session invalid. It simply causes any operation that is supposed to use the connection for actual transfer of bytes to fail. 

Edited 7 hours ago by Rolf Kalbermatter

[bbean]

40 minutes ago, Rolf Kalbermatter said:

Synchronous is not about running in the UI thread. It is about something different. LabVIEW supports so called cooperative multitasking since long before it also supported preemptive multithreading using the underlaying platform threading support.

You are correct...I worded my thought incorrectly.  My thought was that since the Keysight code was using Synchronous VISA calls and both the Keysight / Pendulum were most likely running in the UI thread bc their callers were in the UI thread and the execution system was set to same as caller initially, the VISA Write / Read calls to the powered off instrument were probably blocking the other instrument with a valid connection (since by my understanding there is typically only one UI thread).