[time-nuts] OT: Prologix GPIB and HP3478A...The Answer!

Prologix support at prologix.biz
Wed Nov 28 13:25:13 EST 2007

Hi Chuck,

Good work identifying the problem! Leaving out the bus driver chips was a
conscious decision based on available board space and was not made lightly.
Extensive testing had shown that not to be an issue, so far. In fact, yours
is the first reported case of a drive related problem. However, we consider
even one incident to be one too many, and will be revisiting the decision
during a future release.

Thank you for brining this to our attention.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chuck Harris
Sent: Wednesday, November 28, 2007 7:47 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OT: Prologix GPIB and HP3478A...The Answer!

And the answer is:

My Prologix USB <-> GPIB Version 5.0 adapter has insufficient
pullups to drive my HP3478A DVM.

Once I put another (older) turned on device on the bus, the Prologix
snapped to attention and works as it should.

Abdul, there is a very good reason why the 488 bus driver chips
exist.  The 488 standard *requires* that no listener load the bus
with more than 1 TTL input load, and *requires* that every active
Talker or Controller have a pull up/ pull down resistor on every
data and control pin that is equal to  this:

~     +5V
~      |
~     3.1K
~      |
~  Out-+---->
~      |
~     6.2K
~      |
~     GND

It would appear that my 3478A is in a non loading tristate mode
until it is addressed, and you are relying on the devices on the
bus to do their own pull up.  Clearly that isn't always going to

-Chuck Harris

Chuck Harris wrote:
> Prologix wrote:
>> Hello Chuck,
>> It appears that 3478A sends data continuously as soon as it is addressed
>> talk. Since the Prologix adapter is busy processing GPIB data it is
>> to respond to USB commands. 
> I don't think the 3478A is sending anything at all.
> If it was, it should be showing some of the annunciators on its LCD
> like TLK, LSTN, RMT, SRQ, or something.  But it isn't.
> I think the Prologix isn't capable of driving the 3478A, logic level wise,
> and is waiting in vain for the 3478A to respond to a signal the 3478A
cannot see.

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