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From: Joerg on 11 Mar 2006 16:54
Hello Tauno,

>
> There is an electrical problem: Power.
>
> The GPIB bus is a TTL-level bus with quite low impedance.
> There is simply not enough power available from USB to
> drive the GPIB according to the specifications.
>

Huh? If 2.5 Watts ain't enough then I don't know what you want to drive
with it:
http://www.digi.com/pdf/prd_usb_usbplusseries_appnote.pdf

Ok, it won't power your espresso machine...

Regards, Joerg

http://www.analogconsultants.com
From: cs_posting on 11 Mar 2006 17:35
Tauno Voipio wrote:

> There is an electrical problem: Power.
>
> The GPIB bus is a TTL-level bus with quite low impedance.
> There is simply not enough power available from USB to
> drive the GPIB according to the specifications.

What do you think the bus impedance is?

I think it's fairly high, but dependent on the number of devices. Each
device should have a 3k resistor to 5v and a 6.2k resistor to ground,
of course in parallel with the actual input gate. Load up a bus to its
maximum device count so you have many of these in parallel and you
might see problems, but typically with a USB dongle you have few
devices - most often only 1 device. I don't see this exceeding the
post-enumeration USB current limit.

I also think it would be perfectly legitimate to make a device which
was specified in bold letters right on it, "only for direct connection
to a single instrument without an intervening cable" This is after all
the most common use of the USB-GPIB dongles on the market.

From: Mike Harrison on 12 Mar 2006 07:53
On 11 Mar 2006 14:35:28 -0800, cs_posting(a)hotmail.com wrote:

>Tauno Voipio wrote:
>
>> There is an electrical problem: Power.
>>
>> The GPIB bus is a TTL-level bus with quite low impedance.
>> There is simply not enough power available from USB to
>> drive the GPIB according to the specifications.
>
>What do you think the bus impedance is?
>
>I think it's fairly high, but dependent on the number of devices. Each
>device should have a 3k resistor to 5v and a 6.2k resistor to ground,
>of course in parallel with the actual input gate. Load up a bus to its
>maximum device count so you have many of these in parallel and you
>might see problems, but typically with a USB dongle you have few
>devices - most often only 1 device. I don't see this exceeding the
>post-enumeration USB current limit.
>
>I also think it would be perfectly legitimate to make a device which
>was specified in bold letters right on it, "only for direct connection
>to a single instrument without an intervening cable" This is after all
>the most common use of the USB-GPIB dongles on the market.

...or have a socket for an external PSU.
From: Mike Harrison on 12 Mar 2006 07:54
On Sat, 11 Mar 2006 21:54:31 GMT, Joerg <notthisjoergsch(a)removethispacbell.net> wrote:

>Hello Tauno,
>
>>
>> There is an electrical problem: Power.
>>
>> The GPIB bus is a TTL-level bus with quite low impedance.
>> There is simply not enough power available from USB to
>> drive the GPIB according to the specifications.
>>
>
>Huh? If 2.5 Watts ain't enough then I don't know what you want to drive
>with it:
>http://www.digi.com/pdf/prd_usb_usbplusseries_appnote.pdf
>
>Ok, it won't power your espresso machine...

But can keep your coffee warm...

http://www.ebigchina.com/ebcps/4/pd/1307308.html
From: Mike Harrison on 12 Mar 2006 07:55
On 11 Mar 2006 12:33:49 -0800, cs_posting(a)hotmail.com wrote:

>Dave (from the UK) wrote:
>
>> No thanks - I think I'd rather pick up a National Instruments PCI card
>> on eBay, which would be 100% software compatible with NI.
>>
>> If I really needed USB, and it is hard to see why, then I'd probably
>> still buy a NI one.
>
>One word: laptops
>
>Why you might ask, would one want to control a GPIB bus with a laptop?
>
>1) Salesmen need to demo GPIB instruments without dragging along a
>desktop computer
>
>2) Support engineers need to be able to check out a GPIB instrument
>without dragging along a desktop computer or depending on the
>customer's equipment.
>
>3) When GPIB equipment is installed in a rack and it is necessary to
>try someting out of the ordinary - use a different computer to see if
>the problem is with the rack mount one, or ttemporarily use GPIB to
>debug a system where it is not ordinarily in use, it's a lot easier to
>carry a laptop over to the system, crawl behind the rack with it,
>whaterver, than to do so with a desktop.

4) Space. Many workbenches are short of space - a laptop takes little room and can quickly be
stashed away when not in use.
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