Mohamed,
As I am not an expert on the GPIB commands for an Agilent DCA 86100A (nor do I even know what that is offhand), I can only offer the following advice. I hope it is helpful anyway:
Try looking at the GPIB commands/programming manual that came with the instrument. If you do not have it, there should be an online version on Agilent's website. Also, you can probably download a driver set for this instrument, which should (not guaranteed by any means) contain the proper commands for this instrment. I strongly recommend using the instrument driver if it is available, as this will be the easiest way to solve this and any other problem you have with this instrument, as well as make it MUCH easier for you to program the instrument.