usb-gpib-hs speed when first GPIB device is powered off

[erikj]

I noticed an odd effect with my new USB-GPIB-HS (using an Intel Mac, the latest GPIB drivers and LabView 2009). My GPIB devices (six in total) are arranged linearly, and the first device that has the USB-GPIB-HS plugged in is a Yokogawa multimeter. I was reading values from a device further down the GPIB chain, and found that the speed of reading from that device was much slower when the Yokogawa DMM was turned off. I never noticed a similar effect when I used a PCI-GPIB on the same system. Reading through the manual (and online here) I can see no mention that the first GPIB device needs to be turned on or would have any effect on the GPIB communications speed. Is this normal or expected?

TIA,

Erik. 

[Albert.Geven]

Check the difference with an oscilloscope. Especially the timing of the handshake signals can be affected by a load that is off or on.

It is allowed to have not every instrument switched on but nothing is said about having the same speed. And at least half of the instruments should be on. 

How long is your cabling between instruments. Normally this is not really critical but officialy it should not exceed 2m between instruments.

If you measure the timing it would be nice to see your measurements here.