timing control signal and data acquisition

[Meadow]

Hello Siva,

GPIB or serial isn't really a PC based data acquisition. The data acquisition happens in the instruments and then the data is transfered to the PC through GPIB or serial data port. They transfer the data bit by bit or byte by byte depends on the protocol. GPIB and Serial communication falls under instruments control category.

If you are using PC based data acquisition, the data acquisition happens in the PC based DAQ itself. The data is transfered through PCI or PXI bus inside the computer. GPIB maximum data speed is around 1MB/sec, compared to PCI at 133MB/s.

I hope that this clarifies the difference between the two.

James

[siva0182]

Thanks for the clarifications...

The gpib is supposed to send 1Mb/sec...assuming one data point is 1 byte..i should be able to transfer atleast 10^6 readings per second..

also there are over heads and read commands to be transferred ..so for one data transfer even if i assume 10 bytes of info..transfer rate should be as high as 10^5 reading per sec...But i get only 10 readings per sec even after setting low NPLC settings 

After consulting the keithley support engineer..he asked me to check by using the SCPI commands instead of drivers...

[Dennis_Knutson]

The raw transer rate is just a small part of the overall performance. It takes significant time to actually process a command and perform a measurement. Only then can the transfer start. To expect the performance you are, is just unrealistic.

Why do you not believe you are sending the SCPI commands. That is just ridiculus. All you have to do is look inside one of the functions or turn on NI-Spy. As I said and you don't seem to believe, the overhead of a subVI is at most counted as microseconds. Stop beating this dead horse and work with what you have. 

[siva0182]

i understand what you say ..but i dont have any other choice to prove the keithley support engineer..

[Dennis_Knutson]

Then just go ahead and do it. You don't need permission. You have all of the commands you need - they are right there in the drivers and also in the manual.

[siva0182]

I managed to write the program using the SCPI commands and used GPIB write and read vi's ..

Now since I switched off averaging the time now reduced to 9 msec per reading.

I have also diabled display off,auto zero,auto ranges,source delays trigger delays.I have set NPLC to 0.01.BUt still i am getting only 111 readings per sec  via GPIB .

the specification of keithley 2400 is 1700 readings per sec.

can any one help me in this regard?

I have attahed the vi here.(sourcing 0.1 mA measuring voltage of about 0.5V.)

[Dennis_Knutson]

Have you read the manual about acquiring multiple points or looked at the function in the driver for this? You are acquiring a single point at a time and the spec does not apply to this mode.

Of course, since you insist on using your own code and not the driver, you probably don't need the names of the functions in the driver or the name of the example. However, the manual should explain everything. I think it's in the trigger section.

[siva0182]

My actual case is I read a point and then I switch a relay and then read a point and the whole thing continues in a loop ..

So I think i can't be using muliple point reading at a time...I need to read switch ,read switch.....

So do you still suggest that i can modify using trigger command..

[Dennis_Knutson]

Unless the Keithly can output a trigger to your other instrument (or whatever you are using to switch the relay), then no. You have reached the limits of the instrument and GPIB bus.

[siva0182]

MAy be the instrument but not the limit of the GPIB bus!!!! GPIB can teansfer upto 1MByte/sec