how to increase scan rate of NI Switch SCXI 1130

[hema]

Hi,

I have NI PCI 4070 DMM used with NI SCXI 1130 sitch module. I have connected 10 thermocouples to 1130 module. I am scanning the channels and reading the values in the program using niSwitch and niDMM VIs. I am using software trigger in the program. I have configured Software Trigger in niDMM configur trigger and niDMM configure Multipoint. I get the correct values when i scan using chi->com0, where i goes from 0 to 9. But the problem is that the rate of scanning is very slow.

There is niSwitch Configure scan Rate.vi, here i have given scan delay as 0 second.

It takes one second for one channel when i run the program. why is this , is this because i used software trigger for each channel scan? how to improve the scan rate. ?

[Elizabeth W]

Hello,

I would actually recommend that you use synchronous scanning if you want to maximize the speed of your scan, rather than using software triggers.  If you use synchronous scanning, the DMM will generate a digital pulse (Measurement Complete) each time it completes a measurement, allowing the switch to advance to the next entry in the scan list the instant the DMM has completed its measurement.  The DMM will then take the next meausurement after a specified harware-timed interval.  This will be much more efficient than sending software triggers back and forth to time the scanning.  To set up your application using synchronous scanning, follow these steps:

1. Open the LabVIEW shipping example "niSwitch DMM Switch Synchronous Scanning.vi", found in the NI Example Finder in the folder Hardware Input and Output » Modular Instruments » NI-SWITCH (Switches).
2. Configure the VI to export the Measurement Complete trigger to the AUX connector on the front of the DMM.  It will be exported to pin 6 on the connector, as shown in the image below:

[Elizabeth W]

Sorry for the confusion, I started writing a post and got interrupted and came back to it too late.  You can disregard the last post and here is my final answer:

I would actually recommend that you use synchronous scanning if you want to maximize the speed of your scan, rather than using software triggers.  If you use synchronous scanning, the DMM will generate a digital pulse (Measurement Complete) each time it completes a measurement, allowing the switch to advance to the next entry in the scan list the instant the DMM has completed its measurement.  The DMM will then take the next meausurement after a specified harware-timed interval.  This will be much more efficient than sending software triggers back and forth to time the scanning.  To set up your application using synchronous scanning, follow these steps:

1. Open the LabVIEW shipping example "niSwitch DMM Switch Synchronous Scanning.vi", found in the NI Example Finder in the folder Hardware Input and Output » Modular Instruments » NI-SWITCH (Switches).
2. Physically connect the Measurement Complete output trigger from the DMM to the trigger input of the switch.  How you will do this depends on what type of chassis you are using (PXI/SCXI combo chassis or separate chassis) and what switch terminal block you're using.  If you need assistance with this please provide more details about your hardware setup and I'd be happy to help out.  The following resource may be helpful here: KnowledgeBase 3V07KP2W: Switch/DMM Hardware Configurations.
3. Select valid values for all other front panel controls and run the VI.

I hope this is helpful.  Please let me know if I have misunderstood your application, or if you would like me to go into more detail on any specific part of the solution provided above. 

[hema]

Hi Elizabeth,

thanks for the reply.

I am using separate SCXI 1100 chassis with SCXI 1130 and SCXI 1357 connector and the DMM is PCI DMM 4070.

So i will have to use the trigger cable to initiate hardware timed interval to increase the scan rate.

I first wanted to try with software trigger. but there is no option to increase the scan rate for that.

Thanks, Hema

[Elizabeth W]

Hi Hema,

Hardware triggering is definitely the way to go if you are wanting to maximize your scan rate.  However, I can help you try to increase the speed of your software scan if you would prefer to do that instead.

I would like to get a better idea of whether or not the scan rate you are seeing is within the expected range.  How frequently are you sending a software trigger to advance the scan list in your program?  (This will give me a better idea of what rate you are expecting to scan at).  Also, what resolution do you have the DMM set to?  The higher the resolution of the DMM, the longer each measurement will take.

A good starting point for switch software scanning is the shipping example niSwitch Software Scanning.vi, found in the NI Example Finder under Hardware Input and Output » Modular Instruments » NI-SWITCH (Switches).  You may want to compare this example to your current code and see what scan rate you can attain with this example. 

[hema]

Hi Elizabeth,

I am using a DMM resolution of 1E-7., I am using niDMM send software trigger.vi, each time to scan each channel. i have attached a dummy VI which shows software trigger i had used for checking.

I am looking for scan rate around 10-20 microsecs. I have given no delay in the for loop.

thanks.

[Elizabeth W]

Hi Hema,

Unfortunately, I see several problems here:

• You simply cannot count on that VI to run at a loop rate of 10-20 us per loop iteration.  Software timing is definitely too slow to achieve the scan rate you want. 
• Your switch module has a maximum scan rate of 900 cycles/second.  To achieve 1 cycle per 20 us, you would need a switch with a maximum scan rate of at least 50,000 cycles/second.  I do not know of any switch that can go this fast.
• Even at its minimum resolution of 4 1/2 digits (not sure what you meant by a resolution of 1E-7), the DMM cannot achieve a reading rate this fast.  As stated in the NI 4070/4072 Specifications, the maximum reading rate of the DMM is 10,000 samples/second.

If you use hardware triggering, you will achieve much better scan rates, although you still will not be able to achieve a scan rate of 20 us.  Please let me know if you have any questions about this.