LV 7.1 : Instument I/O Assistant and Expander/Isolator NI GPIB-120A

[reteb]

I just installed the NI Expander/Isolator 120-A and it works flawlessly.
Though, the manual says that now 28 devices could be on the GPIB bus but my LabView 7.1 I/O Assistant prompts me for 30.
I do know that the 120-A has an electrical load of 2 devices, but even then, why does LV I/O Assistant prompts me for making my choice out of 30 devices if only were 28 possible ?
If only 28 out of the prompted 30 were possible, which two ## are unaccessible ?

Thanks, Peter van Daalen.

[TommyTestBoy]

Peter,

The Instrument I/O Assistant will enumerate all VISA resources on your system that it can communicate with, not just GPIB devices. The extra two resources that show up in the list are probably the comm port and printer port on your PC. Do you see COM1 and LPT1 in the list of available instruments?

-Tommy

[reteb]

Yes, apart from the 30 GPIB devices I see my Lpt and my 2 Com ports as separate devices.
In my LV_7.1 I/O Assistant Express.vi the said 30 GPIB devices are really GPIB devices...( They are even numbered GPIB 1..GPIB30 ).

Thanks from Amsterdam for any further help.
Peter van Daalen.

[KvZ]

The Maximum number of devices per bus are 15, the GPIB-120A connect two busses and takes 1 device load on both busses this makes that the maximum number of devices that you can connect becomes: 15 (devices) * 2 (busses) - 2 (the GPIB-120A once on each bus) = 28 (devices).

As for the 30 devices the is expacthed behaviour of GPIB bus extenders. For more information see:

GPIB-120 A Returns Virtual Hardware Instruments

Kind Regards,

Karsten

[reteb]

Thanks to Karsten.
I followed your link and I do understand about the ( two ) virtual instrument ##'s now.

Thanks to the other respondents as well !

Best regards.

Peter van Daalen.