Why must two thirds of the instruments on a GPIB bus be on?

[Anchoman]

Situation:  We are using a GPIB-120B extender to drive up to ~20 Tektronix digitizers on a GPIB bus.  With all 20 instruments on it works fine.
 
Problem:  When using the system with just a few digitizers on (the rest off but still physically connected as part of the GPIB daisy-chains) MAX could not detect the digitizers that were on.  Consequently the controlling software (Labview) failed.
 
In the GPIB-120B manual it states "Where there is GPIB activity, it is recommended that you keep at least two-thirds of the devices on both buses powered on."  Unfortunately, the manual gives no explanation of this.  Doing a Google search shows that there are similar statements for most National Instrument 488.2 devices.
 
Solution:  Turning on most of the instruments on both sides of the extender solves the problem - but we do not want to do this.
 
Questions:  Why must 2/3rds of the instruments be on for the bus to work?  We would like to find a workaround for this but do not want to physically change the GPIB daisy-chain to suit the number of powered on instruments.  A simple fix that is independent of the number of powered on instruments is required.
 
Thank you

 

[NI_Device_user_sb]

Hi, do you think it could be a cable length problem?Cause in the IEEE standard, it is said that you must have up to 2m length between instruments.So if turning off some instruments, it is as if you were having more distance between your instruments.Maybe I'm wrong, but it could be your issue.

[Dennis Knutson]

The requirement for two-thirds to be powered is in the IEEE.488 spec. I believe it is to maintain a minimum impedance for the bus. For maximum speed, you need all instruments powered on.

[Anchoman]

Thank you, that does make sense.  We could go from a pure serial daisy-chain to a seial parallel (daisy-chain / star) confoguration to minimize lengths when instruments are off.

[Albert Geven]

Hi It is indeed the impedance of the system. The impedance is lower for switched on systems (active).And yes you may mix star and daisy chain configurations and please be careful 2m is officially the max length between two instrumentsand although there are 32 address positions you are not allowed to use more than.. 16 or 20 if I remember correctly.