Why such small transfer rate between the PC and ESA-L spectrum analyzer?

[Aleshka]

I use in the activity ESA-L Series Spectrum Analyzer E4411B. The constant reading of the data from a screen of a spectrum analyzer (401 points) on a hard disk of the computer through GPIB-interface implements. For reading of the data is applied the commands sequence: 1) inquiry for data obtaining and 2) data receiving. For this purpose are used the Visual C++ and NI-488.2 API.
For example, part of the code:

while (iter < 250)
{
ibwrt(esa,":INIT;*OPC?",11L); // Trigger a sweep
ibrd(esa,&ok_trigger,4);
ibwrt (esa,"TRAC:DATA? TRACE1",17L); // Request
ibrd (esa,Data_buffer,Size_buffer); // Data reading
iter++;
}

There was a following problem: the too large receptive period of the d
ata in matching with frequency of updating of the data on a screen of a spectrum analyzer (For example, for 250 measurements at Sweep = 4ms theoretically should be 250 х 4 = 1000ms = 1s, and really receives more 10s).
It is supposed that the delay happens owing to long transaction (commands) processing received by a spectrum analyzer. If it so, that I can undertake in this case? Or can, there are also other reasons of delay? Please, explain!
My hardware and program configuration: PC Intel Mobile Celeron 800 MHz, 64 Mb RAM, Windows 98, National Instruments PCMCIA-GPIB Interface Card, National Instruments PCMCIA-GPIB Cable (2 meters), Visual C ++ 6.0, National Instruments NI-488.2 Version 1.60.

Beforehand is grateful
Aleshka

[Ray K]

Aleshka-
You cannot expect to have an instantaneous transfer rate from your instrument. All the data that is being displayed needs to converted to strings and sent over the GPIB bus. If your instrument can handle it, you can speed up the bus timing in MAX. If you want really fast speeds, you should look into getting a PXI system with a PXI spectrum analyzer.

Ray K
NI Applications Engineer