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vrf direct I/O error 811

Question asked by VRFuser on Jan 17, 2005
Griggs, Jon wrote the following:
> Please take the following with a tablespoon of salt... I can't find the reference I was looking for and produced the following from memory.
> Correction would be appreciated...


> While very hazy, I do remember that two things affect communication.
> The first is total cable length.
> The longer the cable, the more capacitance that has to be driven within a specific time window.

> The second is how many instruments are turned on.
> Instruments that are off apply a loading to the bus that is more significant that active instruments (maybe?).

> I think the max allowable between any active instruments at any point in the bus is 4 meters (?).
> If there are inactive instruments, I think that maximum active device to device length shortens...

> Did I set off anyone's B.S. detector?
>

Nope, but here is the text from an Agilent 82350A manual:

** The recommended method for connecting a GPIB system is a linear arrangement
with the system controller (PC) at one end of the system.  However, a GPIB
system can also be connected together in a star, linear and a combination
configuration as long as the total number of devices is <=15 and these
guidleines are followed:
* Minimize cable length as much as possible.  All system devices must have
tri-state drivers and must be powered on.  Systems with devices not using
tri-state drivers are limited to <250Kbytes/sec.  Turning devices on or off
while a system is running may cause faulty operation.
* For operation with data transfer rates <500Kbytes/sec, the total length of all
GPB cables is <= 2 meters times the number of devices connected together, up to
a maximum of 20 meters.
* For operation with data transfer rates >500Kbytes/sec the total length of all
GPIB cables is <=1m times the number of devices connected together, up to a
maximum of 15 meters.
* The length between adjacent devices is not critical as long as the overall
restriction is met.  GPIB bus extenders are available that allow operation over
much greater distances.

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